Mortality rates
Morbidity and disease prevalence
Viral and bacterial diseases
Parasitic infestations
Protozoal diseases
Ectoparasites
Other animal health problems
Disease is the most important constraint on sheep and goat production in the humid tropics of West and Central Africa. However, information on the incidence of disease under traditional production systems is very limited, and little is known about the interaction of disease with other production factors such as husbandry and nutrition levels. Apart from a few village-level studies, largely carried out in the francophone countries, information on disease incidence is largely related to isolated episodes on institutional farms or research stations. Meat inspection records have not generally proven to be useful sources of information because very few small ruminants from the humid zone are slaughtered in abattoirs subject to sanitary control. Veterinary coverage, except in areas near universities and research centres, is also insufficient for monitoring disease situations and trends. More information should become available in future, however, as the role of small ruminants in the agricultural economy of the region is beginning to be stressed and veterinary services and diagnostic facilities are becoming more widespread.
Figures on mortality due to disease must be treated with caution because actual field data are so limited. Deaths occur irregularly in different places and at different times, while most estimates are based on observations over short periods in very limited areas. In three states of southern Nigeria, however, annual mortality rates were calculated from 1965 to 1968 as percentages of the total flocks in the region at the beginning of each six-month period. These figures are given in Table 11.
Table 11. Annual mortality rates among small ruminants in southern Nigeria (%)
|
|
Approximate Population |
Mortality Rates (%) |
||||
|
1965 |
1966 |
1967 |
1968 |
|||
|
Western State |
|
|
|
|
|
|
|
|
Goats |
1500 |
16.3 |
19.2 |
43.6 |
13.6 |
|
|
Sheep |
1500 |
22.2 |
25.6 |
10.3 |
16.9 |
|
Midwestern State |
|
|
|
|
|
|
|
|
Goats |
1500 |
29.4 |
36.2 |
19.4 |
27.0 |
|
|
Sheep |
1500 |
34.2 |
21.4 |
17.4 |
22.9 |
|
Eastern State |
|
|
|
|
|
|
|
|
Goats |
1500 |
15.7 |
20.2 |
- |
- |
|
|
Sheep |
1500 |
15.1 |
25.1 |
- |
- |
Source: Nigeria, Federal Office of Statistics (1972).
This table indicates that there were considerable differences in mortality rates among the three states surveyed and wide fluctuations between years.
Matthewman (1977) recorded annual mortality rates of 10 and 14% in two villages near Ibadan in southwestern Nigeria with a total adult sheep and goat population of just over 200; he estimated preweaning mortality at 15%. In Ghana, the annual mortality rate among sheep and goats throughout the country was estimated at 6.3% (Cockcroft, 1977), but no figures were given for specific climatic zones. Likewise, national figures for Ivory Coast indicate an annual mortality rate between 5 and 7% for adult sheep and up to 21% for recently assembled flocks on private farms, Bonniwell (1978) reported annual mortality rates of 35.6% for village sheep and 7 and 9% for sheep on two commercial farms in Ghana, while deaths among Nungua Blackhead lambs were 84.5 and 89.9% on two farms at the University of Ghana during the period 1971 to 1973 (University of Ghana, 1975).
There is very little information on mortality which suggests the relative importance of various causes of deaths. Vallerand and Brankaert (1975) recorded annual mortality rates among West African Dwarf sheep in Cameroon averaging 32.2% from 1966 to 1970 and 24.1% from 1971 to 1974, and attributed the difference between these two periods to the implementation of a disease control programme in the area. Among the young, they found mortality rates of 44% during the first period and 33% during the second for twins, and 29% during the first period and 21% during the second for single births. Bonniwell (1978) found a peak in mortality in Ghana during the second half of the rainy season, and it is generally considered that mortality rates are higher during the rains (Vohradsky and Sada, 1973). This hypothesis should be further tested, because, if proven correct, it would have important implications for the implementation of animal health measures.
It is possible that morbidity, due to chronic and sub-clinical diseases such as low-grade helminthiasis and pneumonia, has a greater effect on production levels than mortality, but information on morbidity under traditional production systems is virtually non-existent. Visibly sick animals are often slaughtered for home consumption, and such cases may be recorded as deaths, inflating mortality figures and masking the level of morbidity.
Based on numerous studies of sheep in Cameroon, Vallerand and Branckaert (1975) listed the most important disease problems as foot disease, respiratory diseases, enterotoxaemia, toxaemia of pregnancy, parasitic gastroenteritis, including strongylosis and monieziasis, external parasitism and contagious pustular dermatitis. They mentioned that blood parasites were unimportant, although the insect vectors were present in the area: only one case of trypanosomiasis was diagnosed in nine years.
Rombaut and van Vlaenderen (1976), working in Ivory Coast, mentioned the importance of parasitism, presumably helminthiasis, and felt it would be impossible to eradicate this problem in humid tropical areas. Under good husbandry conditions, however, they found that the dwarf Djallonké sheep developed a natural immunity. Two cases of heartwater were diagnosed, but no cases of disease carried by insects, though the vectors were found in the area.
Oppong (1974) referred to the 'pneumonia-diarrhoea complex' as an important cause of losses in Ghana, especially during the rainy season. He also mentioned the occurrence of helminthiasis, tick infestation, heartwater, contagious pustular dermatitis and, under improved feeding regimes, enterotoxaemia. Bonniwell (1978) reported a high incidence of pneumonia and diarrhoea, also in Ghana, together accounting for approximately 50% of all clinical cases recorded. 'Ataxia collapse syndrome', probably toxicosis, was also noted on a commercial sheep farm. Post-mortem analyses at the Kumasi Veterinary Laboratory indicated that haemonchosis, heartwater and peste des petits ruminants were the most important causes of death,' together accounting for about 60% of all mortalities. Heartwater was also found, mostly in sheep imported from Upper Volta.
Peste des petits ruminants
Goat and Sheep Pox
Foot and Mouth Disease
Bluetongue
Contagious Pustular Dermatitis
Other Viral Diseases
Mycoplasma and Pasteurella
Other Bacterial Diseases
Peste des Petits Ruminants (PPR) is a paramyxo-virus, closely related to, but nevertheless distinct from, other morbilliviruses such as measles, rinderpest and canine distemper. It is referred to as kata in Nigeria, or as pseudorinderpest. PPR is often rendered more severe by secondary bacterial pneumonia and concomitant helminthiasis, which has led to the introduction of the descriptive term 'stomatitis-pneumoenteritis syndrome' in Nigeria.
Although PPR has probably been present in West African sheep and goat populations for some time, it was first identified as a distinct disease by francophone workers in the early 1940s in Benin and Senegal and described by Cathou in Benin and Gargadennec and Lalanne (1942) in Ivory Coast. It was called peste des petits ruminants because of its similarity to rinderpest (peste bovine).
Interest in PPR was largely confined to the francophone countries until the 1960s. Zwart and Rowe (1966) found neutralizing antibodies to the rinderpest virus in sera from goats and sheep in northern Nigeria and interpreted this to suggest the presence of PPR in Nigeria. Whitney et al. (1967) described a disease known as kata in southwestern Nigeria with clinical signs similar to those of PPR, although no cross-protection with rinderpest was demonstrated. Later, Rowland and Bourdin (1970) and Rowland et al. (1971) demonstrated cross-protection with rinderpest, and also found that PPR and kata were clinically and pathologically indistinguishable. Isoun and Mann (1972) described a stomatitis and pneumoenteritis complex among sheep in Nigeria which appeared to be PPR, though no virus isolation was achieved. Nduaka and Ihemalandu (1973) described a stomatitis-pneumoenteritis syndrome in eastern Nigeria, which was later shown by Hamdy et al. (1976) to be associated with the PPR virus, and also suggested that abu-nini, a contagious disease of goats and sheep in Sudan involving pleuropneumonia-like organisms (Olte, 1960), might have a similar aetiology.
The clinical signs and post-mortem lesions in small ruminants suffering from PPR resemble those produced by rinderpest in cattle, which led originally to the assumption by some observers that PPR was caused by a form of rinderpest virus adapted to small ruminants. It was also speculated 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. In testing this hypothesis, Mornet et al. (1956) and subsequently other workers failed to produce clinical rinderpest in cattle innoculated with material from PPR cases or with PPR virus, though rinderpest virus antibodies were evoked.
The situation has been clarified more recently by studies performed on PER 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 PER virus is a paramyxo-virus, a fourth and new member of the genus Morbilivirus,
Epidemiology. PPR infection has a four- to five-day incubation period, followed by pyrexia lasting six to eight days. Symptoms include oral necrosis, catarrh, nasal discharge and diarrhoea. In goats experimentally infected with PPR virus, Gibbs et al. (1979) found that nasal excretion of the virus occurred three days after infection and excretion in saliva, urine and faeces at five days. Pyrexia commenced at three days, but no clinical signs of illness were observed until four days. In the same Series of experiments, nasal excretion also occurred at five days. This work would suggest that the disease is spread primarily through the respiratory system, a conclusion previously reached by clinicians observing outbreaks in the field.
Many animals die within a week of the onset of pyrexia, and the terminal stages of illness are frequently complicated by secondary pneumonia. Survivors may develop labial dermatitis, with a scab formation similar to that seen in contagious pustular dermatitis. Young animals are more severely affected than adults, and the limited experimental evidence available supports the general field observation that goats are more severely affected than sheep.
Diagnosis is based on clinical signs, although confusion with bacterial pneumonia complicated by helminthiasis may occur. Differentiation from this syndrome and from contagious pustular dermatitis and pox can be achieved by demonstration through gel diffusion tests of serum antibodies and PPR virus antigens in organs and tissues. The virus may also be isolated from mucosal scrapings of the large intestine.
As in the case of rinderpest, it is generally agreed that there are no carriers of PER, but sub-clinical or unobserved cases may maintain infection and spread it to others. There is no evidence to suggest that either PPR or rinderpest viruses persist in the tissues of recovered individuals, as is the case with measles (see Matumoto, 1969). Although outbreaks are said to be common in the humid zone, and often lead to the loss of entire village flocks, it is not known whether PER is actually endemic within the zone or whether it is introduced in each instance through animals imported for sale or slaughter from the drier regions to the north. In such cases, introduced animals may be held for a few days or weeks before slaughter and contact with local animals is possible. It is an established fact that heavy disease losses occur, largely attributable to PPR, when several groups of sheep or goats from various areas are housed together.
No cases of PPR among wild ruminants have been recorded in West Africa, though Hamdy and Dardiri (1976) found that the American white-tailed deer, Odocoileus virginanus, responds to experimental PPR infection in a similar manner to goats. Nevertheless, indigenous wild ruminants are not likely to play an important role in the dissemination of the disease since they are very few, compared to local sheep and goat populations, and their contact with domestic animals is minimal.
No general survey of the distribution of PPR in West Africa has been undertaken, though it is clear that this is a long-established, common and frequently lethal disease affecting sheep and goats in all climatic zones of the region. In recent years, localized serological investigations of small ruminant populations have shown that a substantial proportion possess antibodies indicating previous exposure to infection. However, rinderpest antigen was apparently used for these serum neutralization tests, and therefore they failed to distinguish between animals which had actually recovered from a PPR infection and those which had merely received tissue culture rinderpest vaccine (TCRV). Subsequently, the development of a serum neutralization test (Taylor, 1979) which distinguishes PPR from rinderpest antibodies has allowed Taylor and Abe Gunde (1979) to demonstrate that between 30 and 40% of small ruminants tested react to the PPR virus, although they possess no rinderpest antibodies unless they have previously been vaccinated with TCRV.
Although the PPR and rinderpest viruses have been shown to be distinct, PPR antibodies confer some degree of protection against rinderpest in cattle. Infection with rinderpest virus will produce a disease indistinguishable from PPR in sheep and goats, though PPR virus produces no clinical symptoms of disease in cattle. For this reason, suspected outbreaks of PPR in areas with substantial cattle populations should be precisely identified, and the use of TCRV to prevent PPR among sheep and goats should be strictly supervised.
Treatment and Control. The treatment of clinical cases of PPR consists mainly of combatting secondary bacterial pneumonia with drugs, but this is unlikely to be effective unless applied at the early stages of the disease. Catho reported that treatment with rinderpest antiserum was useless, but there have been no reports on the value of treatment with PPR antiserum for short-term protection. Treatment of individual animals is also expensive and difficult to carry out on a wide scale.
TCRV has been shown to offer effective protection against experimental PPR virus infections, and field trials with this vaccine, especially in francophone West Africa, are said to have given satisfactory results (Mornet et al., 1956; Bourdin et al., 1970; Bourdin, 1973; Bourdin and Doutre, 1976; Taylor, 1979). A full appraisal of the effectiveness of TCRV is urgently needed and could be carried out at the village level by clearly establishing the presence of a PPR challenge and evaluating the antibody status of vaccinated and control animals before and during the trials. Such trials, which might usefully be conducted in several countries, would firmly establish the effectiveness of TCRV, its dosage rate and the required frequency of application.
If such trials were to indicate that TCRV is not effective against PPR, efforts might be directed towards developing an attenuated vaccine from a PPR virus isolate, though this would be an expensive approach. Ihemelandu and Nduaka (personal communication) have reported that PPR hyperimmune serum was used successfully to prevent, or possibly to control, outbreaks in what they termed 'crisis situations', as, for example, when a station or livestock project is restocked with animals from several areas.
Serological evidence of a second paramyxo-virus, para-influenza 3, has been reported from work with cattle, sheep and goats by Taylor et al. (1975) in northern Nigeria and by the Institut d'Elevage et de Médecine Vétérinaire des Pays Tropicaux (IEMVT) in Chad. No indications were given, however, of the importance of this virus for livestock production.
Goat and sheep pox outbreaks are sporadic, but generally severe when they occur. Three foci of goat pox were reported in Chad by IEMVT, and severe outbreaks of goat and sheep pox occurred in the Ashanti Region of Ghana with the virus identified from specimens by Bonniwell (1977). In this case, it was concluded that the infection had been introduced through animals brought down from the north. IEMVT is exploring methods to produce goat and sheep pox tissue culture vaccines in the testicular cells of newborn lambs.
Foot and mouth disease occurs only sporadically throughout the humid zone, usually introduced through animals brought in from the drier regions to the north. This viral disease takes a mild course among indigenous sheep and goats, and vaccination is not warranted at present except for particularly valuable flocks.
Bluetongue is a viral disease affecting sheep, transmitted by night-flying biting midges, Culicoides spp. Indigenous sheep rarely show any clinical signs, but exotic breeds suffer fever, mouth lesions and lameness which can lead to death. Serological surveys have shown evidence of the disease among goats and sheep in Nigeria, and nine serotypes have been identified by Taylor and McCausland (1976).
Contagious pustular dermatitis is widespread throughout the humid zone, but only occasionally causes serious losses. Diagnosis has been based on clinical grounds, and recently Obi and Gibbs (1978) confirmed the presence of the virus in Nigeria. Venereal balanoposthitis and vulvitis have been described, but whether these are separate diseases or a manifestation of contagious pustular dermatitis has yet to be deter mined.
Pulmonary adenomatosis, or chronic viral pneumonia, occurs among sheep and goats. These viruses have been identified on the basis of serological evidence in Chad. Gibbs et al. (1977) isolated adenoviruses from Nigerian goats naturally infected with PPR, but were unable to type the isolates by using antisera to known bovine or ovine serotypes. They concluded that the adenoviruses were unlikely to play a significant role in the epidemiology of PPR. Similar symptoms are also caused by herpes viruses: Taylor et al. (1977a, 1977b) demonstrated infectious bovine rhinotracheitis antibodies and antibodies to bovine viral diarrhoea in goats and sheep. They showed that unclassified RNA viruses produced a mild disease in experimentally infected animals, but concluded that there was little difficulty in distinguishing this disease from PPR.
Occasional cases of rabies are reported in goats and sheep, probably contracted from rabid dogs. This disease is of no economic significance, but extremely important in terms of public health.
Mycoplasma and Pasteurella are the most common bacteria affecting sheep and goats in the humid zone. Eight species of Mycoplasma have been identified which cause disease among small ruminants: five are associated with pneumonia and the others with various conditions, including mastitis, arthritis and kerato-conjunctivitis. Mycoplasmosis can produce pneumonia, both as a primary infection, particularly under adverse conditions such as overcrowding or during the long rains, and as a secondary infection to viral diseases such as PPR. The mycoplasmoses of small ruminants have been reviewed by Perrau (1976) and Ojo (1977).
Contagious caprine pleuropneumonia (CCPP) does not appear to occur in its classical form among small ruminants in the humid zone, possibly because animals in this area are generally kept in small household flocks, limiting the spread of the disease, rather than in the large flocks typical of the regions further north. Nevertheless, Ojo (1973, 1976a, 1976b) has isolated both Mycoplasma mycoides subsp. capri, which causes CBPP, and M. mycoides subsp. mycoides, which causes contagious bovine pleuropneumonia, from goats in the humid zone. This same researcher has isolated M. argini from goats with arthritis and pneumonia and M. agalactiae from lungs (1976b), although no cases of mastitis have been reported from the humid zone. Vaccination against mycoplasmoses is not practised in the region, and the value of chemotherapeutic treatment has received little attention.
Pasteurella are commonly isolated from cases of pneumonia, most often P. haemplytica and P. multocida. These bacteria cause severe pneumonia, often as a secondary infection to PPR or in situations of stress, such as exhaustion following transit ('shipping fever'), chilling during the rainy season or malnutrition. Endemic pasteurellosis, analogous to haemorrhagic septicaemia in cattle, does not seem to occur among sheep and goats in the humid zone. Ojo (1977) has characterized 200 strains of Pasteurella of caprine origin and identified six serotypes.
Other damaging bacterial infections, such as blackleg, tetanus, footrot and streptothricosis, occur, but there is no clear evidence of their extent or economic importance. Enterotoxaemia (pulpy kidney), tetanus and blackleg are most often reported from institutional flocks, with diagnosis in many cases based on case histories, clinical signs and post-mortem examinations. For intensive fattening or finishing operations, polyvalent vaccine is recommended to prevent these diseases.
Classical footrot has been reported and the causal organism. Fusiformis nodosus, identified, but outbreaks appear to be confined to institutional flocks. Enteropathogenic serotypes of Escherichia coli have been isolated from diarrhoeic infant goats and sheep in institutional flocks in southwestern Nigeria, but their importance as a cause of perinatal losses in village flocks has not been investigated. Streptothricosis has been identified in both goats and sheep, but it causes only minor lesions on the face and ears and seldom results in widespread damage. Anthrax is seldom reported among small ruminants in the humid zone, although animals are vaccinated against this disease in some francophone West African countries. Heartwater, a tickborne infection of Cowdria ruminatum, is of some significance among small ruminants in the humid zone, particularly among imported goat breeds.
Corynebacteria, Streptococci, Staphylococci and other pyogens are frequently reported in association with pneumonia, foot abscesses or ectoparasite damage, though these infections are incidental in themselves and not a serious cause of loss. Other bacterial infections, such as tuberculosis and nocardiosis, are occasionally reported, but there have been no reports of vibriosis, listeriosis or abortions caused by Chlamydia infections.
Diseases of public health significance include brucellosis and salmonellosis. Serological surveys indicate that a low level of brucellosis infection exists among small ruminants throughout the humid zone (see Gidel et al., 1974; Falade et al., 1974), but no systematic efforts have been made to identify the Brucella biotypes involved, nor to assess to what extent brucellosis contributes to reproductive wastage or constitutes a hazard to human health. Various Salmonella serotypes have been identified throughout the humid zone, but their importance has not been assessed.
Many research workers rank helminthiasis, especially in the form of parasitic gastroenteritis, along with PPR and pneumonia as a major constraint on increased sheep and goat production in the humid zone. However, these observations are often based on experience with institutional flocks, which are kept in relatively large numbers on enclosed pastures, often with little rotation. By contrast, under traditional husbandry systems small ruminants are generally kept in small household flocks of less than 10 animals of varying ages, foraging extensively or, in some localities, tethered each day on fresh land. Under these conditions, faeces are deposited over a wide area and the chances of susceptible animals encountering and ingesting infective third-stage worm larvae are considerably less than is the case in institutional flocks.
There is a wide fluctuation in larval challenge throughout the year, with larva survival and development greatly reduced during the dry season. During this period, animals are not exposed to larval antigens and thus suffer a loss of immunity developed from previous exposure to parasitic worms. This will result in the maturation of inhibited larvae in the gut mucosa if histotropic larvae are present, and also in greater susceptibility to new infestations picked up from the herbage when the rains begin. This syndrome has been observed in northern Nigeria (van Veen, 1978; van Geldorp and van Veen, 1976), but may not occur in the humid zone where the dry season is much shorter. As already mentioned, young animals are more susceptible to helminth infestation than adults and animals which first begin grazing during the rainy season, when the larval challenge is greatest, may be more prone to acute helminthiasis than those weaned during the dry season. In the temperate zone, lambs born in the early spring begin grazing just when larval challenge is most severe so that parasitic gastroenteritis becomes a serious problem, but under traditional husbandry systems in the humid zone mating is not controlled and parturition occurs year round.
Parasitic gastroenteritis may also differ in importance among sheep and goats, though no valid comparisons between the two species have been made. Although the same helminth parasites have been found in both species in the humid zone, it would seem logical that sheep would suffer heavier infestations because they graze succulent grasses which provide a favourable environment for the development and survival of worm larvae, while goats prefer to browse trees and shrubs.
Treatment of seriously affected animals is provided only in those areas where veterinary services and anthelmintic drugs are available, and this has little impact on the general situation. The effectiveness of treatment may also be reduced by the occurrence of drug resistance. In northern Nigeria, van Veen has accumulated convincing evidence that resistant strains of H. contortus emerge following regular dosage with the imadazole group of anthelmintics. Preventative measures, such as rotational grazing and a treatment regime aimed primarily at young animals, are practised in the temperate zone, but would be difficult to carry out in a situation where grazing is extensive and uncontrolled, where breeding is not seasonal and age groups are not clearly identified. Since the actual importance of parasitic infestations has not been ascertained under traditional management conditions, it is not clear whether the cost of widespread control and treatment measures would be justified in terms of increased production and economic returns.
The most important helminth parasites identified among small ruminants in the humid zone are the nematodes, including Haemonchus contortus, Trichostrongylus colubriformis, T. axei, Cooperia, pectinata, C. curticei, Ostertagia marshali, Gaigeria pachyscelis, Chabertia ovina, Skrjabirema ovis, Trichuris globulosus, T. ovis, Oesophagostomum columbianum, Strongyloides papillosus and Bunostomum spp. (see Okon and Enyenihi, 1975; van Veen et al., 1975). Of these, H. contortus is considered the most serious, particularly affecting young animals during the rainy season.
In northern Nigeria, clinical haemonchosis was diagnosed among young lambs in the latter part of the dry season and attributed to the development of histotropic larvae associated with high stocking rates (van Veen, 1978), though no similar observations have been recorded in the humid zone. Under experimental conditions, however, Okon and Enyenihi (1977) demonstrated that rainfall was the most important factor in the development and survival of infective H. contortus larvae. Eggs spread on experimental plots developed into infective third-stage larvae when the mean daily rainfall was 3 mm or more. These survived from 28 to 63 days, with survival reduced when the rainy season was interrupted by short intervals of dry weather. From an examination of 480 goats taken from eight states in Nigeria, Okon (1975) found that Trichuris spp. infestations were greater during the dry months and that T. globulosis occurred more often than T. ovis. Increased worm egg counts were found among sheep after parturition on the Accra plains in Ghana (University of Ghana, 1976) and in Zaria, northern Nigeria (van Geldorp and van Veen, 1976), though neither of these areas is in the humid zone and neither observation involved West African Dwarf sheep.
Cestodes found among sheep and goats in the humid zone include Moniezia expansa, M. benidini, Avitellina centripunctata and Stilesia globipunctata. Moniezia spp. are often reported among young animals, with M. expansa the more common species. There is some disagreement concerning their importance: some authorities consider them relatively harmless, while others claim that they cause serious problems, perhaps overlooking concurrent nematode infestations. No reports of hydatidosis (Echinococcus spp.) were encountered, though cysts of other tapeworm genera were sometimes noted in post-mortems. The epidemiology of these infestations and their importance in relation to small ruminant production have received little attention.
Trematodes identified among small ruminants in the humid zone include Fasciola gigantica, Schistosoma bovis and Paramphistomum spp. Liver fluke infestation (F. gigantica) occurs in both sheep and goats and is occasionally listed in abattoir records as a reason for condemning liver, but it is not as important a problem among small ruminants as among cattle. S. bovis has been reported in Senegal and Paramphistomum spp. in Ghana (Oppong, 1973).
Protozoa identified among sheep and goats in West Africa include Coccidia spp. Anaplasma ovis, Babesia ovis, Theileria ovis, Eperythrozoon ovis, Trypanosoma congolense, T. vivax, T. brucei and, indirectly, Toxoplasma. None Of these causes serious losses among indigenous West African Dwarf sheep and goats, though trypanosomiasis can be a major problem for animals brought down from the north or for exotic imported breeds.
Tsetse flies, which carry trypanosomiasis, are found throughout the forest and derived savanna of the humid zone. The distribution of the three tsetse groups, morsitans, fusca and palpalis is shown in a map prepared for OAU/STRC (1973). The indigenous West African Dwarf goats and sheep live under conditions of high tsetse challenge. They become infested with trypanosomes without showing serious clinical symptoms of the disease and are thus classified as 'trypanotolerant'. The pathology of the disease was reviewed by Losos and Ikede (1972), who also recorded the results of experimental T. brucei infection among sheep (Ikede and Losos, 1972, 1975). Ikede (1974a) described ocular lesions which were observed during the same series of experiments.
Although little is known about the mechanism of trypanotolerance, it appears that animals of the West African Dwarf breeds inherit a predisposition, but must develop tolerance to the disease through exposure. In East Africa, exotic breeds of sheep have been shown to suffer higher rates of natural infection, with more severe anaemia and greater weight loss, than indigenous breeds. This situation has been confirmed by experiments carried out under FAO (1976) on animals born and reared under tsetse challenge conditions.
It is possible that trypanosome infection among tolerant animals results in poor health which may lower their resistance to other diseases and parasites. Clinical signs may include anaemia, reduced growth and weight gain and poor fertility. T. congolense and T. vivax are haemoflagellates, primarily confined to the blood, which cause anaemia. However, T. brucei. also affects the intercellular body fluids, causing degenerative, necrotic and inflammatory changes. For example, experimental infections of sheep with T. brucei have been shown to cause testicular degeneration, implying impaired fertility. However, no systematic studies have been carried out among small ruminants under traditional management systems nor of the effects of low-grade trypanosomiasis on productivity.
Several species of coccidia have been identified by oocyst culture in Ghana, and coccidial oocysts are commonly mentioned in faecal worm egg counts throughout the humid zone. However, no symptoms occur under extensive traditional husbandry systems where the disease is overshadowed by helminthiasis. Occasional clinical cases are reported among young kids and lambs, especially when they are housed and managed intensively.
Serological evidence of the presence of toxoplasmosis has been found in southwestern Nigeria (Falade, 1974), but without evidence of any associated disease condition. Although evidence of the presence of other protozoal infections is available, such as babesiosis, anaplasmosis and theileriosis, information is limited to occasional reports of disease outbreaks. There is no systematic evidence of their prevalence or of the losses they may cause.
Ticks are widespread, with Amblyomma spp. the most common. They cause local lesions and damage from blood-sucking and transmit a number of diseases, such as heartwater. Dipeolu (1975) also recorded a case of tick paralysis in a sheep experimentally infested with Amblyomma larvae, though it is not known whether this occurs among the general population of small ruminants in the humid zone.
Mites cause sarcoptic and psoroptic mange, particularly among dwarf goats under traditional husbandry conditions. Louse infestation is commonly reported, and severe flea infestations occur occasionally. Van Veen and Mohammed (1975) identified Linognathus stenopsis and Ctenocephalides felis strongylus from louse and flea infestations of small ruminants in northern Nigeria. The sheep nasal fly (Oestrus ovis), which carries myiasis, is common throughout the zone. Ogunrinade (1977) reported nasal fly infestations among dwarf goats in southwestern Nigeria throughout the year, but with a peak during the wet season. Biting midges (Culicoides and Lasiohelia) are common throughout the zone at all seasons. They carry bluetongue and probably a number of other pathogenic viruses. Other biting flies, such as Stomoxys occur throughout the zone, but appear to cause little damage to small ruminants.
Uncontrolled grazing and crowded housing are factors contributing to the maintenance and spread of ectoparasite infestations. The treatment of affected animals is haphazard, and control measures, such as routine dipping, are not widely practised.
Metabolic diseases and mineral or vitamin deficiencies or imbalances, such as hypocalcaemia, hypomagnesaemia, pregnancy toxaemia and acidosis, are reported occasionally among institutional flocks, but diagnosis is generally based on clinical evidence only and not supported by biochemical findings. Bloat may also occur among animals grazing legume swards following enforced overnight fasting. None of these conditions appears to be of any significance under traditional management systems, but they may become more important with intensification, involving selected forages and increased growth rates.
Fungus infestations occur among small ruminants, though these have never been investigated systematically. Skin conditions caused by dermatophytes are reported occasionally, and systemic infestations are sometimes discovered in the course of post-mortems or meat inspections (Ikede, 1974b). Mycotic abortions do not appear to be important.
The presence of poisonous plants throughout the zone is reasonably well documented, but there are no indications of the extent of loss which results. This is likely to be minimal under traditional husbandry conditions with extensive grazing and a free choice of forage plants. Nwude and Parsons (1977) have compiled a list of plants found in Nigeria which are known to contain poisonous factors. Amagee (1977) has described and demonstrated experimentally a condition called tournis à Brysocarpus (Brysocarpus staggers) among sheep and goats who have eaten the young shoots of B. coccineus, a shrub described botanically by Brunel (1977) which is widespread in West Africa.
Losses from snakebite occur, and deaths from road accidents are an important factor in towns or villages situated near busy highways. Injuries may also be inflicted by local farmers when animals stray on growing crops.
