K.A. Majiyagbe, D. Shamaki, D. Kulu and T.K.C. Udeani
National Veterinary Research Institute
Vom near JOS, Plateau State, Nigeria
Materials and methods
Discussion and conclusion
An investigation was conducted in the small ruminant producing areas of the northern part of Nigeria and the Adamawa Province of the Cameroon (Garoua and its suburb) to assess the prevalence of peste des petite ruminants (PPR) and rinderpest (RP) antibodies in apparently normal and non-vaccinated sheep and goats. This study was aimed at providing a better understanding of the epidemiology of PPR in relation to a clinically similar disease, RP.
The virus-specific monoclonal antibody (Mate) based competitive ELISA (enzyme-linked immunosorbent assay) technique was used to analyse 702 serum samples from the surveyed area for PPR and RP.
The results show that overall, 46.7% (328/702) had PPR antibodies while 18.2% had RP antibodies. For Cameroon, (N = 320), 35% and 3.1% had PPR and RP antibodies respectively, while for Nigeria (N= 382), the corresponding values were 56.5 and 30.9%.
Comparison of the spectrum of antibody profiles showed that some animals reacted solely to either PPR or RP virus infection, while others simultaneously possessed antibodies for both viruses. This observation has serious implications for the on-going Pan-African Rinderpest Campaign (PARC) program to eradicate rinderpest from the African continent.
Présence d'anticorps contre la peste des petits ruminants et la peste bovine chez des petits ruminants du Cameroun et du Nigéria ne présentant pas de signes cliniques de maladie
Une enquête a été réalisée dans les zones d'élevage de petits ruminants de la région septentrionale du Nigéria et de la province de l'Adamaoua au Cameroun (Garoua et sa périphérie) afin d'évaluer la prévalence des anticorps contre la peste des petits ruminants et contre la peste bovine chez des ovins et des caprins bien portant en apparence et non vaccinés. Cette étude avait pour objectif d'aider à mieux comprendre l'épidémiologie de la peste des petits ruminants par rapport à une infection cliniquement semblable, la peste bovine.
La technique immuno-enzymatique basée sur les anticorps monoclonaux du virus a été utilisée pour analyser 702 échantillons de sérum prélevés dans la zone d'étude. Les résultats révèlent que, dans l'ensemble, 46,7% des sujets abritaient des anticorps contre la peste des petits ruminants et 18,2% des anticorps contre la peste bovine. Ces chiffres étaient respectivement de 35% et de 3,1% au Cameroun (n = 320), et de 56,5% et 30,9% au Nigéria (n = 382).
En comparant entre eux les spectres des profils des anticorps, on a noté que certains animaux réagissaient uniquement au virus de la peste des petits ruminants ou à celui de la peste bovine, alors que d'autres possédaient des anticorps contre chacun de ces deux virus. Cette observation a des conséquences importantes sur le programme de la Campagne panafricaine contre la peste bovine qui s'emploie à faire disparaître cette maladie du continent africain.
Livestock are very important for both the subsistence and economic development of the African continent. They provide a flow of essential food products throughout the year. In some countries they are a major source of government revenue and export earnings. They also sustain the employment and income of millions of people in rural areas, contribute draught energy and manure for crop production and are the only food and cash security available to many Africans (Brumby, 1990). In Nigeria, as in other African countries, small ruminants (sheep and goats) contribute a substantial proportion of the nation's meat supply.
Jahnke (1982) stated that 21.5% of the nearly 104 million sheep and 25.4% of the 125 million goats in tropical Africa are located in the humid and subhumid zone. Some 13.6% of the sheep and 16.2% of the goats are located in the humid zone, while 7.9% of the sheep and 9.2% of the goats are found in the subhumid zone. There are an estimated 22 million sheep and 34.5 million goats in Nigeria (FDLPCS, 1991). The indigenous breeds of sheep in order of importance are Yankassa (60%), West African Dwarf (WAD, 20%), Uda (10%) and Balami (10%). For the goat breeds, the order is Red Sokoto goat (50%), West Africa Dwarf (45%) and Sahel (5%) (Osinowo, 1992).
In sheep and goats, PPR is considered the most important single cause of morbidity and mortality. It is a highly contagious rinderpest-like viral disease that particularly affects goats, but also affects sheep. The disease is characterised by fever, oculonasal discharge, necrotic stomatitis, bronchopneumonia and gastro-enteritis. The disease is caused by the PPR virus, a member of the Morbillivirus genus which includes rinderpest, canine distemper and measles. PPR is endemic in the humid zone and to a lesser extent in the subhumid zone of the West African subregion. Its economic importance in these areas is enormous. It can quickly decimate whole flocks either by itself or in combination with other diseases, such as pneumonia or gastro-intestinal parasitism (Majiyagbe, 1985). In susceptible flocks, morbidity may be 100% and mortality greater than 90%, especially amongst animals under six months of age. This has caused severe economic hardship to many small ruminant producers. Clinically, PPR occurs as an acute disease in goats, while in sheep it is generally benign. Surviving animals usually develop a dual immunity to PPR and RP viruses.
In practice, tissue culture rinderpest virus (TCRV) vaccine is used in immunoprophylaxis against PPR in the absence of a homologous vaccine. Control of PPR is therefore paramount to efficient small ruminant production in the West African subregion.
Currently, the PARC (Pan African Rinderpest Campaign) to control and subsequently eradicate rinderpest from the African continent is being carried out in over 22 countries. The target animals for the campaign are cattle. Small ruminants, being animals of the Artiodactyla group, are also susceptible to rinderpest virus and are hence occasionally infected with bovine rinderpest. However, the PPR virus causes a non-clinical disease in bovines; infected cattle sero-convert and become immune to RP and PPR viruses. This will interfere with successful TCRV vaccination and has serious implications when cattle are sero-surveyed to determine the level of protection against rinderpest.
To provide a better understanding of the epidemiology of PPR and RP in small ruminants, an investigation was conducted in small ruminant producing areas of the northern part of Nigeria and the Adamawa Province of Cameroon (Garoua and its suburbs). Its purpose was to assess the prevalence of PPR and RP antibodies in apparently normal and non-vaccinated sheep and goats.
A total of 702 sheep and goat sera samples were collected from 352 survey regions in Nigeria and 320 survey regions in Garoua and its environs in the Adamawa Province of Cameroon. Serum was aseptically separated from the blood clot and kept chilled while being transported to the laboratory for analysis. Details of samples collected and species distribution are shown in Table 1.
Serum testing for PPR and RP antibodies
A competitive ELISA (C-Elisa) for the detection of antibodies to RP and PPR viruses was used for serum analysis. This is a monoclonal antibody (Mab) based test. Two types of Mab are used, one specific against the hemagglutinin (HA) protein of RP virus and the other specific against the H-protein of the PPR virus. Unlike the indirect ELISA and the virus neutralisation test (VNT), the RP C-Elisa detects only antibodies to RP virus and gives no cross-reactivity with antibodies to PPR virus.
Table 1. PPR and RP antibodies in clinically normal small ruminants from Nigeria and Cameroon (%).
Cameroon-samples from Boklé (IRZ, LANAVET), Lagdo, Ngong and Garoua Abbatoir.
Nigeria-samples from Bauchi State (Soro and Alkaleri) and Plateau State (Vom, Jos and Eviron).
Vaccinated control samples from If e and Shika Research Farms.
The Mab used in the PPR C-Elisa also gives a high level of specificity to the PPR virus. However, some anti-see to rinderpest do compete against this PPR specific Mab. The test was performed as outlined in the C-Elisa kit manual (Majiyagbe et al, 1991; Anderson et al, 1991). The ELISA test reagent kits were supplied in two parts, Buffers + Conjugate, from the joint FAO/IAEA (Food and Agriculture Organization of the United Nations/International Atomic Energy Agency) Agricultural Laboratory in Seibersdorf, Austria and RP and PPR antigens kits from the Pirbright Laboratory, Pirbright, Surrey, UK.
Table 1 shows that just under half of the samples had PPR antibodies, while one fifth had RP antibodies. The rest were negative to both viruses.
Because of the discriminating nature of the monoclonal antibodies used in the test, it was possible to distinguish between specific PPR and RP virus induced antibodies in these animals' serum samples. Comparison of the spectrum of antibody profiles showed that some animals reacted solely to either PPR or RP virus infection.
Serum samples collected from small ruminant producing areas in Nigeria and the Cameroon showed that some of the animals possessed antibodies to PPR and RP viruses. None of the animals were less than one and a half years of age and they had not been vaccinated with either TCRV or homologous PPR vaccine. The source of these antibodies can therefore only be from an unapparent infection or from immunity gained through recovery from a field outbreak. Animals that recover from field outbreaks posses a durable immunity, as an infected animal either dies or survives. Antibodies can therefore be found in the blood of surviving animals.
The West African subregion is considered an endemic zone for PPR. The disease is considered the single most important cause of morbidity and mortality in small ruminants, especially goats. The most effective form of protection against PPR is through vaccination. Since the causal agent of PPR is a morbillivirus related to rinderpest virus, the use of TCRV vaccine has been shown to be effective in offering protection against PPR; a single vaccination confers immunity for at least 12 months (Majiyagbe et al, 1991; Rao et al, 1984). Vaccinated animals are therefore able to withstand the virulent PPR virus in the field.
Field experience, however, shows that these animals are rarely vaccinated. Their survival therefore depends on being able to overcome a field outbreak of PPR or infection with the closely related RP virus. Observations from the last RP outbreak in Nigeria showed that very few goats and sheep exhibited evidence of infection (Majiyagbe, 1985). This was probably due to prior PPR exposure or infection with a mild RP virus strain. A small ruminant adapted strain of RP virus is know in India (Dardiri et al, 1976; Rao et al, 1984) and there is evidence that this strain circulates within cattle, sheep and goats. Experimental studies (Anderson et al, 1991) have shown that cattle may be infected by contact with the PPR virus, but will not exhibit any symptoms and will resist RP. Similarly Anderson et al (1991) have shown that goats with circulating antibodies to PPR virus will not respond to rinderpest vaccination.
In the absence of vaccination, the presence of PPR and RP virus antibodies indicates that PPR and RP are circulating in the population, even if only at low levels. This observation has serious implications for the on-going PARC seromonitoring program, particularly in the assessment of immunity levels to RP after vaccination. In a situation where the PPR virus infects cattle before RP vaccination, the animal would not respond to the vaccination and would be considered an animal at risk and test negative in the RP C-Elisa test used in evaluating sero-conversion. Our own results show that a similar situation occurs in small ruminants and this could influence the results of seromonitoring in the current PARC program.
The goal of the current PARC program is to eradicate RP from the African continent. For a country to be able to declare itself free from disease, a sufficiently high level of immunity (>95%) must be achieved in the national herd and vaccinations against RP must cease.
The contribution of PPR virus immunity to animals' resistance to RP must be properly quantified. Appropriate tests such as the C-Elisa must be used for a meaningful evaluation of survey results.
We would like to thank Dr Zyam and Dr Abdoulaye Nakano, the Director, LANAVET (Laboratoire National Vétérinaire), Boukle Garoua for their support and provision of laboratory facilities during sample collection. The technical assistance of Mr S Igbabab and Mr O Ochai is appreciated.
The permission of the Director, National Veterinary Research Institute, Vom, to publish this report is also acknowledged.
The work was supported by the FAO/IAEA/SIDA (Food and Agriculture Organization of the United Nations/International Atomic Energy Agency/Swedish International Development Agency) Research contract No. 6559/SD.
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