A serological survey of rinderpest antibody in wildlife and sheep and goats in northern Tanzania

Population regulation of African buffalo in the Mara–Serengeti ecosystem

Context The processes regulating ungulate populations have been the focus of numerous studies. For the African buffalo (Syncerus caffer Sparrman) population inhabiting the Mara–Serengeti ecosystem, rinderpest was the primary regulatory factor up to the mid-1960s. Following reduction of rinderpest and buffalo population increase, interspecific competition for food, notably with cattle and wildebeest (Connochaetes taurinus Burchell), was thought to be the primary regulatory factor in the ecosystem. Aims We analysed buffalo population trends and the relationship between buffalo population growth and rainfall and density dependence in the Mara–Serengeti ecosystem and discuss the findings in the context of the key ecosystem processes governing buffalo population dynamics in African savannas, namely, food limitation, competition, predation, disease and land use changes. Methods We analysed buffalo population dynamics in the Mara–Serengeti ecosystem in relation to rainfall and density dependence feedback between 1984 and 2010. Key results Buffalo population growth was both significantly density-dependent and positively correlated with the dry season rainfall after, but not before, a severe drought in 1993. Buffalo numbers crashed by 48.6% in 1984–85 and by 76.1% in 1993–94 during severe droughts when food availability was lowest and competition with the more numerous cattle and wildebeest was highest. Conclusions Recovery of buffalo numbers to pre-drought levels took 8–9 years after the 1984–85 drought but was much slower, with buffaloes numbering merely 36% of their 1993 population (12 895 animals) 18 years after the 1993–94 drought despite intermittent periods of high rainfall, probably due to demographic and/or reproductive factors, heightened competition with livestock, land use changes in the adjoining pastoral ranches, lion predation and recurrent severe droughts. Implications Our findings demonstrate how food limitation caused by droughts associated with the hemispheric El Niño–Southern Oscillation can cause severe declines in and threaten the persistence of large ungulate populations. The findings also portray how density-dependent food limitation, competition, predation, land use changes and other factors can accentuate the effect of droughts and greatly prolong population recovery.

A disease-mediated trophic cascade in the Serengeti and its implications for ecosystem C

The removal of rinderpest had cascading effects on herbivore populations, fire, tree density, and even ecosystem carbon in the Serengeti ecosystem of East Africa.

Tree cover is a fundamental structural characteristic and driver of ecosystem processes in terrestrial ecosystems, and trees are a major global carbon (C) sink. Fire and herbivores have been hypothesized to play dominant roles in regulating trees in African savannas, but the evidence for this is conflicting. Moving up a trophic scale, the factors that regulate fire occurrence and herbivores, such as disease and predation, are poorly understood for any given ecosystem. We used a Bayesian state-space model to show that the wildebeest population irruption that followed disease (rinderpest) eradication in the Serengeti ecosystem of East Africa led to a widespread reduction in the extent of fire and an ongoing recovery of the tree population. This supports the hypothesis that disease has played a key role in the regulation of this ecosystem. We then link our state-space model with theoretical and empirical results quantifying the effects of grazing and fire on soil carbon to predict that this cascade may have led to important shifts in the size of pools of C stored in soil and biomass. Our results suggest that the dynamics of herbivores and fire are tightly coupled at landscape scales, that fire exerts clear top-down effects on tree density, and that disease outbreaks in dominant herbivores can lead to complex trophic cascades in savanna ecosystems. We propose that the long-term status of the Serengeti and other intensely grazed savannas as sources or sinks for C may be fundamentally linked to the control of disease outbreaks and poaching.

Diseases are known to play important roles in regulating and structuring populations, but the consequences of disease outbreaks for entire communities and ecosystems are not as well understood. The Serengeti wildebeest were historically kept at low numbers by the rinderpest virus, but underwent a population explosion (irruption) after rinderpest was eradicated in the 1960s. We examined nearly a half-century of data to test the hypothesis that this irruption was responsible for a decline in the frequency of fires in this ecosystem (through increased grazing and a reduction in fuel loads), and that this in turn increased the density of trees. We found strong evidence for this indirect link between rinderpest and tree density, and less support for the role of other factors such as elephants and climate. We also investigated the consequences of this chain of events for ecosystem carbon, and suggest that the combined effects of increased grazing intensity by wildebeest, reduced fire, and increasing tree density may have shifted the Serengeti from being a net source to a net sink for carbon. This would imply that seemingly small ecological perturbations such as disease outbreaks have the potential to profoundly affect ecosystem function.

Re-infection of wildlife populations with rinderpest virus on the periphery of the Somali ecosystem in East Africa

We report surveillance for rinderpest virus in wildlife populations in three major ecosystems of East Africa: Great Rift Valley, Somali and Tsavo from 1994 to 2003. Three hundred and eighty wild animals were sampled for detection of rinderpest virus, antigen or genome and 1133 sampled for antibody in sera from Kenya, Uganda, Ethiopia and Tanzania from 20 species. This was done modifying for wildlife the internationally recommended standards for rinderpest investigation and diagnosis in livestock. The animals were selected according to susceptibility and preference given to gregarious species, and populations were selected according to abundance, availability and association with livestock. Rinderpest virus, antigen and/or genome were detected in Kenya; within Tsavo, Nairobi and Meru National Parks. Serological results from 864 animals (of which 65% were buffalo) from the region were selected as unequivocal; showing the temporal and spatial aspects of past epidemics. Recent infection has been only in or peripheral to the Somali ecosystem (in Kenya). Our evidence supports the hypothesis that wildlife is not important in the long-term maintenance of rinderpest and that wildlife are infected sporadically most likely from a cattle source, although this needs to be proven in the Somali ecosystem. Wildlife will continue to be a key to monitoring the remaining virus circulation in Africa.

Rinderpest seroprevalence in wildlife in Kenya and Tanzania, 1982-1993

Eight hundred and thirty five serum samples collected from eight wild artiodactyl species in Kenya and Tanzania between 1982 and 1993 were tested for virus-neutralising (VN) antibodies to rinderpest (RP) virus. Antibodies were found in 116 of 344 buffaloes (Syncerus caffer) but not in the other species including 349 wildebeest (Connochaetes taurinus). Most of the antibody positive buffaloes were from the Maasai Mara-Serengeti ecosystem (MM-SE) and would have had opportunity for exposure to the virus during the epidemic of rinderpest in buffalo confirmed there in 1982. Buffalo born after 1985 did not have antibody indicating that virus stopped circulating in this population at or around that time. This second demonstration that RP virus disappears from the MM-SE is further evidence that these species are not permanent reservoirs of this virus. Re-infection of wildlife is transient and they remain valuable sentinels for infection in nearby domestic livestock.

Rapid competitive enzyme-linked immunosorbent assay for detection of antibodies to peste des petits ruminants virus

Peste des petits ruminants (PPR) is a contagious viral disease of small ruminants that is of economic importance in Africa, the Middle East, and Asia. We developed a rapid competitive enzyme-linked immunosorbent assay (rapid c-ELISA) for the diagnosis and surveillance of PPR. This assay detects PPR virus (PPRV) antibodies in serum samples by quantifying the amount of monoclonal antibody (MAb) P-3H12 after 30 min of incubation of a serum-MAb conjugate mixture on plates coated with a PPRV recombinant nucleocapsid protein (rPPRV-N). We tested 249 PPRV-positive serum samples and 733 PPRV-negative serum samples from field ruminants. The threshold of percent inhibition (PI) was determined to be <50 on the basis of the mean PI plus 3 standard deviations for sera from PPRV-negative ruminants. The relative specificity and sensitivity of the rapid c-ELISA were 98.5% (722 of 733 serum samples) and 93.4% (234 of 249 serum samples), respectively. The rapid c-ELISA sensitively detected PPRV antibodies in hyperimmune sera (virus neutralization test [VNT] titer, >512), even at dilutions > or = 512 in normal goat serum, and as early as 6 to 13 days postinfection from 12 goats, each of which was infected with one of the four PPRV lineages. Hyperimmune sera from animals experimentally vaccinated with rinderpest virus gave positive results by the rapid c-ELISA when the rinderpest virus VNT titers were >512, although the rapid c-ELISA titers were very low (2 to 16). However, the rapid c-ELISA was negative when the rinderpest virus VNT titer was < or = 128. The rapid c-ELISA developed in the present work provides a short turnaround time and could be a useful tool for the diagnosis of PPR and screening for PPRV in the field.

Use of participatory epidemiology in studies of the persistence of lineage 2 rinderpest virus in East Africa

In 1994, rinderpest virus of African lineage 2 was detected in East Africa after an apparent absence of more than 30 years. In 1996, a disease search, based on participatory epidemiological techniques supplemented by serological and virological analyses, was undertaken in southern Somalia and north-eastern Kenya to collate past and current epidemiological information about rinderpest-compatible disease events, and to test the hypothesis that African lineage 2 rinderpest virus persists in populations of transhumant cattle in the Somali ethnic areas. The findings in Afmadu in Lower Juba led the search for rinderpest to the communities in the Bardera area and then on to the Kenya/Somalia border areas between Mandera and El Wak. The herders had a specific knowledge of the clinical signs of rinderpest and provided detailed and accurate descriptions of cases. They differentiated between classical acute rinderpest and a milder syndrome characterised by an ocular discharge and diarrhoea, few oral lesions, corneal opacity and occasional mortality. The studies provided evidence for the endemic occurrence of rinderpest back to at least 1981, with a periodicity of five years in the incidence of the disease. After a period of high mortality in 1992 to 1993, around Afmadu, herders reported a mild disease, with occasional increases in mortality, from other areas of Lower Juba and the Gedo Region. Reports by herders of a rinderpest-compatible disease in the El Wak area were pursued until active cases were located and rinderpest was confirmed.

Recombinant hemagglutinin protein of rinderpest virus expressed in insect cells induces humoral and cell mediated immune responses in cattle

Rinderpest virus causes a highly contagious and often fatal disease in domestic and wild ruminants. The surface glycoproteins, hemagglutinin (H) and fusion (F) proteins of this enveloped virus are known to confer protective immunity in cattle. We have reported the generation of a recombinant baculovirus expressing H protein and studied its protective properties in cattle. In this report, we demonstrate that the recombinant baculovirus encoded H protein expressed in insect cells gets incorporated into extracellular baculovirus. Single administration of low doses of purified recombinant extracellular virus with or without adjuvant induces virus neutralizing antibody responses and bovine leukocyte antigen (BoLA) class II restricted helper T cell responses in cattle.

Rinderpest antibody detected in sheep and goats before an outbreak of rinderpest reported in cattle in northern Tanzania

In January 1997, serum samples from 1346 adult sheep and goats were tested by a competitive ELISA to determine the prevalence of rinderpest in the northern zone of Tanzania. Seroconversion rates of 20%, 13%, 9%, 7% and 3% in sheep and goats were recorded in Ngorongoro, Monduli, Hai, Arumeru and Simanjiro districts, respectively. The low profile and insidious nature of the rinderpest virus involved caused very mild disease in cattle in some of these area. The mild signs associated with this outbreak of rinderpest resulted in difficulty in its diagnosis. In these circumstances, the presence of rinderpest antibody in sheep and goats served as a valuable and effective indicator of the rinderpest outbreak in cattle.

Expression in baculovirus vector system of the nucleocapsid protein gene of rinderpest virus

The rinderpest (RV) nucleocapsid (NP) gene segment was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV) adjacent to the polyhedrin promoter. The expression of NP protein in Sf9 cells was confirmed by indirect immunofluorescence and by Western blotting analysis with monoclonal antibodies. Recombinant RV-NP protein was purified by ultracentrifugation on a sucrose density gradient, and used as an antigen for an enzyme linked immunosorbent assay to detect anti RV-NP antibody. Both IgM and IgG antibodies against RV-NP were detected in the sera of rabbits infected with the L strain of RV. The pattern of development of IgG anti RV-NP antibody closely correlated with that of virus neutralizing antibody. In rabbits inoculated with recombinant vaccinia virus expressing RV-H gene (RRV-H), anti RV-NP was not detected. The results indicated that the baculovirus vector system can be used for the preparation of the diagnostic antigen of rinderpest as well as to distinguish between natural infection and vaccination with RRV-H.