by R. O. Drummond
The first three articles in this series on ticks and tick-borne diseases have shown that ticks are a problem because they transmit diseases, produce paralysis or toxicosis, and cause physical damage to livestock. Although only relatively few of the more than 700 species of ticks in the world are of importance to man and his domestic animals, these few species must be controlled if livestock production is to meet world needs for animal protein. Losses due to tick infestations can be considerable. For example, in Australia alone in 1974, losses due to the cattle tick (Boophilus microplus) were estimated to be US$ 62 million (Springell, 1974). Such losses could be cut considerably by adopting effective tick control measures. The main weapon for the control of ticks at present is the use of chemical acaricides. In this article the need for tick control is reviewed and techniques for applying acaricides to animals, classes of acaricides, strategies of tick control, and precautions when using acaricides are discussed.
Need for tick control
There are three major reasons for controlling ticks on domestic animals: disease transmission, tick paralysis or toxicosis, and tick-caused physical damage.
Disease transmission
Ticks are responsible for the transmission of a large variety of diseases that affect livestock. The major diseases include babesiosis, anaplasmosis, theileriosis, and heartwater; in addition, there are other diseases of lesser importance. The major tick-borne diseases of livestock, their symptoms, and present methods for their control were briefly reviewed by Bram (1975). Generally the ticks become infested with the causative organisms while they are feeding on infected animals. Then the organism may be transmitted from stage to stage in the tick (an example is Theileria parva transmitted by Rhipicephalus appendiculatus), or from the female tick through the egg to the larvae - an increase of several thousandfold in vector potential (an example is Babesia equi transmitted by Anocentor nitens). When the next stage or generation subsequently feeds on another animal, the organism is transmitted to that animal if it is susceptible to the disease. Therefore, the transmission of diseases by ticks can only be stopped by killing the ticks before they feed sufficiently on susceptible animals and transmit the organism. The most common method of killing ticks is by the use of chemical acaricides.
R.O. Drummond is Location and Research Leader at the Department of Agriculture, Agricultural Research Service, P.O. Box 232, Kerrville, Texas 78028, United States.
Mention of a proprietary product or a pesticide in this article does not constitute an endorsement or a recommendation by the U.S. Department of Agriculture or by Fao.
Tick paralysis or toxicosis
Several species of ticks cause paralysis or toxicosis in livestock because they inject a toxin into the animals while feeding on them. Examples are paralysis caused by the feeding of Dermacentor andersoni, sweating sickness caused by Hyalom- ma truncatum, Australian tick paralysis caused by Ixodea holocylus, and tick toxicosis caused by Rhipicephalus species. Unless ticks are controlled, animals may exhibit severe reactions such as swelling, necrosis of skin, rise in temperature, etc. Some may die from paralysis or toxicosis.
Dipping cattle in dipping vat. Note complete coverage
Tick-caused physical damage
Tick infestations may cause physical damage to livestock other than disease, paralysis, or toxicosis. Included are “tick worry”, the irritation unrest, and weight loss due to massive infestation of ticks; the direct injury to hides due to tick “bites”; a loss of blood due to the feeding of ticks; the invasion of tick-infested animals by bacteria, fungi, and other pathogens; and the predisposition of tick-infested animals to infestation by other arthropods. An example of this last category is the infestation of ears of cattle in southern Texas by larvae of the screwworm fly (Cochliomyia hominivorax) that develop from eggs laid by female flies attracted to wounds in the ears created by infestations of the Gulf Coast tick (Amblyomma maculatum).
Methods of controlling ticks with chemicals
Acaricides used to control ticks on livestock or in the environment should be applied in such a manner that the ticks will be killed, the treatments will not harm livestock or applicators, the tissues of treated animals will not contain illegal residues, and the environment will not be adversely affected. Both Barnett (1961) and Shaw et al. (1970) have reviewed the control of ticks with acaricides.
Control off the host
Since all species of ticks spend a considerable portion of their lives off the host, these off-the-host stages of certain species can be controlled by applying acaricides to buildings, to livestock holding areas, or to the natural environment of the ticks. However, the environment must be treated with a sufficient amount of acaricide to produce maximum control. Unfortunately, many of the acaricides used for tick control are also excellent general-purpose insecticides and therefore may also be toxic to a number of the beneficial arthropods found in the same area inhabited by ticks. It is therefore important to determine that any treatments applied for the control of ticks will not harm other organisms. In addition, the effort and expense of treating large areas of land for the control of ticks plainly limit the general use of this technique. However, acaricides applied to such areas as stables, barns and small pastures can be used for the control of certain species of ticks.
Application to the host
The most popular method of controlling ticks on livestock is the application of acaricides directly to the animal host. It is important that application techniques be thorough and that the acaricides be highly effective against ticks without injuring the host.
Dipping vat
The usual way of treating large numbers of animals with acaricides is to immerse them in acaricides in dipping vats. The many different vats that have been designed all include three basic components: (1) an approach area, (2) a vat or tank in which animals are immersed, and (3) a drainage area. The approach area should be designed so as to enable the animals to enter the dip area one at a time. Often the approach area contains a footbath to remove dirt and other debris from the animals' feet before they are dipped. The vat or tank portion should be constructed of reinforced concrete or some other strong, imprevious material that will not crack or leak, and should be designed to hold a sufficient volume of liquid so that the animals can be immersed completely. Such immersion is essential for adequate control of ticks. Usually the vat is long enough to ensure that animals leaping from the entrance point cannot jump across the deepest portion of the vat, and is deep enough so that the animals will not injure themselves by hitting the bottom. The opposite end of the vat or tank consists of a series of steps to enable animals that have been immersed in vat fluids to walk steadly out of the dipping vat.
The drainage area should be so constructed that excess fluids draining from treated animals will flow back to the dipping vat proper. Sometimes these fluids are routed through a sump tank where debris, manure, and other materials are removed so that the fluid may return to the dip tank as clean as possible.
In general, dipping vats provide a highly effective method of treating animals with acaricides for tick control. However, their immobility, high initial cost of construction, and the cost of the acaricides may make vats impractical for many small ranching operations. Also, dipping vats must be managed carefully so that the dips are maintained at the proper concentration and the cattle are dipped properly.
Spray
Because of the disadvantages of the dipping vat for small operations, sprays are the most commonly used method of treating animals with acaricides for the control of ticks. Spraying equipment is highly portable, and only small amounts of acaricides need to be mixed for a single application. Spraying equipment may consist of a simple device such as the standard bucket pump; these handoperated pumps will apply acaricides under a pressure of 27 to 45 kg. In most areas, hand-operated pumps have been replaced by motor-driven pumps capable of generating pressures as high as 90 to 136 kg. However, spraying is generally less efficient in controlling ticks than immersion in a dipping vat because of problems associated with applying the acaricide thoroughly to all parts of the animal's body. The key point with spraying equipment is that application is only as through as the operator. Special care is needed to treat the ears, axillae, and other relatively inaccessible areas on animals.
Spray race or spray-dip machine
The spray race and spray-dip machine are a compromise between spraying and dipping in that these devices utilize smaller amounts of acaricides than dipping vats, and the animals are treated individually by acaricides applied under pressure through a system of nozzles directed to all areas of the animal's body. A spray race is usually permanently installed and includes an entrance chute and a drain pen that collects excess fluids which are filtered and recirculated through the pump. A spray-dip machine is portable and self-contained, and is so operated that the animals remain in the machine long enough for excess fluids to drain back into the collecting tank beneath the spray chamber. However, both are mechanical devices and suffer from the ills common to all such devices. They must be constantly cared for and maintained in peak condition if they are to apply acaricides adequately and efficiently to livestock.
Hand dressing
In certain instances, as when a species of tick inhabits a limited area of an animals's body, acaricides may be applied to these areas by hand. For example, larvae and nymphs of Otobius megnini are found only in the ears of cattle and other animals. Also, all stages of Anocentor nitens are found in the ears and nasal diverticula of horses.
The application of insecticides with aerosols and in oils, smears, and dusts by hand to limited body areas is time-consuming and laborious, but in certain instances it may be more effective and economical (in terms of cost of acaricide) than treating the entire animal.
Other methods
Certain other methods of controlling ticks with chemical acaricides have sometimes been used. For example, oral treatments or injections of some chlorinated hydrocarbon insecticides have controlled B. microplus on cattle. Also, oral treatments with animal systemic insecticides have controlled several species of ticks feeding on livestock. However, allowing animals to treat themselves on a continuing basis with a systemic presents problems, and the method will probably not be practical on a large scale, though it may be useful in certain limited situations.
Another example is the tick control achieved through the use of insecticide-impregnated plastic ear tags or horn bands. This treatment method is similar to the “dog collar technique” used to control fleas and ticks on pets.
Infestation of Gulf Coast tick, Ablyomma maculatum, in the ear of a cow
Hand spraying. Application is only as thorough as the operator
Acaricides
Several general groups of chemical compounds effectively kill ticks on livestock.
Arsenicals
Water-soluble forms of arsenic and arsenic-containing compounds, usually As2O3, have been used for many years in dipping vats to control ticks, especially ticks of the genus Boophilus. Arsenic is inexpensive, stable, and water-soluble, and there is an accurate vat-side test. Arsenic dips were used successfully to eradicate Boophilus ticks from the southern United States. Unfortunately, arsenic has a very short residual effectiveness (less than one to two days), and in most areas of the world Boophilus ticks have become resistant to arsenic.
Chlorinated hydrocarbons
Chlorinated hydrocarbon acaricides have been used extensively throughout the world for the control of ticks. Of particular interest are benzene hexachloride (1, 2, 3, 4, 5, 6-hexachlorocyclohexane; the gamma isomer of this compound, called lindane, possesses the greatest acaricidal activity); and toxaphene (chlorinated camphene containing 67 to 69 percent chlorine). Chlorinated hydrocarbon acaricides are generally cheap, have long residual effectiveness, and are good general-purpose insecticides for the control of a variety of livestock pests. They are also not highly toxic to livestock, though some animals have been poisoned by treatment with lindane. Unfortunately, most chlorinated hydrocarbon insecticides create residues that remain in tissues of treated livestock for fairly long periods. Also, a number of species of ticks have developed resistance to these acaricides. Nevertheless, both lindane and toxaphene are used in many areas of the world for the control of a variety of ticks.
Spraying of cattle with power-operated sprayer
Organophosphorous compounds
Because resistance to arsenicals and chlorinated hydrocarbons has developed in many species of ticks, the only available acaricides in certain areas of the world are organophosphorous compounds. Most of the large number of such acaricides available (Drummond et al., 1974) are highly effective against ticks at very low concentrations. However, their residual effectiveness is usually shorter than that of chlorinated hydrocarbons, and the risk of causing acute toxicity in livestock is greater. Several tick species are now known to be resistant to organophosphorous acaricides; resistance is acutely present in M. microplus in Australia and several other countries (Wharton, 1974).
Other compounds
Carbamate acaricides are used in special situations when ticks have become resistant to most organophosphate and chlorinated hydrocarbon acaricides. In addition, several other new acaricides have appeared on the market to be used for the control of resistant ticks. Especially promising are the formamidines that have demonstrated outstanding acaricidal activity (Roulston, 1973). The use of chemical acaricides continues to produce resistance in ticks. It is obvious that ticks, especially B. microplus, are highly adaptable insects and will probably respond to any challenge posted by a new acaricide. It is therefore imperative that the effective acaricides presently used be applied correctly and efficiently so as to prolong their usefulness as tick control agents.
Control strategy
In any situation the strategic (or most effective) use of acaricides for the control of ticks will depend upon a number of factors including the life cycle of the species of tick to be controlled, the urgency of the need in terms of disease transmission, the seasonal activity of the tick, and government regulations.
Certainly an acaricide must be directed against the susceptible stage of the tick. The strategy in controlling a one-host species such as B. microplus, which attaches to animals as larvae and moults on the host, consists of treating animals at 14- to 21-day intervals so the females are killed before they can finish engorgement. The strategy in controlling a three-host species such as Amblyomma americanum consists of treating animals at 7- to 10-day intervals so as to kill adults moving from the environment to attach on the animals. The strategy in controlling Otobius magnini, another one-host species that slowly engorges in the ears of livestock, may consist of a monthly or bimonthly treatment because of the low reinfestation pressure and low biotic potential of this species. Since the strategy of tick control depends so heavily on the biology of the species to be controlled, the need for correct identification of ticks and current information on their biology cannot be over-emphasized.
Urgency of need in terms of disease transmission can be illustrated by the need for control of Rhipicephalus appendiculatus, which transmits East Coast fever to cattle. Since the disease is transmitted from stage to stage in the life of the tick, an infective adult can infect a susceptible animal within two or three days after it attaches. Therefore, with an acaricide that has a short residual activity, cattle must be treated every three to five days to prevent transmission of the disease.
The seasonal activity of the tick should logically determine the time of application of acaricides to livestock or to the environment. Treatments should be applied when they will provide the maximum benefit in terms of reduction of tick populations. A “strategic” scheme for treatment does not necessarily mean treating animals at the time tick populations are at their peak. It could well mean treating when animals are only lightly infested so as to reduce the populations that survive through stress periods such as dry seasons, low temperatures, etc. Again, considerable information concerning the life history, seasonal appearance and biology of ticks must become available before such “strategic” treatment can be effectively utilized.
The use of tick control measures in government-sponsored or regulated eradication campaigns is usually dictated by the need for complete, thorough and frequent treatment of livestock on a compulsory schedule. In the southern United States a compulsory scheme that included the dipping of cattle every two weeks in 0.18 to 0.22 percent arsenic was successful in eradicating B. annulatus. Additional factors were pasture rotations and quarantines to prevent movement of livestock.
Precautions
Acaricides must be used to control the many species of ticks that affect livestock, but these acaricides can be toxic to livestock and humans, can create illegal residues in tissues of animals, and can be destructive to the environment if they are not used and handled in a safe and correct manner. The following are some precautions to follow when using acaricides for the control of ticks (modified from U.S. Department of Agriculture, 1967).
Use only those acaricides recommended and approved for use on livestock by a competent authority, usually a government official, recognized agricultural agent or adviser, livestock production specialist, veterinarian, or other person with specialized training. Often lists of recommended acaricides are published by responsible government agencies.
Use a formulation of the acaricide that is approved and especially designed for use on livestock. There are too many examples of poisoning and death of livestock due to treatment with a recommended acaricide in a formulation designed for use on plants. Especially, only those formulations designed specifically for dipping vats should be used in dipping vats.
Follow the label directions exactly. The label contains all the information on dilution, time of retreatment of animals, antidotes for poisoning, methods of disposing of unused insecticide, and other important facts.
Be sure that spraying equipment is clean and working properly, and especially that it provides sufficient agitation to allow for thorough mixing of acaricides.
Be aware of safe practices when mixing or applying acaricides. Do not eat, drink or smoke during application; wear special clothing that can be changed after the day's application is finished; use simple precautions when mixing and preparing acaricides; if accidentally exposed to an acaricide, wash it off thoroughly and change clothes immediately; do not store acaricides near food or in other than original containers.
Learn to recognize signs of acaricide poisoning in livestock and humans to avoid delay in applying antidotal measures.
Dispose correctly of all containers, unused concentrate, and used diluted acaricide to avoid contamination of the environment.
The safe use of acaricides is essential to an efficient, well-run programme for the control of ticks. Although millions of animals are treated and millions of kilograms of insecticides are used yearly, this safe use should not be taken for granted. To avoid accidents and misuse, it is necessary to continually review and employ safe use precautions and procedures.
References
Barnett, S.F. 1961. The control of ticks on livestock. Rome Fao. Fao Agricultural Studies No. 54.
Bram, R.A. 1975. Tick-borne livestock diseases and their vectors. 1. The global problem. Wld Anim. Rev. (FAO), (16): 1–5.
Drummond, R.O., Gladney, W.J. & Graham, O.H. 1974. Recent advances in the use of ixodicides to control ticks affecting livestock. Bull. Off. int. Epiz., 81(1–2): 47–63.
Roulstion, W.J. 1973. Prospects for chemical control of Boophilus microplus in Australia. Proc. 3rd Int. Congr. Acarol., 1971: 693–695.
Shaw, R.D., Thorburn, J.A. & Wallace, H.G. 1970. Cattle tick control. London, St. Martins Press. 65 p.
Springell, P.H. 1974. The cattle tick in relation to animal production in Australia. Wld Anim. Rev. (FAO), (10): 19–23.
U.S. Department of Agriculture. 1967. Safe use of agricultural and household pesticides. Washington, D.C. Agriculture Handbook No. 321. 65 p.
Wharton, R.H. 1974. Ticks with special emphasis on Boophilus microplus. In Pal, R. and Wharton, R.H., eds. Control of arthropods of medical and veterinary importance, p. 35–52. New York, Plenum Press.
by R.H. Wharton
The resistance of ticks to acaricides poses an increasing threat to livestock production in many countries because of their almost complete dependence on acaricides for tick control. Resistance has led to instability and increased costs in areas where the onehost cattle ticks Boophilus microplus and B. decoloratus have acquired resistance to a variety of toxic chemicals. The point has now been reached where such resistance must be expected in these ticks within five to 10 years of the introduction of any new type of acaricide, unless control practices are changed. In Africa, resistance could ultimately lead to a much greater dislocation of the cattle industry than elsewhere because of the more serious problems caused by the three-host ticks (Rhipicephalus, Amblyomina and Hyalomma) and the diseases they transmit.
Prior to the development of arsenic in the 1890–1910 period, cattle owners in North America, Latin America, Australia and Africa had no practical method of limiting tick numbers on their cattle. In the United States arsenic provided the opportunity for cattlemen in the southern states to retain their markets in the north through a campaign that eventually succeeded in eradicating B. annulatus; in Africa the frequent application of arsenic permitted the development of highly productive cattle industries, while in Central and South America and Australia arsenic allowed European breeds of cattle to thrive in the tropics and subtropics in association with B. microplus.
Arsenic was used for 30 to 40 years before resistance developed in Boophilus ticks. This occurred at about the same time prior to World War Two in Africa, Australia and South America. The organochlorines DDT, BHC (lindane) and toxaphene (campheclor) overcame the arsenic resistance problem during the 1945– 55 period, and for the first time provided cattle owners with chemicals that were highly toxic to ticks and were safe and easy to use.
The organochlorines were supplemented or replaced by the organophosphorus (OP) and carbamate chemicals in the 1955–70 period. These in turn were replaced in some areas by formamidines and related chemicals after 1970. These rapid changes were brought about by resistance, unacceptable organochlorine residues in meat, and the competition that developed for the acaricide market. The competition continues and adds to the complexity of the resistance problem, and focuses attention on the need for authentic information on acaricide resistance and its significance in relation to control. This applies particularly to Africa because of the different rates at which resistance has developed in the one-host Boophilus ticks and the two-and three-host Rhipicephalus ticks, and because the recognition of resistance to a particular acaricide does not necessarily mean that resistance is widespread or that the acaricide is no longer valuable for tick control.
The resistance problem also focuses attention on the need for alternate methods of tick control that are less dependent on acaricides.
The author is Officer-in-Charge of the Division of Entomology, Long Pocket Laboratories, CSIRO, Private Bag No. 3, Indooroopilly, Queensland 4068, Australia.
Types of resistance
Resistance is of four basic types - arsenic, DDT, toxaphene-BHC-dieldrin and organophosphorus-carbamate (OP). They are not related biochemically, but ticks that have developed resistance to one type can subsequently develop resistance to one or more of the other types. Thus, in South Africa Whitehead (1959) described an arsenic/DDT/BHC-resis tant strain of B. decoloratus which is also resistant to pyrethrum. In Australia, significant proportions of OP-resistant B. microplus have been found with resistance to arsenic, DDT and dieldrin.
There is no evidence to suggest that resistance to arsenic, DDT or the toxaphene group can be divided into different forms. But many forms of OP resistance have been described in B. microplus. They have different toxicological and biochemical responses and have been given special strain names derived from the locality where they were originally collected (see table). Strains with similar characteristics have subsequently been recognized from other areas of the same country, from different countries, and even from different Boophilus species.
The many OP-resistant strains of B. microplus have caused considerable confusion. Figures 1, 2 and 3 illustrate the complex biochemical characteristics and interrelationships of the various Australian strains, their resistance factors to selected OP acaricides, and the mortality these acaricides produce when sprayed on cattle infested with the different strains. Resistance is caused by changes in the sensitivity of tick cholin-esterase (AChE) to inhibition by OP chemicals and/or by the tick's ability to detoxify the acaricide. These may be accompanied by striking changes in the amount of AChE activity demonstrable in larvae. This is illustrated by the following examples:
Ridgelands ticks have a low level of AChE activity (21) compared with susceptible ticks (100), but the AChE is moderately insensitive to inhibition by OP acaricides.
Biarra ticks (42) have more of a highly insensitive AChE.
Mt. Alford detoxify chlorpyrifos in addition to having highly insensitive AChE.
Mackay have normal AChE but detoxify coumaphos.
The resistance factors (RF) and mortality in spraying trials illustrate the progressive nature of OP resistance. Thus Ridgelands ticks have an RF of 2 to chlorpyrifos (=Dursban) which produces 99 percent mortality when sprayed on cattle; Biarra RF is 6 and mortality is 96 percent, while Mt. Alford RF is 110 and mortality is only 46 percent.
Resistance resembling Ridgelands has developed in a number of countries (see table), but it is not possible to relate other forms reported in B. microplus to those characterized from Australia. Some of the South American strains resemble Biarra in being resistant to coumaphos and ethion, while resistance to chlorpyrifos, which occurs in several Australian strains, has developed in Rio Grande do Sul, Brazil (Wharton and Roulston, 1976).
OP-resistant Boophilus microplus (Mt. Alford strain) on a Hereford from a herd which had been treated with chlorpyrifos (Dursban) every 10 to 15 days for over four months
Geographical and chronological records of tick resistance to acaricides*
| Species | Arsenic 1900- | DDT 1946- | Toxaphene-BHC- dieldrin group 1947- | Organophosphorus-carbamate group 1955- | ||||
|---|---|---|---|---|---|---|---|---|
| Amblyomma hebraeum | Rhodesia South Africa | 1975 1975 | South Africa | 1975 | South Africa | 1975 | ||
| Amblyomma variegatum | Zambia | 1975 | Tanzania Zambia | 1975 1975 | ||||
| Boophilus decoloratus | Kenya Malawi Rhodesia South Africa Zambia | 1953 1969 1963 1938 1975 | South Africa | 1954 | Kenya Malawi Rhodesia South Africa Uganda Zambia | 1964 1975 1969 1948 1970 1975 | Rhodesia South Africa1 “Berlin” = “Ridgelands” | 1975 1966 |
| Boophilus microplus | Argentina Australia Brazil Colombia Jamaica South Africa | 1936 1936 1948 1948 1948 1976 | Argentina Australia Brazil Venezuela | 1953 1953 1953 1966 | Argentina Australia Brazil Colombia Ecuador Guadeloupe India Madagascar Malaysia Martinique Trinidad Venezuela | 1953 1953 1953 1966 1966 1961 1964 1963 1967 1961 1969 1966 | Argentina “Goya” “Las Guerisas” Australia “Ridgelands” “Biarra” “Mackay” “Mt. Alford” “Gracemere” “Bajool” “Tully” “Ingham” Brazil “Ridgelands” “Minas Gerais” Colombia “Ridgelands” “Guaimarito” Venezuela “Ridgelands” “Guaimarito” | 1970 1964 1963 1966 1967 1970 1970 1972 972 1973 1963 1969 1967 1970 1967 1970 |
| Haemaphysalis leachii | South Africa | 1976 | ||||||
| Hyalomma marginatum | Spain | 1967 | ||||||
| Hyalomma sp. (rufipes, truncatum) | South Africa | 1975 | South Africa | 1975 | ||||
| Ixodes rubicundus | South Africa | 1976 | ||||||
| Rhipicephalus appendiculatus | Malawi Rhodesia South Africa Zambia | 1975 1975 1975 1975 | Kenya Rhodesia South Africa Tanzania Uganda Zambia | 1968 1966 1964 1971 1968 1975 | South Africa | 19751 | ||
| Rhipicephalus evertsi | Kenya Rhodesia South Africa Zambia | 1975 1975 1975 1975 | Kenya Rhodesia South Africa Tanzania Uganda Zambia | 1964 1966 1959 1970 1970 1975 | South Africa | 1975 | ||
In South Africa, recent reports indicate that two forms of OP resistance have developed in B. decoloratus and R. appendiculatus (C.J. Howell, personal communication). the first is Ridgelands and the second is characterized by resistance to chlorfenvinphos (=Supona) and may resemble Biarra from Australia.
Confusion about OP resistance outside Australia is due to lack of published information, different methods of evaluation by various research workers, and probably to the fact that most field populations of “resistant” ticks contain both resistant and susceptible ticks. Before the resistance can be characterized, the field sample may need to be selected in the laboratory to produce a more homogeneous strain. This is illustrated in Figure 4, which shows the dosage mortality response of a sample suspected to be resistant, as compared with the responses of susceptible and “pure” Biarra strains and of the sample after culturing and selection in the laboratory. The field and F1 populations were clearly heterogeneous; the resistance factors (calculated from the LC50 values) of 8 and 10 respectively do not represent the true potential of the resistance which proved to be Biarra with a resistance factor of 34.
Resistance is determined genetically, with DDT resistance being controlled by a recessive autosomal gene, dieldrin resistance by a dominant autosomal gene, and the various OP resistances by incompletely dominant autosomal genes (Stone, 1972). It is a pre-existing phenomenon with acaricide selection favouring the survival of mutant resistant individuals. Selection in the laboratory to “purify” a heterogeneous field sample hastens the process that would ultimately occur in the field.
Occurrence, distribution and significance of resistance
Geographical and chronological records of resistance, from governmental or commercial sources, are biased toward countries with facilities and expertise for the recognition of resistance (see table).
The only records outside Africa, Australia and South America are on Hyalomma marginatum in Spain and Boophilus microplus in Asia. B. microplus has been transported to most tropical and subtropical areas from its Asian origin and has developed resistance in almost all areas where it is sufficiently abundant to warrant control measures. A notable exception is Mexico.
Also notable for a lack of resistance are Ixodes ricinus in Europe and the North American Amblyomma and Dermacentor ticks, for which there are no substantiated records of resistance (Wharton and Roulston, 1970). The dog tick (Rhipicephalus sanguineus) has developed resistance to lindane in several parts of the world, but is not important in livestock production.
The significance of resistance is not easy to quantify. Although a supply of efficient acaricides is available at reasonable cost, confusion regarding resistance and the costs involved in changing to a new acaricide are major problems. Arsenic resistance was a threat to production because no alternatives were available. But DDT and toxaphene-type resistance were relatively minor problems for B. microplus since OP acaricides were being developed as competitive alternatives.
The toxicological response of larval ticks may be documented by exposing larvae to different concentrations of acaricides in impregnated filter paper packets; resistance is detected when larvae survive exposure to concentrations that kill susceptible larvae
Figure 1. Biochemical characteristics of eight OP-resistant strains of Boophilus microplus in Australia
Figure 2. Resistance factors of eight OP-resistant strains1 of Boophilus microplus in Australia
Figure 3. Mortality in spraying trials of cattle infested with eight OP-resistant strains of Boophilus microplus in Australia
Figure 4. Dosage mortality responses of a field strain of Boophilus microplus as submitted, and following selection in the laboratory, compared with responses of susceptible and Biarra OP-resistant strains
In Australia, when organochlorines were banned for use on cattle in 1962 because of residues found in meat, several excellent and cheap OP acaricides were available. As OP resistance developed first to dioxathion, then to coumaphos and ethion, and finally to chlorpyrifos and bromophos ethyl, control problems increased. An almost frantic search for alternative acaricides has resulted in an array of new acaricides, a formamidine (chlordimeform), an imminopyrolidine (clenpyrin), a thiourea (chloromethiuron) and a carbamate (promacyl). Mt. Alford resistance in 1970 was alleviated by adding the formamidine chlordimeform to existing OP acaricides. The mixtures were effective, but chlordimeform was mainly a “stopgap” to buy time for the newer acaricides. Unfortunately, these are not as efficient as were several OP acaricides before resistance, and they cost two to three times as much (Wharton and Roulston, 1976). This excludes the recently marketed formamidine-type chemical amitraz (Taktic), which is highly effective against all strains, but has the disadvantage of being unstable unless buffered to pH 10. Since resistance is not universal and OP acaricides are still effective in many areas, the difficulties facing the cattle owner in selecting an acaricide are obvious. Where resistance has progressed to the highest level and non-OP acaricides have been used since 1970–71, there is a haunting fear that resistance to these will become evident at any time.
In South America the significance of resistance is less apparent, but the fact that amitraz and another new type of acaricide, the dithietane nimidane (Abiquito), are being marketed in Brazil (Amaral et al., 1974) suggests that some cattle owners are facing control problems similar to those in Australia.
In Africa the different rates of development of resistance in Boophilus and Rhipicephalus ticks led to the use of arsenic-DDT, arsenic-BHC, toxaphene or BHC-OP mixtures, and more recently mixtures of OP-acaricides. Toxaphene and BHC have been highly effective over many years, but their future was jeopardized when resistance was observed in R. evertsi and R. appendiculatus in some localities in South Africa, and more recently in Kenya, Uganda and Tanzania. Recent reports from Africa indicate that resistance to toxaphene is widespread in countries where R. appendiculatus and R. evertsi are abundant, and has extended to Amblyomma and Hyalomma species in South Africa (see table). Toxaphene and certain OP acaricides have been banned in Kenya because of resistance. Mixtures of OP acaricides have in turn been affected, and the recent introduction of amitraz in South Africa suggests that serious control failures have occurred with these mixtures. Nevertheless, toxaphene is still used and may continue to be effective where resistance fails to develop, or where resistant individuals comprise only a small proportion of the population. In R. appendiculatus collected from different properties in South Africa, Baker and Shaw (1965) reported resistance factors from 2.5 to 14 in R. appendiculatus, indicating that they were examining heterogeneous populations (Figure 4). Control was still adequate on a property with “low” resistance but unsatisfactory on one with “high” (14 ×) resistance. Under these circumstances it would probably have been premature to change the acaricide on the property with “low” resistance. The finding of resistance does not necessarily mean that practical control has failed; the effective life of any acaricide should be prolonged as much as possible to conserve acaricides for the future.
In Tanzania R.J. Tatchell (personal communication) has also found “low” resistance in R. appendiculatus. This “low” resistance differs from the low levels of resistance illustrated in Figure 2, which may be controlled by using increased concentrations or shorter intervals between treatments. The former approach was used in Australia with chlorpyrifos to control Biarra OP resistance, and more recently with the carbamate promacyl, which controls all OP-resistant B. microplus, despite low levels of resistance. Increased costs are associated with this approach, but they may still be less than changing to a completely new type of acaricide.
Detection, identification and characterization of resistance
Resistance is usually recognized because of failure to obtain a satisfactory kill of the parasitic stages on a treated animal. Failure is frequently due to inadequate treatment and many reports of resistance are unfounded. There is no doubt about resistance when cattle continue to be infested with large numbers of engorged ticks after frequent treatments. But the response of parasitic ticks to understrength acaricides is very similar to their response to low-level resistance or to high-level resistance in the early stages of its development. Thus field spraying tests must be conducted under standardized conditions where resistance is suspected (Baker and Shaw, 1965).
Confirmation of resistance must be made by laboratory tests. Many methods of testing the effects of acaricides have been developed, mainly by chemical companies in screening programmes. The stages most commonly used are the engorged females and the unfed larvae. The former usually provide the most useful information on potential acaricides, but unfed larvae are generally accepted as the logical stage to document resistance. Several engorged females provide sufficient larvae to test against a range of concentrations of several acaricides. The response of susceptible ticks provides the baseline when resistance is suspected. Two methods of testing have been used extensively: the Shaw method in which larvae are “dipped” for short periods in commercial formulations; and the Stone and Haydock (WHO) method in which larvae are enclosed in acaricide-impregnated filter paper packets. The latter has been recommended by FAO (1971) as a tentative method for detecting acaricide resistance.
When resistance to a new type of acaricide is found, the “resistant” population may need selection and culturing in the laboratory before its full potential and toxicological responses can be documented (Figure 4). With this information it should be possible to select discriminating doses that will kill all susceptible larvae (usually LC99 × 2); field samples are then exposed to these doses to determine whether resistance is present (FAO, 1971).
Figure 5. Levels of resistance of Hereford, Shorthorn and zebu × British cattle in the wet tropics and subtropics of Australia
SOURCE: After Wharton, 1974
Alternate methods of tick control
When considering alternate methods of tick control, it is important to remember that the cattle industries of countries where acaricide resistance is a problem have all been dependent on acaricides to develop those industries. This does not mean that alternate methods are not available for some ticks or may not eventually be developed, but the era of highly effective acaricides has encouraged cattle owners and many scientists involved in animal production to believe that the supply will continue. Unfortunately, the adaptability of ticks limits the commercial return from acaricides and could lead to a situation in which chemical companies may find the development of acaricides not economically viable.
Approaches to insect control based on sterile males, genetic manipulations or pheromone attractants offer little promise except for attractants which could be useful for domestic pets, or for ticks with specialized attachment sites (Gladney et al., 1974). Pest management systems will require detailed knowledge of the ticks' ecology and the epizootiology of the diseases they transmit. These objectives are within reach for B. microplus and Babesia (Mahoney, 1974), but not for Anaplasma or any other tick-borne disease association. The strategic use of acaricides to maximize their effect, the adoption of pasture rotation and alteration of the physical character of vegetation (including conservation) are well-established principles (Barnett, 1961) that have had little impact on tick control.
The most logical method of alleviating tick depredations would be to capitalize on host-parasite relationships that evolved in nature. Cattle survived in Asia and Africa despite Babesia, Theileria and their Boophilus and Rhipicephalus vectors. Host resistance, expressed by an animal's ability to prevent the maturing of large numbers of ticks, and disease immunity, are survival mechanisms for the host and for external and internal parasites. The problem is not only to utilize these attributes, but also to increase productivity. Resistance to B. microplus is associated primarily with zebu (Bos indicus) cattle. Considerable progress has been made in evolving resistant Bos indicus × Bos taurus beef and dairy cattle that limit the effects of ticks while retaining high productivity (Turner, 1975; Hayman, 1974; Mason, 1974). Similar approaches may ultimately benefit the more complex and demanding African environment. In this context the domestication and harvesting of game have obvious merit (King and Health, 1975).
Utilization of host resistance, while offering an attractive approach to tick control, raises many questions even with the relatively simple B. microplus-Babesia association (Figure 5). Resistance is an acquired characteristic and each animal develops its own level of resistance in response to tick challenge; the level may be high (as in most zebu cattle) or low (as in most European cattle), but a wide range of resistance occurs in all breeds of cattle. It is heritable, and selection and breeding for tick resistance are possible not only in zebu × European breeds, but also within European breeds. However, selection for resistance or culling for susceptibility must at present be based on tick numbers surviving on cattle exposed either naturally or artificially to tick challenge. This raises obvious problems for the cattle producer who is concerned about the effects of these ticks on production. Markets for tick resistance are being sought, but intensive studies on the cattle-tick relationship highlight its complexity and the difficulties of either finding a simple marker for resistance or enhancing the low resistance exhibited by European breeds of cattle. Nevertheless, the prejudices against zebu cattle in Australia are dying, and recognition of their virtues and potential in crossbreeding programmes indicates a more rational and hopeful prospect for using this approach to improved tick control with less dependence on acaricides.
References
Amaral, N.K., Nonmany, L.G.S. & Carvalho, L.A.D. 1974. Acaricide AC 84,633: First trials for control of Boophilus microplus. J. econ. Ent., 67: 387–389.
Barnett, S.F. 1961. The control of ticks on livestock. Rome, FAO. FAO Agricultural Studies No. 54. 115 p.
Baker, J.F.A. & Shaw, R.D. 1965. Toxaphene and lindane resistance in Rhipicephalus appendiculatus, the brown ear tick of equatorial and southern Africa. J. S. Afr. vet. med. Ass., 36: 321–330.
FAO. 1971. Recommended methods for the detection and measurement of resistance of agricultural pests to pesticides. Tentative method for larvae of cattle ticks, Boophilus spp. FAO Method No. 7. FAO Plant Prot. Bull., 19: 15–18.
Gladney, W.J., Grabbe, R.R., Ernst, S.E. & Oehler, D.D. 1974. The Gulf Coast tick: evidence of a pheromone produced by males. J. med. Ent., 11: 303–306.
Hayman, R.H. 1974. The development of the Australian milking zebu. Wld Anim. Rev. (FAO), 11: 31–35.
King, J.M. & Health, B.R. 1975. Game domestication for animal production in Africa. Wild Anim. Rev. (FAO), 16: 23–30.
Mahoney, D.F. 1974. The application of epizootiological principles in the control of babesiosis in cattle. Bull. Off. int. Epiz., 81(1–2): 123–138.
Mason, I.L. 1974. Maintaining crossbred populations of dairy cattle in the tropics. Wild Anim. Rev. (FAO), 11: 36–43.
Stone, B.F. 1972. The genetics of resistance by ticks to acaricides. Aust. vet. J., 48: 345–350.
Turner, H.G. 1975. The tropical adaptation of beef cattle: an Australian study. Wld Anim. Rev. (FAO), 13: 16–21.
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.
Wharton, R.H. & Roulston, W.J. 1970. Resistance of ticks to chemicals. A. Rev. Ent., 15: 381–404.
Wharton, R.H. & Roulston, W.J. 1976. Acaricide resistance in Boophilus microplus in Australia. Proceedings of the Hemoparasite Workshop, Cali, Colombia, 1975. Cali, Centro Internacional de Agricultura Tropical. (Mimeographed)
Whitehead, G.B. 1959. Pyrethrum resistance conferred by resistance to the blue tick. Nature, 184: 378–379.
