*. Based on papers presented by W Ferguson and A A Ademosun and discussions led by D H Hill.
Cattle diseases
Sheep and goat diseases
The major disease risks to cattle in the subhumid zone have long been recognized as rinderpest, trypanosomiasis, contagious bovine pleuropneumonia (CBPP) and dermatophilosis (streptothricosis). With the development of a tissue culture vaccine for rinderpest and a fairly effective freeze-dried vaccine for CBPP, tsetse-transmitted trypanosomiasis has remained the major development constraint, restricting the use of much of the subhumid zone by cattle, and to a lesser extent by sheep and goats. As such, it will be treated in a separate chapter of this report.
In addition to the major diseases, tick-borne diseases are common and heartwater in particular takes a heavy toll of susceptible animals. Several of the endemic bacterial diseases, e.g., brucellosis and tuberculosis, could assume greater importance as husbandry systems become more intensive, as could parasitic gastroenteritis which already takes a heavy toll whenever ruminant livestock are concentrated on wet pastures. On the other hand, increased sedentarization should limit the wide dispersal of many infectious diseases, and changes in production systems should affect the incidence of nutrient deficiencies and metabolic disjunctions.
In francophone countries, many diseases are controlled by vaccines provided free to the livestock owners by the national governments. However, drugs such as anthelminthics have to be paid for by the producers, so the economic justification for such treatments must be considered at the producer level as well as at the national level when planning animal health programmes. For example, their costs might be justified for the treatment of young stock, but not for adult animals. Similarly, the cost of chemoprophylactics for trypanosomiasis might be kept to a minimum by treating only the most vulnerable livestock populations.
In general, money for veterinary treatment might be more wisely invested in improved animal feeding. Yet large sums are often allocated for veterinary drugs in cattle production projects without clearly specifying the local disease problems. Similarly, although most small-ruminant diseases have been identified and described, little is known of their relative importance or the cost/benefit ratios of veterinary treatment. As a result, it appears that too much is being spent at present on veterinary drugs. Important topics requiring further research include the relative importance of different animal diseases, the interaction of disease with nutritional status and other factors, and the seasonality of disease incidence.
Rinderpest
Rinderpest first appeared in the subhumid zone between 1890 and 1900. The disease is caused by a highly infectious virus producing a febrile, catarrhal condition with inflamation of the mucous membranes. Lacrymation, becoming muco-purulent, is the usual first sign of the disease. Inflammation of the buccal mucosae gives rise to necrotic areas of the gums producing a cheese-like material and a characteristic smell from affected animals. Diarrhoea, and even dysentery, follow with a high mortality in susceptible animals.
In some instances, mortality rates as high as 90 or 100% were recorded from the 1890s to the 1920s in parts of West Africa. The seriousness of losses from rinderpest led to the establishment of the Veterinary Research Laboratories at Vom in Nigeria in 1924, where virulent virus and hyperimmune sera were prepared for use in immunization campaigns according to methods which had been developed in Kenya and India.
The virus/serum double inoculation method (DI) of immunization was highly effective, though at a cost of a 3 to 5% mortality rate amongst vaccinated cattle. It was followed in the 1940s by the use of an attenuated goat-adapted virus vaccine which was as effective as the DI method and much safer, mortality on the average being below 1%. However, these vaccines never reached more than 30 to 40% of the cattle population and there was always a residual pool of susceptible unvaccinated animals, usually under three years old. Rinderpest continued to occur until a major international eradication campaign was decided upon through the Organization of African Unity (OAU). This was the 15th joint international action sponsored by the OAU and hence was called Joint Project (JP) 15. Vaccinations were carried out for three full years, from 1962 to 1965, reducing the disease to occasional sporadic outbreaks.
When rinderpest was prevalent, the highest incidence of the disease was in the Sudan and Sahel zones. Rinderpest also occurred sporadically in the subhumid zone, though its incidence was restricted by the relatively low cattle numbers, due to the prevalence of trypanosomiasis. In forest areas it was introduced to the susceptible trypanotolerant population by trade cattle moving towards the southern markets.
The present tissue culture vaccine is safe and effective for all breeds and classes of cattle, but the current danger is that the dramatic reduction in the incidence of the disease will generate complacency and -premature relaxation of vaccination programmes. Inadequate maintenance of immunity after the JP 15 campaign could all too easily lead to bigger rinderpest outbreaks than those experienced in the past, particularly where more intensive production systems and concentrated livestock populations have been established.
Contagious Bovine Pleuropneumonia
Contagious bovine pleuropneumonia (CBPP) is caused by the organism Mycoplasma mycoides which affects the lungs and pleura causing a lobular pneumonia and sero-fibrinous pleurisy. The course of the disease is seldom acute: it is usually insidious and slow, with few clinical symptoms at the outset to enable early recognition. CBPP spreads through inhalation of infected droplets in the breath of infected animals. Morbidity and mortality can be high, although it may take many months for infection to spread amongst animals in a herd. Chronically ill animals may live for a long time, and some recover but retain necrotic sequestra in their lungs which can break down at any time under stress and renew the infection. Postmortem lesions of lobular pneumonia, red and grey hepatization with distended septa ('marbling') and much sero-fibrinous pleural exudate, and even strong fibrinous attachment between parietal and visceral seral mucosae, are diagnostic.
The incidence of CBPP persists in most West and Central African countries in spite of the development of effective freeze-dried vaccines. The FAO/VVHO/OIE Animal Health Yearbook for 1977 indicates a low sporadic incidence of the disease in Senegal, Guinea and Sierra Leone, a moderate incidence in Chad, Upper Volta and Nigeria and a higher incidence in Mali, Ivory Coast and the Central African Empire. In Niger, Togo, Benin, Nigeria, Cameroon and Central African Empire, the disease is said to be confined to specific regions
Given the persistence of CBPP, the incidence of this disease could rapidly revert to serious proportions if control measures were to become relaxed. However, there is now growing confidence in prospects for its satisfactory control, largely due to the availability of an effective and stable vaccine based on the strain Mycoplasma mycoides T1 44, which appears to confer an immunity lasting more than 12 months (Lindley, 1978), There is also evidence that this vaccine, if used correctly over a period of years, could lead to the elimination of the disease altogether. The use of this vaccine, combined with the control measures recommended by the FAO/OIE/OAU Expert Panel on CBPP in 1971, has been made the basis of an International Campaign for the Eradication of CBPP in West Africa.
The T1 vaccine is inoculated into the tip of the tail and sometimes, particularly if inoculation techniques are faulty, causes severe reactions which may result in the loss of part, or all, of the tail. This can discourage cattle-owners from taking their animals to immunization camps and, as the animals are normally vaccinated for rinderpest at the same time, there is a risk that cattle owners' rejection of CBPP vaccine will lead to an increase in the number of animals susceptible to rinderpest as well (Chalmers, 1975).
In general, however, where adequate supplies of T1 vaccine are available to a mobile and competent veterinary service, the prospects for control of CBPP are favourable. This disease is therefore no longer considered a major deterrent to the development of more intensive Livestock production systems in the subhumid zone.
Brucellosis
Brucellosis in cattle is caused by the bacterium Brucella abortus which affects the reproductive tract. It causes abortion in the second half of pregnancy due to inflamation of the cotyledons and placenta. Infection is commonly spread through affected foetal membranes and uterine exudates, but can also he transmitted by an infected bull at service. Diagnosis may he made by isolation of the organism from an aborted foetus or foetal membranes, or serologically from the dam.
Insufficient is known about the incidence of brucellosis, though the disease appears more widespread among settled cattle than in mobile herds and it is more common in some areas than in others. Brucellosis may he more common in the subhumid zone than in the drier areas to the north, and the incidence of the disease may increase substantially, along with abortion rates, if large numbers of animals are introduced into the zone.
Brucellosis is also a zoonosis: people are affected by the disease when they consume milk from infected cows. However, most milk in the subhumid zone is soured before it is consumed and souring kills Brucella (Eze, 1977), so the human population is generally not at great risk.
Dermatophilosis
Dermatophilosis, formerly known as streptothricosis, is one of the most important cattle diseases in West Africa from an economic standpoint, although it has been the subject of relatively little research. It is caused by the organism Dermatophilus congolense and can also affect sheep and horses, though in West Africa it usually only affects cattle. The organism can be isolated from the skin of cattle showing no clinical signs: it requires some triggering mechanism to become pathogenic and start growing in the dermis. This mechanism might be a scratch or insect trite penetrating to the dermis or continuous or repeated wetting which softens the epidermis. This causes a chronic local dermatitis with proliferation of the epidermis and a serous exudate which gives rise to small raised patches of skin and matted hair in 'paint brush' fashion. Lesions commonly occur first in the perineal region or on the udder, on the back near the hump, on the rump or on the hind legs. Lesions may increase in size, producing hard plaques of debris and matted hair with painful inflamed areas underneath. They may gradually extend over the body and in serious cases cover most of the skin, interfering with its function and sometimes leading to death. The symptoms do not normally progress to this extent in Zebu cattle, hut chronic infections can greatly impair the condition of an animal, causing emaciation, lowered productivity and a greatly devalued hide. In addition to economic losses in breeding herds, there is evidence that the disease interferes with marketing efficiency.
A conference on dermatophilosis was held in Ibadan, Nigeria in 1973, which emphasized the limited contribution of past research. Research has tended to concentrate on the possible role of ticks in transmitting Dermatophilus congolense or triggering a change in the organism from skin commensal to pathogen by causing local host tissue damage (Zlotnik, 1955; Plowright, 1956). The possible role of high environmental humidity has also been investigated (McAdam, 1961).
Although associated in the literature with the wetter tropical regions, dermatophilosis occurs more frequently in Nigeria in the drier northern zone. It is seldom seen among the trypanotolerant West African Shorthorn cattle of the derived and lowland rainforest, and it has been suggested that these breeds have a level of resistance to the disease. In the northern areas, the disease appears more frequently during the rainy season, though there have been insufficient studies of its epidemiology in the field under traditional husbandry conditions. Some individual animals seem to be resistant, while others in the same herd are highly susceptible, though occasional inspections are inadequate to reveal why this should be so. Spontaneous cures occur in many cases, following the cessation of the rains? but it is not known what proportion represents permanent recoveries.
Exotic stock are particularly susceptible to dermatophilosis, which can lead to severe debilitation and often death. First generation crosses with exotic stock are also more susceptible than local animals. An outbreak at the University of Ife farm in Nigeria, for example, during the rainy season of 1978, caused heavy losses among the Friesian/Holstein, Brown Swiss and Jersey cattle, while the White Fulani were hardly affected.
Tick-Borne Diseases
Tick-borne diseases, such as anaplasmosis, babesiosis and heartwater, are assuming greater importance in West Africa, especially in areas where exotic cattle have been imported in substantial numbers and where management systems are becoming more intensive (FAO, 1977c).
Anaplasmosis or gall sickness, caused by the protozoa Anaplasma marginale in cattle and A. ovis in sheep and goats, causes a low-grade fever with anaemia, loss of appetite, eventual emaciation and sometimes death. Babesiosis, also called piroplasmosis, redwater or tick fever, is caused by Babesia bovis and B. bigemina in cattle and B. ovis in sheep. It is a febrile disease characterized by anaemia and jaundice, and abortion in pregnant animals. Mortality is not high, but full recovery from debility and anaemia takes a long time. Cowdria (Rickettsia ruminantium) affects cattle, causing fever, nervous symptoms and excess pericardial fluid which gives rise to its common name of 'heartwater'. Mortality occurs fairly frequently, particularly among exotic cattle.
Although exotic animals are not likely to be imported into the region on a large scale for some time, sedentarization and more intensive production systems based on indigenous animals can be expected to lead to increased incidence of tick-borne diseases, as well as mechanical damage to teats and perineal regions caused by ticks. More research is needed within the region on the biology, ecology, host relationships, population dynamics and disease relationships of the most important tick species: Boophilus annulatus, B. decoloratus, B. geigyi, Amblyoma variegatum and Hyaloma spp.
An undefined Theileria problem has also been recognized in the region (Mussman, 1978), probably caused by pathogenic Theileria mutans, so that the characterization of cattle Theileria in West Africa now requires urgent study (FAO, 1977c).
Parasitic Gastroenteritis
Infestation by intestinal worms causes anaemia, trace mineral deficiency, irritation of the gut, toxin production, impairment of protein digestion and poor utilization of nutrients. This leads to progressive weight loss and even death, particularly in young animals. Infestation is more serious during the rainy season. The most common genera of nematodes causing this condition are Trichostrongylus, Ostertagia and Haemonchus.
The extent of losses suffered as a result of worm infestation is often underestimated. These are particularly serious in areas where anthelmintic drugs are used irregularly or not at all and where cattle converge at watering points.
Other Viral Diseases
Foot and mouth disease occurs throughout the subhumid zone, but it is not of major importance in extensive production systems. Whenever more intensive production is introduced, however, the detection and control of foot and mouth disease will become more important.
Other potentially dangerous viral diseases known to exist in West Africa include Rift Valley fever (Ferguson, 1959; Kemp et al., 1973). Highly virulent outbreaks of this disease occurred in Sudan and Egypt in 1976 and 1977. A slow wave of lumpy skin disease has also affected animals in West Africa in recent years, originating in East Africa and progressing through Somalia, Ethiopia and Sudan.
Reproductive Disorders
Low reproductive performance is a widespread problem throughout West Africa, although satisfactory reproductive performances are occasionally recorded, for instance among trypanotolerant Muturu herds (Ferguson, 1967b). The relatively late age at first calving and long calving intervals of the Zebu breeds lead to low calving rates. In some herds, abortion rates are also high. Reproductive performance appears to be lowered by disease and nutritional deficiencies, as shown by Pullan on the Jos Plateau (1978). Pockets of high brucellosis incidence have been identified, and leptospirosis, trichomoniasis and vibriosis also lower reproductive performance, though their extent is not known. Mineral deficiencies, in particular an inadequate intake of phosphorus, are also likely to play a role in poor reproduction, as does sub-acute or intermittent trypanosomiasis. These factors all need closer study.
One of the most serious diseases affecting sheep and goats is the rinderpest-like disease observed in many areas of West and Central Africa, referred to variously as peste de petits ruminants (PPR) or stomatitis pneumoenteritis complex (SPC). This disease has been responsible for heavy losses, particularly among young animals and in the wetter areas. In Nigeria, it is characterized by ulcerative stomatitis, pneumonia and enteritis, resulting in severe diarrhoea.
Some research workers originally hypothesized that PPR was caused by a rinderpest virus adapted to small ruminants, or perhaps that a caprinized rinderpest vaccine, widely used in West Africa before the introduction of tissue culture vaccines, might have escaped and established itself in small ruminant populations. Ali (1973) reported that a concentration of cattle, sheep and goats around the Rahad River in Sudan during the dry season led to a rinderpest outbreak among all three species. However, the situation has been clarified by studies performed on PPR virus isolates by Gilbert and Monnier (1962), Bourdin and Laurent-Vautier (1967), Laurent (1968), Hamdy et al. (1975, 1976) and Gibbs et al. (1979). This work has clearly indicated that the PPR virus is a paramyxo-virus, a fourth and new member of the genus Morbillivirus, though closely related to the rinderpest virus. Small ruminants inoculated with tissue culture rinderpest vaccine have been shown to develop an immunity to PPR.
Sheep and goats are also severely affected by parasitic gastroenteritis. Fabiye (1973) studied seasonal fluctuations of nematode infestations in goats in the Zaria area of Nigeria over a year and found increased infestation during the wet season. High counts of Haemonchus and Strongyloides were recorded at the beginning of the rains, while high counts of Gaigeria, Oesophagostomum and Trichostrongylus were recorded later in the season.
Trypanosomiasis affects sheep and goats, though the dwarf breeds of the coastal heft and the southern parts of the subhumid zone enjoy a degree of trypanotolerance. Isoun and Anosa (1974) examined the effect of experimental T. vivax infection on the reproductive organs of sheep and goats and presented data which indicate that trypanosomiasis has a serious effect on reproductive performance.
Both sheep and goats are also affected by bacterial pneumonia and other respiratory diseases. Goats suffer from contagious caprine pleuropneumonia, while sheep in particular are affected by foot-rot especially during the rains, sheep pox and contagious pustular stomatitis.
