Cow's face infested with various stages of Boophilus microplus
This article is the first of a series of five on the control of tick-borne diseases and their vectors. Subsequent articles will consider the epizootiology of some of the tick-borne diseases; the Australian methods of immunization against anaplasmosis and babesiosis; the chemical control of ticks; and acaricide resistance and alternate methods of tick control.
by Ralph A. Bram
The tick-borne diseases of livestock constitute a complex of several diseases whose etiological agents may be protozoal, rickettsial, bacterial or viral; their single common feature is that they can all be transmitted by ticks. Tick-borne diseases are present throughout the world, but are most numerous and exert their greatest impact in the tropical and subtropical regions.
In many countries, they are the major health impediments to efficient livestock production. On a global basis, the economic toll caused by tickborne diseases is staggering. Better global control of tick-borne diseases of livestock and their vectors would contribute substantially to improved meat and milk production (Barnett, 1974a and 1974b).
Although over 60 tick-borne agents may be pathogenic to livestock throughout the world, relatively few are recognized as being of economic significance. The following is a brief resumé of the major tick-borne haemotropic diseases.
R.A. Bram is Tick Control Officer in the Animal Production and Health Division, fao, Rome, on secondment from the Animal and Plant Health Inspection Service, U.S. Department of Agriculture.
Babesiosis, or tick fever, is a febrile disease of domestic and wild animals characterized by extensive erythrocytic lysis leading to anaemia, icterus and haemoglobinuria, and which can be fatal. The disease is caused by protozoan parasites of the genus Babesia transmitted by a variety of tick species. There are at least 14 distinct species of Babesia from various vertebrate hosts (Riek, 1968). In cattle, Babesia bigemina is distributed through Central and South America, Europe, Africa, Australia and Asia; Babesia bovis has been reported from Europe, Africa, Asia and the Far East; Babesia argentina (which may be synonymous with B. bovis) is found in southeast Asia and Australia, and from Mexico down through Latin America.
The successful treatment of babesiosis depends on early diagnosis and prompt administration of a number of drugs. This is particularly difficult where range cattle are involved. Protection depends on premunition (with or without specific chemotherapy) and continued exposure to infected ticks in order to maintain the protection established by inoculation. Prevention of babesiosis in enzootic areas depends on the elimination of the tick vector by regular dipping of cattle with an acaricide at intervals of two weeks or less, depending on the local ecology of vector species.
The eradication of Texas cattle fever (caused by Babesia bigemina) was accomplished in the United States by the elimination of the tick vectors Boophilus microplus and Boophilus annulatus.
Bovine anaplasmosis is an infectious, noncontagious haemotropic disease of cattle characterized in the acute form by fever, anaemia, weakness, constipation, yellowing of the mucous membranes, lack of appetite, depression, dehydration, and laboured breathing. Animals surviving an acute attack often make a slow recovery, resulting in losses in milk or meat production. Generally, mortality is between 5 and 40 percent, but may reach 70 percent during a severe outbreak. The causative agent, Anaplasma marginale, may be biologically transmitted by 20 or more species of ticks and may also be mechanically transmitted by a variety of biting fly species, particularly horse flies of the family Tabanidae.
Electron micrograph of a bovine erythrocyte infected with Anaplasma marginale (courtesy R.L. Sealock, Animal Parasitology Institute, ARS-USDA)
Anaplasmosis is present in most tropical and subtropical regions, and is also found in some temperate areas. The epizootiology of anaplasmosis is complicated by the life-long carrier state which occurs in animals that have recovered from the clinical disease. As with babesiosis, treatment of anaplasmosis depends on early diagnosis and prompt administration of an appropriate drug. Anaplasmosis immunization, using either killed vaccine, attenuated live vaccine, or live Anaplasma centrale vaccine, has been practised with varying results in many parts of the world.
East Coast fever
East Coast fever is a cattle disease caused by the protozoan parasite Theileria parva. Recent research in east Africa indicates that virulent strains of Theileria mutans may also be involved in the East Coast fever syndrome. The pathogen is transstadially transmitted by at least nine tick species; however, the principal vector is the brown ear tick, Rhipicephalus appendiculatus. East Coast fever is enzootic in east Africa and has been reported from Tanzania, Kenya, Uganda, Mozambique, South Africa, Rhodesia, Zaire, Rwanda, Burundi, Malawi and the Sudan. The disease probably exists in Ethiopia, Zambia and southern Somalia. South Africa and Mozambique have reportedly eradicated the disease through tick control.
No practical therapy exists for East Coast fever. Although great progress is being made in the development of a method of immunization by a UNDP-supported regional project in Muguga, Kenya, for which FAO is the executing agency (Cunningham, 1974), no practical technique is yet available for widespread application. Control of East Coast fever depends, therefore, on preventive tick control by frequent spraying or dipping of cattle using an appropriate acaricide, preferably at intervals of less than seven days depending on the vector species and its biology in different geographical areas.
Other species of Theileria act as causal agents of theileriasis in a number of mammalian hosts. Of particular note is Theileria annulata, which has a broad geographical distribution and numerous strains with a wide range of virulence. An excellent review of theileriasis has been presented by Barnett (1968).
Heartwater is a septicaemic, infectious disease of cattle, as well as of sheep, goats and other ruminants. The causative agent is a rickettsia, Rickettsia ruminantium, which is transmitted by ticks of the genus Amblyomma. The enzootic areas of the disease correspond to the distribution of the vector species in southern, eastern and western African countries and in Madagascar. It has also been reported from the Sudan.
Treatment of heartwater with one of several appropriate drugs will result in recovery of a large proportion of infected animals if applied early in the disease. Immunization for heartwater has been used in enzootic regions for a number of years. It is now common in some areas to infect animals deliberately, and apply chemotherapeutic agents upon the first appearance of symptoms of the disease. Attempts have been made to control heartwater by the eradication of the vector species, but this has proved to be a formidable task. Other incompletely defined tickborne rickettsial pathogens of livestock have also been recognized.
Distribution of Boophilus microplus (after wharton, 1974)
Tick-borne virus diseases
A number of tick-borne virus diseases exist which are generally more regional than the diseases of haemoprotozoal etiology. Several viruses, mainly in the Togavirus group, are known to be spread by ticks as well as other arthropods. These are currently under investigation. Nairobi sheep disease, which has recently been found to be identical with Ganjam virus in India, is a widespread tick-borne disease in east Africa. Louping ill is a tick-borne virus disease affecting cattle and sheep in northern England and Scotland, and it has been reported from the U.S.S.R. and Czechoslovakia. Ticks of the genus Ornithodoros may be significant in the transmission of African swine fever virus from feral to domestic swine; however, other means of spread are of even greater significance, particularly when the disease is introduced into a new area.
In addition to being efficient vectors of diseases agents, ticks cause grave economic damage in certain areas. For example, it has been estimated that there is a loss of 1–3 ml of blood for every cattle tick completing its life cycle on an animal. Furthermore, tick infestations cause irritation, damage hides, and predispose animals to bacterial and fungal infections, as well as screw-worm attack, in the wounds left by tick bites. Recent studies in Australia indicate that the total annual loss caused by the cattle tick amounts to about A$5 per head of cattle, or 4 percent of the gross value of cattle slaughtered in 1972/73.
FAO is encouraging countries to intensify their efforts to control ticks and tick-borne diseases, in order to improve the efficiency of livestock production. As a general principle, control should be organized as a responsibility of government animal health authorities; decisions affecting control initiatives would thus be made within the perspective of a country's total animal health commitment.
Before intensification of tick and tick-borne disease control can begin, several preliminary criteria should be satisfied. First, a countrywide survey must be made to determine the incidence and distribution of the various tick-borne diseases. An economic assessment of the benefits expected from improved control should be made at the same time. In addition to the classical blood smear examination for the detection of haemoprotozoa, modern serodiagnostic techniques - such as the fluorescent antibody tests, card agglutination tests, complement-fixation reactions and immunodiffusion tests - should be utilized whenever possible to define disease incidence and distribution precisely.
Second, a systematic entomological survey is necessary to determine which tick species are of veterinary significance within a country, as well as their distribution and incidence. This survey will undoubtedly point out deficiencies in information on the biology and population dynamics of the tick species in different ecological zones, and will stimulate long-term investigations to provide the data necessary for the execution of tick control activities. Before a programme is considered, such basic factors as target species, the biology and population dynamics of these species in different areas, the role of wildlife in maintaining tick populations, and acaricide susceptibility, must be thoroughly understood. An assessment of existing control facilities and anticipated additions to these will also be necessary.
A comprehensive plan should be developed for a coordinated, countrywide control scheme. On the basis of the preliminary epizootiological and entomological information (such as that developed by Mahoney and Ross, 1972), decisions can be made concerning such basic considerations as the application of immunization techniques, possibilities for treatment, methods of tick control, necessity for limiting animal movements and establishment of tick eradication zones. These decisions will also be tempered by the availability of financial and manpower resources and technical expertise, and by the economic benefits expected from the control scheme.
The possibility of including largescale immunization in a control programme will obviously depend on the diseases to be controlled. Operationally effective methods of immunization are available only for babesiosis and anaplasmosis, and even these have their shortcomings. If the total control scheme includes tick and disease eradication zones, present methods of immunization are excluded from these areas.
Methods of treatment may be available, but impractical for large-scale application because of management practices. For example, early treatment of a producing dairy animal for babesiosis infection is possible, whereas treatment of a beef animal under range conditions would be difficult, if not impossible.
Cattle emerging from acaricide dip vat in Tanzania
Although chemical control of ticks will, for the foreseeable future, form the basis of any control programme, alternate methods of control should also be incorporated whenever feasible. Raising tick-resistant cattle, adopting special management techniques and practising strategic dipping are all methods which belong in a fully integrated programme (Wharton, 1974).
In some programmes, quarantine regulations must be established to control the movements of infected animals. This is particularly important when tick eradication in defined zones is planned. In general, nationwide tick or protozoal tick-borne disease eradication is not to be encouraged as a programme objective in developing countries. Usually, programmes which aim to achieve eradication eventually falter, with the result that large populations of susceptible livestock are left unprotected and subsequently succumb to one of the tick-borne diseases. The result can be an epizootic of major proportions. As recently as 1973, the Australian Cattle Tick Control Commission Inquiry concluded that eradication of the cattle tick on a national basis was not practicable in that country; it did, however, indicate that eradication on a regional basis in certain parts of Australia was feasible. On the other hand, Mexico has just established a major eradication programme based on sound scientific principles and with the strong financial and organizational support of the Government. A national tick or tick-borne disease eradication effort should only be attempted when a long-term commitment of finances, manpower and dedication is assured.
On a global basis, the FAO Animal Production and Health Division is increasing its efforts to promote and coordinate improved tick and tickborne disease control. Immediate objectives will concentrate on improving methods of vector control, treatment and immunization; the long-term objective will be to integrate these improved techniques into effective disease control programmes. Over 14 regional and country projects, which are supported by the United Nations Development Programme with fao as the executing agency, will continue to provide basic information and operational applications in different regions. Additional projects will continually be developed and implemented.
On the basis of recommendations of the FAO Expert Consultation on Research on Tick-borne Diseases and their Vectors (fao, 1975a) and the Joint FAO/Industry Task Force on Tick and Tick-borne Disease Control (FAO, 1975b), a Global Acaricide Resistance Monitoring Programme is being planned as a Regular Programme activity of the Animal Health Service. Major features of this programme include: (1) FAO Acaricide Resistance Test Kits, to be distributed to cooperating countries; (2) establishment of a World Acaricide Resistance Reference Centre which will provide cooperating countries with baseline data for registered acaricides; and (3) the annual dissemination of data on the global status of acaricide resistance.
Finally, also as a result of recommendations of the Expert Consultation, FAO intends to increase its activities in information dissemination. The existing FAO Information Circular on Tick-borne Diseases of Livestock will be continued and expanded; a practical field manual on the control of ticks and tick-borne diseases is in the planning stage; and efforts are being made to establish practical, field-oriented training courses on the control of ticks and tick-borne diseases of livestock.
Such tick-borne diseases as babesiosis, bovine anaplasmosis, East Coast fever and heartwater cause severe economic damage to livestock production.
Control of tick-borne diseases within a country requires the preparation of a comprehensive plan for coordinated, countrywide action. The plan should include immunization, treatment and tick control, and should be based on scientifically sound data collected during preliminary epizootiological and entomological surveys. In general, nationwide tick or protozoal tick-borne disease eradication is not to be encouraged as a programme objective in developing countries.
On a global basis, FAO is increasing its efforts to promote tick and tick-borne disease control. This will be accomplished by existing and future regional and country projects; establishment of a Global Acaricide Resistance Monitoring Programme; and the increased dissemination of disease control information.
Barnett, S.F. 1968. Theileriasis. In Weinman, D. and Ristic, M. Infections blood diseases of man and animals. II. p. 269–328. New York, Academic Press.
Barnett, S.F. 1974a. Economical aspects of protozoal tick-borne diseases in livestock in parts of the world other than Britain. Bull. Off. int. Epiz., 81(1–2): 183–196.
Barnett, S.F. 1974b. Economical aspects of tick-borne disease control in Britain. Bull. Off. int. Epiz., 81(1–2): 167–182.
Cunningham, M.P. 1974. A report of the activities of the FAO immunological research on tick-borne cattle diseases and tick control project. Bull. Off. int. Epiz., 81(1–2): 161–166.
FAO, 1975a. Report of the FAO Expert Consultation on Research on Tick-borne Diseases and their Vectors, Rome, Italy, 6–8 May 1975. Rome.
FAO. 1975b. Summary report of the Joint FAO/Industry Task Force on Tick and Tick-borne Disease Control, Rome, Italy, 9 May 1975. Rome.
Mahoney, D.F. & Ross, D.R. 1972. Epizootiological factors in the control of bovine babesiosis. Aust. Vet. J., 48: 292–298.
Riek, R.F. 1968. Babesiosis. In Weinman, D. and M. Ristic. Infectious blood diseases of man and animals. II, p. 220–268. New York, Academic Press.
Wharton, R.H. 1974. Ticks with special emphasis on Boophilus microplus. In Pal, R. and Wharton, R.H. Control of arthropods of medical and veterinary importance, p. 36–52. New York, Plenum Publishing Corporation.
This article is concerned with aspects of the epizootiology of the more important tick-borne diseases of cattle: babesioses, theilerioses, anaplasmoses and cowdriosis. These are considered in the order in which the causal agents were discovered and described. Some of the opinions expressed are not necessarily accepted by all workers, and some statements, especially on parasites of the family Theileriidae, are based on unpublished work.
by Gerrit Uilenberg
A knowledge of the epizootiology of tick-borne diseases is of fundamental importance for their effective control. It is only by understanding how the different pathogens are transmitted, what factors influence transmission, and in what ways the animal defends itself against the clinical effects of infection that one can hope to devise successful methods of disease control. This is especially true in areas where complete eradication of the tick vectors is not feasible due to acaricide resistance, or human problems such as poor dip management, irregular attendance of herds at dips or sprays, etc., or because wild animals serve as reservoirs for tick populations.
Anaplasma and Cowdria are not protozoa like Babesia and Theileria, but belong to the order Rickettsiales. Nevertheless, all these infections share a few common points in their epizootiology. The simplest epizootiological model is provided by bovine babesiosis, caused by Babesia argentina and Babesia bigemina in Madagascar and Australia where the only vector is the one host tick, Boophilus microplus. In practically the whole of Madagascar and in much of the tropical north of Australia the climate is favourable for this tick. Wild animals do not serve as reservoirs for either the tick or the babesial parasites. Cattle, after recovery from their first infection, remain carriers for a considerable period, and the opportunity for a proportion of the tick population to be infected is excellent at all times. Because of favourable climatic conditions, the tick population remains at a high enough level to infest all calves shortly after birth, and as a result all young calves become infected with both species of Babesia. However, since young calves up to the age of about nine months are more resistant to the clinical effects of the infections than older animals, and since protective antibodies from the dam are transmitted to the calf via the colostrum, there is no mortality and no apparent signs of disease. Immunity among the adult cattle is maintained and reinforced by frequent reinfection. This situation in which Babesia infection is widespread but does not constitute a disease problem is described as enzootic, there is no apparent babesiosis, but all animals are infected and are resistant to clinical illness. The whole of Madagascar can be considered as enzootic for these babesial parasites of cattle, but in Australia the climate becomes less and less favourable for the tick as one goes south toward the drier interior and toward the cooler subtropical coastal regions, where the tick population falls to a level so low that not all calves acquire the Babesia infections. Here the proportion of susceptible adults is variable, and these animals will show clinical disease when exposed to infected ticks, often resulting in mortality. This situation may be described as epizootic when the disease is apparent, and may lead to outbreaks.
Where seasonal climatological factors favour a temporary increase in the tick population, or where a herd with many susceptible animals is moved into a region with more ticks, the epizootic situation becomes unstable. However, as one moves farthere away from the enzootic areas, the tick population density becomes lower until only sporadic cases of babesiosis are seen; one finally reaches the tick-free area where the vector cannot survive and the whole cattle population is susceptible.
G. Uilenberg is project manager of a Tanzanian FAO/UNDP project for the improvement of the control of ticks and tick-borne diseases of livestock. His address is c/o UNDP, P.O. Box 9182, Dar es Salaam, Tanzania.
Movement of cattle from free areas into epizootic and enzootic areas, and from epizootic into enzootic areas, will result in disease, as will, of course, the importation of susceptible cattle from other countries. Control of ticks changes the picture and may create epizootic or free situations in isolated farms inside an enzootic region, depending upon the efficacy of tick control. Paradoxically, the best method for controlling babesiosis in an enzootic area is not to control ticks! But where humpless cattle (Bos taurus) are concerned, it is necessary to limit the numbers of ticks in an enzootic area, since the animals will suffer from tick-worry and loss of blood through the sheer numbers of ticks. This is less important for the zebu breeds (Bos indicus), which have the ability to limit tick numbers by immunological means (e.g., Seifert, 1971). Therefore, in enzootic areas tick control may not be necessary on zebu cattle, while control of ticks on humpless cattle may in many instances be limited to reducing the tick population to a level where their numbers have no direct harmful effect, but still remain sufficiently high to ensure infection of all young calves.
Theileria parva macro- and microschizonts in lymphnode
Intensive control of ticks with the aim of eradicating them is not usually feasible in enzootic areas, and will only serve to create an epizootic situation or a temporary free pocket in the infested zone, with disease problems becoming apparent where there were none before. But in epizootic areas or in those that have been made epizootic by a partial reduction of tick numbers, control of babesiosis will always be necessary. Such control can be achieved by immunization against Babesia or by intensification of tick control measures with a view to eradication. The latter should preferably be applied to a whole region rather than to single herds.
It should be borne in mind that because zebus are more resistant to tick infestation than European breeds, a situation that is epizootic for the combination zebu-Boophilus-Babesia may be enzootic for European cattle-Boophilus-Babesia. Humpless breeds of cattle will thus require more intensive tick control than zebus in the same area.
A feature of some tick-borne infections (e.g., Babesia and Theileria spp.) is that the infection is not passed on to the host as soon as the tick begins to feed; a few days may elapse before the infective particles in the salivary glands of the tick multiply and become mature. This so-called prefeeding period is especially important in the case of East Coast fever (Theileria parva). In situations where tick eradication is not feasible, dipping twice a week with modern acaricides that have a short residual effect may still prevent disease transmission. By this means, ticks that have attached after the last dipping will be killed before the end of the prefeeding period.
Most mammalian species are hosts to one or more species of Babesia. The only stage in the mammalian host that is known with certainty is the piroplasm that occurs in the red blood cells. Because the defence mechanisms in the host prevent the parasite from multiplying at a harmful rate, an equilibrium between the host and the parasite is reached and the parasite is thus able to maintain itself for an extended period. The situation may resemble that which occurs in trypanosome infections where certain antigens of the parasite, that stimulate the formation of protective antibodies by the host, change before the parasite is completely eliminated by these antibodies; new antibodies are then produced by the host, and the parasite changes its antigenic constitution again in order to maintain itself.
While there is some experimental evidence of such antigenic variation in Babesia, it appears that these changes are not so numerous and so frequent as in some of the trypanosomes. Babesia infections seem to be eliminated by the host after varying intervals (sometimes a few years) if there are no new infections. As long as the animal is a carrier of a Babesia species, and sometimes for an even longer period, it is protected by its immune mechanism against new infections. This protection is not always complete because antigenically different strains of a species can occur, but it is usually sufficient to prevent fatal disease in cattle. Clinical, even fatal disease, is occasionally seen in carriers, when the defences are broken down by an intercurrent disease (e.g. in cattle, by rinderpest), or other causes of stress.
Left: Theileria parva piroplasma in blood
From an economic point of view the most important babesioses are those of cattle. The foremost of these is that caused by Babesia argentina (synonymous with B. berbera, B. bovis pro parte, Françaiella caucasica, and a few other names), which is usually transmitted by species of the tick genus Boophilus. As Boophilus spp. are one-host ticks (where moulting from larva to nymph and from nymph to adult takes place on the same host individual), transmission is normally from one generation to the next and the infection is passed on through the egg of the infected female to the larvae of the next generation.
Babesia argentina is widespread in tropical and subtropical countries wherever Boophilus spp. occur, although there is at least one curious gap in its distribution: it appears not to have been reported in East Africa where both Boophilus decoloratus and B. microplus occur and research in blood parasites of cattle has been going on for a long time. Cattle of the zebu breeds are significantly less susceptible to the clinical effects of Babesia argentina infection (although not to the infection itself) than are cattle of European breeds (e.g., Daly and Hall, 1955). B. argentina is not only an important cause of disease in epizootic areas, but it is also one of the main causes of mortality in susceptible cattle imported from abroad.
Babesia bigemina, another cause of babesiosis or redwater in cattle, has roughly the same distribution as B. argentina because Boophilus ticks are also among its most important vectors. The pathogenic importance of B. bigemina is less than that of B. argentina. Its special position in many textbooks and reports is perhaps due to the fact that it is much more easily diagnosed than B. argentina because of its large size and its occurrence in far greater numbers in the peripheral blood. It is also more amenable to treatment than B. argentina. Nevertheless, animals do die of B. bigemina infection, and immunization of susceptible animals before exposure may be recommended.
Other babesioses of cattle are more restricted in their distribution, and of less economic importance. Two species that may be mentioned are Babesia divergens (= B. bovis pro parte), transmitted by Ixodes ricinus, and B. major, transmitted by Haemaphysalis punctata. The latter parasite is almost as large as B. bigemina. The reports that suggest that B. bigemina is transmitted by H. punctata may be due to mistaken identification.
Species of the family Theileriidae are widespread among ungulates. Transmission is from stage to stage and not through the egg to the next generation. The larva ingests infected blood, and after moulting transmits the disease as a nymph; if the nymph feeds on an infected animal the infection is then transmitted by the next stage, the adult. One-host ticks, in which all three stages stay on the same individual host animal, are therefore not vectors. Trans-stadial transmission also means that those species of ticks of which the immature stages do not normally feed on the final host play no role in transmission, even though in the laboratory certain of these species have been shown to be capable of transmitting the disease.
|Anaplasma marginale in blood||Babesia bigemina in blood|
The life cycle of the Theileriidae in the final host is more complex than that of the Babesiidae, in as far as the latter is known. After the infective particles are injected into the host during tick feeding, the first visible stages are the macroschizonts in lymphoid cells of the regional lymphnode. The macroschizonts contain a number of nuclei, and give rise to microschizonts. The microschizonts contain many small nuclei which after the microschizonts break up invade the red cells and form the piroplasms, the infective stage for the tick. After the regional lymphnode is invaded, other lymphnodes are also invaded, and lymphocytic cells containing schizonts can be found in other organs as well.
The most pathogenic of the Theileriidae is Theileria parva, the cause of East Coast fever in cattle. This disease is fortunately limited in distribution to eastern and southern Africa, and is of primary importance there as the most important cattle killer in many areas. The vector is Rhipicephalus appendiculatus, a three-host tick, and the distribution of the disease generally coincides with the distribution of this tick species. Other species have been proved capable of transmitting East Coast fever experimentally, but play no significant role in nature (e.g., Barnett, 1968).
The epizootiology of East Coast fever is different from that of the babesioses. The parasite has apparently not been adapted to cattle long enough to render it apathogenic in enzootic regions where there is a considerable calfhood mortality, even in indigenous zebu breeds. Although figures of calfhood losses of 30 percent or more have been reported, reliable statistics are not available. Mortality in epizootic situations (where tick numbers are too low to infect all young calves) can be catastrophic and depends on the proportion of immune animals and the size of the tick population which fluctuates from year to year as a result of rainfall. Mortality in susceptible adult cattle may be over 90 percent even in zebu breeds. Another difference from the babesioses is that, with some exceptions, recovered animals do not usually remain carriers of the parasite and are normally not infective to ticks.
In areas where R. appendiculatus occur, the African buffalo is a carrier of a theilerial parasite which is pathogenic to cattle and causes corridor disease in the latter. Buffaloes act as carriers of this parasite, and may remain infective to ticks for at least three years. On transmission to cattle, macroschizonts develop, but there are very few or no piroplasms, and the disease usually dies out in cattle. On a few occasions however it has been possible to transmit corridor disease from cattle to cattle with R. appendiculatus and even to transform the behaviour of the parasite in such a way that it resembles classical Theileria parva. Since the buffalo parasite, which has been given the name T. lawrencei, is serologically identical to T. parva, and since transformation into a parasite resembling T. parva is possible and also because a degree of cross-immunity between strains of T. lawrencei and T. parva occurs, it is likely that T. lawrencei of buffalo is the original parasite which has later adapted to cattle after being introduced into Africa.
Captive African buffalo is studied for its role in cattle theilerioses. The dangerous horns of this animal have been removed to facilitate handling.
Recently transformed strains may give rise to the carrier state in cattle. The author shares the view with barnett and Brocklesby (1966) that T. parva of cattle and T. lawrencei of buffalo are biologically different strains of the same parasite, which should be called T. parva, as this was the first name given. For the sake of facility, and taking into account the biological differences between T. parva and T. lawrencei, one might consider them as subspecies, and the classical cattle parasite would then be called T. parva parva and the buffalo parasite T. parva lawrencei.
Cross-immunity between different strains of T. parva is not always complete, and cattle recovered from disease caused by one strain may still die of another, although a partial cross-immunity may also exist. The total number of significantly different strains which exist in nature is still unknown. At the East African Veterinary Research Organization in Kenya, where East Coast fever immunization studies are being undertaken, it has been found that a combination of three strains, one a transformed buffalo strain and the other two from cattle, gives protection against all field strains tested so far in the laboratory. On field exposure of animals immunized with this cocktail, protection is not always complete, but preliminary results are encouraging. The technique of immunization employed is to infect the animals with infective particles derived from ticks, and to treat them during the early part of the incubation period with a tetracycline.
Research on immunizing with parasites grown in tissue culture has also reached a promising stage. Strains may also differ in virulence, although it does not appear to be conclusively known whether the degree of virulence is fairly stable in nature for a particular strain.
The second most important species of Theileria in cattle, with an even wider distribution, is T. annulata, which causes bovine theileriosis in northern Africa, southern Europe, the Near East and the Far East, including the Indian subcontinent. It is transmitted by species of the genus Hyalomma, of which H. detritum and H. anatolicum (= H. excavatum) are among the most important vectors. These are two-and three-host ticks and their immature stages occur on cattle. Although the mortality caused by T. annulata is on the average lower than that of East Coast fever, the disease is of great importance and is a major obstacle to the development of the cattle industry.
Immunologically different strains of T. annulata exist. Strain differences in virulence also occur, as in T. parva. Indigenous zebu breeds may be more resistant than imported breeds. Recovered animals remain carriers of the parasite. As far as is known, there is no wild host reservoir for T. annulata, but the domestic water buffalo can be infected. Although research on the disease has been going on for a long time, satisfactory answers to several epizootiological questions remain to be provided. Since there are different tick vectors, with a different biology from one region to another, its epizootiology will not be the same everywhere. Immunization is carried out in some countries with parasites grown in tissue culture, but more research is needed to evaluate the efficacy of present methods.
Theileria sergenti, a species occurring in cattle in the Far East resembling T. annulata, is transmitted by the three-host tick Haemaphysalis longicornis (= H. bispinosa pro parte = H. neumanni). It has been shown that T. sergenti is not transmitted by Hyalomma spp., while T. annulata is not transmitted by Haemaphysalis longicornis. The real pathogenic and economic significance of T. sergenti is still rather poorly documented.
Diluting cattle sera for serological testing for theileriosis
The fourth named theilerial species of cattle is Theileria mutans. This was originally described as a non-pathogenic species from South Africa and the name has since been given to nonpathogenic Theileria of cattle in other parts of the world. It has been shown in South Africa to be transmitted by Rhipicephalus appendiculatus, as well as by R. evertsi. However, it has recently been found that the species called T. mutans in East Africa is not, or is only rarely, transmitted by R. appendiculatus, but that Amblyomma variegatum is an efficient vector. It remains to be demonstrated whether the East African T. mutans is really T. mutans.
The issue has been complicated recently by the discovery of apathogenic Theileria species of cattle in East Africa, transmissible by R. appendiculatus, but serologically different from both T. parva and the East African “T. mutans” (Burridge et al., 1974, and unpublished experiments in Tanzania). It has also been shown that the East African “T. mutans” is not as devoid of pathogenicity as was generally accepted, and that fatal infections do occur; apart from its own importance as a pathogen, such fatal infections with high parasitaemias confuse the East Coast fever picture and must have been often diagnosed in the past as T. parva.
An apathogenic Theileria occurring in cattle in Europe has also been called T. mutans until now, but recent serological and morphological studies have shown that it is quite distinct from the East African “T. mutans”. Transmission is by the tick Haemaphysalis punctata. Other parasites called T. mutans in the Far East and in Australia are transmitted by Haemaphysalis longicornis and are also unlikely to be the real T. mutans; since the vector is the same as that of T. sergenti, they may in fact belong to that species. Sera from Australian carriers, tested against East African T. mutans antigen, have not shown any reaction, but there is a serological relationship with the European T. mutans (unpublished data). The systematic position of the Theileria species in East Africa is not clear, and a comparative study with the South African T. mutans should be carried out.
Another apathogenic member of the Theileriidae is Haematoxenus veliferus, which is widespread in tropical Africa and in Madagascar. Amblyomma variegatum has recently been shown to be a vector in Tanzania. Natural infection has been found in wild African buffalo. There is no serological cross reaction with T. mutans or T. parva. Although parasitaemia may on occasion infest over 5 percent of red cells, it is unlikely to be incriminated as a cause of clinical disease.
Many points in the taxonomy and epizootiology of cattle Theileria remain to be clarified. In particular, the theileriosis complex in East Africa - where T. parva parva, T. parva lawrencei and T. mutans cause the “East Coast fever syndrome,” and unidentified Theileria species and H. veliferus are also involved - will have to be unravelled.
In small ruminants at least three species are known: Theileria hirci, which is pathogenic, and two species, T. ovis and Haematoxenus separatus, which as far as is known at present are nonpathogenic.
In the mammalian host the only known stages of Anaplasma, which are classified as Rickettsiales, are the forms in red cells where they occur as small darkly staining bodies.
The most important species is Anaplasma marginale in cattle, which has a very wide distribution in tropical, subtropical and even temperate zones. A less pathogenic species, A. centrale, was found in South Africa early in the century. Although immunological differences exist between both, they have certain antigens in common and A. centrale is extensively used to immunize against A. marginale. The morphological difference between the two species is in the site within the red cell. In the United States a new genus, Paranaplasma, with two species, has been separated from A. marginale, both on immunological and morphological grounds, but the validity of these species has not yet been generally accepted.
Transmission of A. marginale is normally by ticks, of which several general and species have been incriminated. Although transmission through the egg has been reported from time to time, most workers have had negative results, and transmission appears to be mainly transstadial. Boophilus spp. can be incriminated on field evidence as major vectors, but the results from carefully controlled transmission experiments have been negative. As they are one-host ticks, it had of course been assumed that transmission would be transovarial, through the egg. However, positive results have recently been obtained with B. microplus in Madagascar and Australia in transstadial experiments, a small percentage of ticks apparently transferring spontaneously from one host animal to another during their cycle on the host (Uilenberg, 1970; Connell and Hall, 1972; Leatch, 1973). It has still to be determined whether transfer of one-host ticks from one host individual to another occurs frequently enough in the field to explain the vector role of Boophilus.
Besides ticks, biting insects may play a significant role in mechanical transmission in some circumstances, particularly where large tabanid flies might be the cause of seasonal outbreaks. Transmission experiments with the smaller insects, such as mosquitoes and stable flies, have in most instances given negative results. Anaplasmosis is also mechanically transmitted with injection needles and surgical instruments if these are improperly used on a series of animals. The epizootiology of anaplasmosis where Boophilus spp. are the main vectors can be compared to the simple model of Babesia-Boophilus. In enzootic situations, young calves, which are less susceptible than adults, all become infected. Although anaemia in young calves does occur occasionally (mainly in imported humpless cattle), on the whole there is not much of a disease problem, and the disease becomes important only in epizootic situations and in susceptible cattle imported into infected areas. Despite the fact that the transfer of certain antibodies from the immune dam to the calf through the colostrum has been demonstrated, it remains to be conclusively proved that protective antibodies are included. It also has to be shown unequivocally that indigenous zebu breeds are more resistant than imported cattle breeds. After an animal has recovered from infection it probably remains infected for life, and is protected against the clinical effects of further infections. Although immunologically distinct strains might exist, their importance under field conditions is unknown Strains of different virulence exist but the stability of the degree of virulence of a particular strain is not really known.
Transmission studies are carried out by feeding infected ticks in earbags
Cattle can be infected with anaplasms harboured by certain African antelopes, but there is not conclusive proof that these organisms are strains of A. marginale. Deer can act as reservoirs of A. marginale. However, anaplasmosis in cattle is just as prevalent in many regions without ungulates (e.g. in Madagascar, where it is enzootic) as in areas with game. Although small ruminants are susceptible to Anaplasma ovis, little research has been done on anaplasmosis of sheep and goats.
The causal agent of cowdriosis, Cowdria ruminantium, belongs to the Rickettsiales. The only stage known in the mammalian hosts is that occurring in endothelial cells of blood vessels. Diagnosis is easiest in smears of the cortex of the brain, where the organism can be seen in groups in the capillaries. As removal of the brain from a carcass is not often done in the field, the number of diagnosed cases does not reflect the actual importance of the disease.
Cowdriosis, or heartwater, is an important African disease of cattle, sheep and goats. It is transmitted trans-stadially by several species of Amblyomma, of which A. variegatum is the most widespread. The disease probably exists wherever Amblyomma spp. of ruminants occur in Africa, including the island of Madagascar, and possibly also in some of the Caribbean islands, where A. variegatum has been introduced. Proen vectors are A. hebraeum (southern Africa), A. variegatum (widely distributed), A. pomposum (a highland tick of eastern and southern Africa). A. gemma and A. lepidum (ticks replacing A. variegatum in lower rainfall areas of eastern Africa).
Due to difficulties in working with the disease, comparatively little research has been carried out on cowdriosis. Until quite recently the organism could only be reliably propagated in ruminants and it has not been cultured in vitro. Since there is no serological test available, laboratory diagnosis is only possible after death.
Rousselot (1957) has described the situation in regard to cowdriosis in certain parts of western and central Africa as follows: “This disease exists wherever there is someone in the laboratory used to its microscopical diagnosis. If this person is transferred, the disease is transferred with him. It appears mysteriously at the site of his new assignment, after having disappeared from his previous post.”
There is evidence of short-lived resistance of very young animals to the clinical effect of the disease, even in those born to susceptible dams. In situations with a sizable population of the vector, only a certain proportion of young adults is susceptible (Uilenberg, 1971), suggesting that perhaps a very small percentage of the tick population is infected. This may be due to the fact that the host remains infective to ticks for only a short period after recovery. It is possible that truly enzootic situations exist, without apparent disease, where vector numbers are very great, especially where indigenous breeds which are less susceptible are involved. But since control of such large numbers of Amblyomma is necessary to prevent festering wounds and abscesses caused by their long mouthparts, and since total eradication of these three-host ticks (which also feed on numerous other domestic and wild hosts) is almost impossible to achieve, the situation is usually an epizootic one in which the only control method is intensive dipping or spraying.
Tick control may be combined with artificial immunization, but present methods are still too cumbersome for application on a large scale in practical circumstances, at least as far as cattle are concerned.
Certain indigenous breeds of zebu cattle and sheep are more resistant than imported breeds, presumably through natural selection over the centuries; but zebu breeds imported from other continents appear to be as susceptible as European breeds. Strain differences in virulence have been reported. Little is known with certainly about immunological differences between strains; partial differences have been reported.
Once the animal has recovered from infection, it is thought to be immune for several years. Whether the immunity is sterile or not is unknown, but recovered animals do not remain infective to ticks. Field observations indicate that animals may die suddenly of cowdriosis when exposed to certain stresses, but there is no experimental proof of a hypothetical carrier state.
Some wild African ruminants can be infected with heartwater and might play a role as reservoirs for domestic ruminants. However, the situation in Madagascar, where there are no wild ruminants and where heartwater is common and widespread, appears to indicate that the presence of wild reservoirs will not significantly influence the picture.
Tick-borne diseases of cattle make havoc of the most ambitious plans for importing exotic breeds into tickinfested countries, or for upgrading a local breed by distributing improved animals from well-managed ranches that adopt good tick control. Many examples may be quoted of the disastrous results of importation or distribution where not effort was made to anticipate and prevent massive losses from theilerioses, babesioses, anaplasmoses or cowdriosis. Animals should be immunized prior to exposure. If this is impossible, and if it is known that tick control will be inadequate, it is often a waste of time and resources to import or distribute them, unless they can be supervised closely by competent staff for immediate appropriate treatment when they become ill. But no treatment exists for theileriosis, and treatment is often too late in the case of cowdriosis.
The importation of susceptible cattle of breeds with no inherent resistance, costing large amounts of money, should be preceded by a thorough evaluation of the epizootiological situation regarding tick-borne diseases (and some other diseases, such as dermatophilosis, as well) at the point of destination, so as to assess the possibility of taking appropriate control measures. If this proves impossible, no importation should be made.
Barnett, S.F. 1968. Theileriasis. In Weinman, D. and Ristic, M. Infectious blood diseases of man and animals. Diseases caused by Protista. Vol. II. The pathogens, the infections and the consequences, p. 269–328. New York and London, Academic Press.
Barnett, S.F. & Brocklesby, D.W. 1966. The passage of “Theileria lawrencei (Kenya)” through cattle. Brit vet. J., 122: 396–409.
Burridge, M.J., Brown, C.G.D., Crawford, J.G., Kirimi, I.M., Morzaria, S.P., Payne, R.C. & Newson, R.M. 1974. Preliminary studies on an atypical strain of bovine Theileria isolated in Kenya. Res. vet. Sci., 17: 139–144.
Connell, M. & Hall, W.T.K. 1972. Transmission of Anaplasma marginale by the cattle tick Boophilus microplus. Aust. vet. J., 48: 477.
Daly, G.D. & Hall, W.T.K. 1955. A note on the susceptibility of British and some zebu-type cattle to tick fever (babesiosis). Aust. vet. J., 31: 152.
Leatch, G. 1973. Preliminary studies on the transmission of Anaplasma marginale by Boophilus microplus. Aust. vet. J., 49: 16–19.
Rousselot, R. 1957. Biotopes des ixodes en Afrique Noire Française. (Influence sur la pathologie, en fonction du climat, de la répartition et de la densité des espèces.) Bull. Off. int. Épiz., 47: 645– 652.
Seifert, G.W. 1971. Variations between and within breeds of cattle in resistance to field infestations of the cattle tick (Boophilus microplus). Aust. J. agric. Res., 22: 159–168.
Uilenberg, G. 1970. Note sur les babésioses et l'anaplasmose des bovins à Madagascar. IV. Note additionelle sur la transmission. Rev. Élev. Méd. vét. Pays trop., 23: 309–312.
Uilenberg, G. 1971. Etudes sur la cowdriose à Madagascar. Première partie. Rev. Elev. Méd. vét. Pays trop., 24: 239–249.
by L.L. Callow
In Australia, three parasites - Babesia argentina, B. bigemina and Anaplasma marginale - cause a disease complex commonly known as tick fever. All three organisms are transmitted to cattle by the cattle tick, Boophilus microplus. To our knowledge there is no other vector of any consequence in Australia. The three parasites have a worldwide distribution, generally in warm, moist environments that favour Boophilus. In some countries anaplasmosis is also widespread in areas where B. microplus does not exist. Other species of ticks and biting insects act as vectors.
Vaccination against babesiosis and anaplasmosis is a well-established procedure in Australia. In recent years, more than a million doses of vaccine have been supplied annually. The unique methods by which vaccines are produced are described in this article. Because the epizootiology of the disease complex determines whether or not economic levels of disease will occur, this will be considered briefly. Remarks about B. argentina should be considered relevant to B. bovis and B. berbera because of the probable synonymity.
The three parasites causing tick fever are transmitted in different ways by B. microplus. Both species of Babesia infect engorging ticks, and are transmitted via the egg to the next generation. In this generation B. argentina is transmitted to cattle shortly after infestation by larvae, whereas the transmission of B. bigemina is delayed for at least nine days until the ticks are nymphs and adults. A. marginale is transmitted within the same generation of B. microplus. Although it is classified as a one-host tick, Boophilus can transfer from one host to another much more readily than was once supposed, and in doing so may also transfer anaplasmosis.
The young animal tends to be resistant to tick-borne diseases, and this has an important influence on epizootiology. There are two components in the resistance. One is colostral in origin and is conferred if the dam is immune. Whereas this effect is lost within two months, the other component, which is physiological in nature, may be present for much longer. This type of resistance wanes slowly after about nine months. Cattle infected during the resistant period rarely suffer fatal attacks, but nevertheless develop levels of immunity.
The author is Officer-in-Charge of the Queensland Department of Primary Industries, Tick Fever Research Centre, Brisbane, Australia.
Effect of vector density
In common with other arthropodtransmitted diseases, vector density determines the rate at which cattle become infected with Babesia and Anaplasma. The question has been very thoroughly examined for B. argentina in recent years by CSIRO workers in Australia (Mahoney and Ross, 1972; Mahoney, 1973). Where the tick population is high, most cattle become infected very early in life. In areas less favourable for the propagation of Boophilus, although ticks may be constantly present, their numbers are often insufficient to infect a high proportion of cattle in the first year or two of life. The low infectivity of some tick populations results from the surprising fact that the majority of Boophilus do not carry parasites. One infected larva in two or three thousand is not uncommon in southern Queensland. The reduction in the transmission rate caused by low vector density is compounded by a tendency of populations to become progressively less infected with parasites as the tick numbers decrease. At times this leads to the complete elimination of Babesia and Anaplasma from the environment.
Stability and instability
Two situations can thus be broadly defined. The first, termed enzootic stability, is associated with frequent transmission of the parasites. In many tropical countries, transmission may be continuous throughout the year. Indigenous cattle suffer minimally from tick-borne disease, but unprotected cattle brought into these areas are immediately infected and often suffer acutely. Enzootic instability is an appropriate description of a host-parasite imbalance resulting from infrequent transmission. Disease is seen when the susceptible part of a herd encounters ticks carrying a virulent infection. Enzootic instability similar to that observed in areas of Australia almost certainly exists in large areas of Latin America and possibly in parts of central Asia.
When Babesia and Anaplasma are present in a region, it does not follow that a vaccination programme is essential. If enzootic stability is present and no susceptible cattle are being imported, there should be little evidence of clinical babesiosis and anaplasmosis. If, however, susceptible cattle are being introduced to improve the local cattle industry, or conditions of enzootic instability exist, babesiosis and anaplasmosis are likely to be a problem.
Calf donor of Babesia argentina vaccine after splenectomy and infection, but before collection of its blood. The vaccine will contain one hundred times more parasites than those in the infective dose
It is not difficult to determine whether or not vaccination is required to protect imported cattle. Losses of 50 to 100 percent of unprotected cattle have followed their introduction to tropical areas where the transmission rate of tick-borne diseases is high. Decisions on what vaccines should be used, and on whether economic loss due to enzootic instability warrants their use, are not made so easily. In all situations the correct identification of the parasite or parasites causing significant loss is essential. This is often difficult, particularly in developing countries. Morphological similarities between B. argentina and B. bigemina, inappropriate specimens, substandard equipment and inexperienced diagnosticians can result in mistaken identification. Imported cattle must often overcome barriers other than tick-borne diseases to survive in their new environment. Nutritional and environmental stress, and exposure to other new parasites often confuse the situation. Even in developed regions of Australia, cattle exposed for the first time to excessive numbers of B. microplus sometimes die from the effects of the ticks per se. These mortalities have been wrongly attributed to failure of vaccination against babesiosis and anaplasmosis. This occurred in Bolivia, when 23 of 80 imported Herefords died from acute anaemia following sudden and heavy infestations with Boophilus.
Carotid artery exteriorized and clamped off prior to canulation and exchange transfusion procedure which allows the calf to survive after its heavily infected blood has been collected for vaccine
|(Left) Babesia argentina in a thin blood smear from a vaccine donor|
(Right) Babesia bigemina in a thin blood smear from an experimental animal
(Below) Anaplasma centrale in a thin blood smear from a vaccine donor
In Australia, as a result of careful study of specimens received at the laboratory over a long period, we know that B. argentina is the major pathogen, that incidence of A. marginale has increased from 7 to more than 20 percent in a period of 10 years, and that B. bigemina can more or less be ignored because of low pathogenicity. These observations have allowed the development of effective control by vaccination in Australia.
Risk due to enzootic instability
Direct evidence for enzootic instability is provided by the observation of disease affecting indigenous cattle, mainly in the age group of one to three years. Incidence is usually seasonal, the peak coinciding with maximum tick activity. The extent of the losses should indicate whether or not vaccination is warranted. In remote areas where disease incidence cannot be effectively observed, evidence for enzootic instability can be obtained by serological surveys. In an FAO study performed in a mountainous region of Bolivia, antibodies against B. argentina were found in 38 percent of mature cattle. The 62 percent that had not been exposed were considered to be at risk. The symptoms that local farmers described in their cattle strongly suggested infection with tick-borne disease. Vaccination would be advantageous in such a situation.
Another factor in deciding whether or not vaccination is warranted is the composition of the herds in a region. Probably as a result of thousands of years of close association, some breeds of zebu cattle are not seriously affected by the parasites. Although it would be advisable to vaccinate previously unexposed zebutype cattle being introduced into a heavily infected environment, it may not be necessary to protect them in conditions of enzootic instability where the challenge is not as severe.
Tick eradication programmes
Another indication for vaccination is in tick eradication programmes. Experience in Australia has been that Boophilus can be suppressed readily, but is a most difficult pest to eradicate completely. Repopulation of a region with ticks after only two or three years of freedom can cause serious losses when babesiosis and anaplasmosis return. No tick eradication programme should be attempted without first ensuring that adequate supplies of vaccine will be available if needed.
For over 75 years, vaccination procedures of varying effectiveness have been used. Until the 1960s the carrier donor system was used in Australia to provide vaccine. Carriers of Babesia held at the laboratory were used to vaccinate against the species occurring in the field. For anaplasmosis, however, the immunizing agent used against A. marginale was frequently A. centrale. When laboratory services were not available, herd animals were selected as donors on the assumption that they carried the parasites against which protection was required. Similar systems have been used in Algeria, Israel, South Africa, Sri Lanka and Sweden, and also in some South American countries.
Developments in Australia
Increasing dissatisfaction with the carrier donor method in Australia during the 1950s resulted in investigations that showed that carrier donors of Babesia provided infective vaccine in only 60 to 70 percent of cases (Callow and Tammemagi, 1967). This finding was followed by the development of highly infective but relatively attenuated vaccines against babesiosis and anaplasmosis (Callow and Mellors, 1966; Callow, 1971).
The salient features of the Australian vaccines are as follows:
Each dose of vaccine contains 10 million viable parasites, about one hundred times the infective dose. Large numbers of parasites are produced in splenectomized calves.
Repeated passaging in splenectomized calves has two desirable effects: B. argentina and possibly A. centrale undergo a decrease in virulence; there is a loss of infectivity of the parasite for the tick vector.
B. bigemina is not routinely included in vaccine because epizootiological studies showed it to be a minor cause of disease. However, infective vaccine of reduced virulence can be prepared by utilizing relapse parasitaemias provoked by splenectomizing carriers of this parasite.
Parasitized erythrocytes are suspended in a cell-free, plasmabased diluent. The erythrocyte component of the vaccine averages 0.1 ml. The reduced volume of whole blood in vaccine minimizes the risk of breeding females developing antibodies against incompatible blood. These do not harm the mother, but may cause a haemolytic syndrome in newborn calves after they have ingested colostrum. The diluent is especially designed to preserve the infectivity of the vaccine.
Vaccine is dispatched in ice inside insulated containers to prevent deterioration in transit.
The vaccine strain of B. argentina is changed periodically. Babesial immunity is reinforced by a second vaccination, and it is greatly improved when this is made with a strain different from that used at the initial infection.
Use of vaccine
In recent years approximately 1.3 million doses of vaccine have been supplied to vaccinate 8 000 herds of cattle. Most of the vaccine required in the past was monovalent B. argentina, but recently the demand for A. centrale has increased sharply so that about 25 percent of the vaccine currently being supplied contains this organism. Most of the vaccine is used in cattle under 12 months of age living in conditions of enzootic instability. Although one vaccination is probably sufficient for these conditions, a second is often given about six months after the first. The incidence of severe reactions is so small that close supervision of cattle following vaccination is seldom practised. About 12 reports of breakdown of immunity following natural challenge of vaccinated cattle are received each year. Related to the estimated number of cattle vaccinated, this represents an incidence of less than 0.25 percent. A recent study showed that unvaccinated cattle were 16 times more likely to suffer clinical attacks than vaccinated animals. Haemolytic disease of calves was a problem some years ago (Langford et al., 1971) when the vaccine was wholly comprised of blood, and farmers considered that maintenance of immunity required frequent vaccinations. The introduction of the cell-free diluent and less frequent vaccinations have reduced the incidence of this condition to negligible proportions.
Applicability of Australian findings for other regions
The progress made in Australia in dealing with babesiosis and anaplasmosis has resulted from the resources of a developed country being applied to the solution of a problem of economic significance. In evaluating the usefulness of this approach for solving similar problems in developing countries, a number of questions must be considered.
The first questions concern whether or not the procedures are within the capability and resources of a developing country. Elaborate equipment is not necessary, but it is essential that some special training be provided for persons responsible for vaccine production. Great depth of knowledge is not essential because the principles are not difficult. However, personnel should be responsible and alert because there is a risk of spreading disease in a live vaccine based on fresh bovine blood.
A significant problem for some developing countries is to obtain animals known to be free of tick-borne infections and to protect them from natural infections during vaccine production. Special tick-free facilities at the laboratory - and possibly breeding programmes within them - may have to be established to satisfy this requirement.
Four-litre plastic bags of vaccine components held in cold room at 2–4°C
In Australia vaccine is produced continuously to meet a large demand. Because it is perishable, new batches must be prepared every week. This could be a difficult task in a developing country. It could also be extravagant in many instances because the annual demand might not exceed a few thousand doses required for protecting imported cattle. A solution is the preparation of batches followed by viable preservation in the frozen state. Effective procedures are now available (Dalgliesh and Mellors, 1974), and have been used for several years in the FAO project in Bolivia, where Australian-type vaccine is held in liquid nitrogen until required.
Other questions concern the immunological similarities of the Babesia transmitted by B. microplus. Will cattle immunized in one area survive challenge in another, and could a single vaccine be used in more than one region? Cattle immunized in Australia and exported to Southeast Asia do not suffer from tick-borne diseases. Some years ago, cattle immunized in Australia were exported to Trinidad without difficulty. Recently, McCosker (1975) found that Australian strains of B. argentina could be used as vaccine in Bolivia. This was followed by a laboratory study showing that Australian and Bolivian strains of B. argentina were serologically identical. Other recent studies (Goldman and Rosenberg, 1974) have shown immunological similarities in B. bovis, B. argentina and B. berbera. A vaccine prepared with any of these parasites could be applicable wherever Boophilus transmits babesiosis.
Display of vaccine bags, method of packing in ice and insulating material
“Boosting” with a heterologous strain of B. argentina raises the level of immunity, but is not always considered necessary in Australia. In more tropical environments, where the challenge is intense, two vaccinations with different strains would be an advantage. Inclusion of B. bigemina in vaccine has not proved necessary in Australia and Bolivia because of the low pathogenicity of field strains, but in other environments protection against this species may be required. Both B. bovis and B. bigemina are pathogenic in South Africa. In Australia, A. centrale has protected successfully against A. marginale. Again because of the probability of stronger challenges, A. centrale may not be completely satisfactory in some tropical regions. Its combination with relatively avirulent laboratory strains of A. marginale alone, specially processed for vaccine, might be necessary.
Immunity against babesiosis and anaplasmosis is obtained in Australia with reliably infective vaccines produced by manipulating strains of Babesia and A. centrale in the laboratory. These methods could have worldwide application. They could be adopted totally in countries where the epizootiology of the diseases is similar to that in Australia. Several alternatives exist for other regions where it would be difficult or unnecessary to maintain continuous production. These include limited production and the establishment of frozen stores of vaccine, or provision of vaccine by a foreign centre engaged in regular production. As an alternative to isolating and processing indigenous strains of parasites for vaccine, these might be obtained from Australia. Strains have been found to be immunogenic in regions far removed from this country. Vaccine strains currently used in Australia are also of reduced virulence and are incapable of being spread by ticks.
Callow, L.L. 1971. The control of babesiosis with a highly infective attenuated vaccine. Proc. XIX World Vet. Congr. Mexico, 1: 357–360.
Callow, L.L. & Mellors, L.T. 1966. A new vaccine for Babesia argentina infection prepared in splenectomized calves. Aust. Vet. J., 42: 464–465.
Callow, L.L. & Tammemagi, L. 1967. Vaccination against bovine babesiosis. Infectivity and virulence of blood from animals either recovered from or reacting to Babesia argentina. Aust. Vet. J., 43: 249–256.
Dalgliesh, R.J. & Mellors, L.T. 1974. Survival of the parasitic protozoan, Babesia bigemina, in blood cooled at widely different rates to - 196°C. Int. J. Parasit., 4: 169–172.
Goldman, M. & Rosenberg, A.S. 1974. Immunofluorescence studies of the small Babesia species of cattle from different geographical areas. Res. Vet. Sci., 16: 351–354.
Langford, G., Knott, S.G., Dimmock, C.K. & Derrington, P. 1971. Haemolytic disease of newborn calves in a dairy herd in Queensland. Aust. Vet. J., 47: 1–4.
Mahoney, D.F. 1973. Babesiosis of cattle. Australian Meat Research Committee Review, 12: 1–21.
Mahoney, D.F. & Ross, D.R. 1972. Epizootiological factors in the control of bovine babesiosis. Aust. Vet. J., 48: 292–298.
McCosker, P.J. 1975. Control of piroplasmosis and anaplasmosis in cattle. A practical manual. p. 1–64. Rome, FAO. FAO Animal Health Programme, Bolivia.