Disease, predation and demography: assessing the impacts of bovine tuberculosis on African buffalo by monitoring at individual and population levels

Abstract

Summary 1.Understanding the effects of disease is critical to determining appropriate management responses, but estimating those effects in wildlife species is challenging. We used bovine tuberculosis (BTB) in the African buffalo Syncerus caffer population of Kruger National Park, South Africa, as a case study to highlight the issues associated with estimating chronic disease effects in a long-lived host. 2. We used known and radiocollared buffalo, aerial census data, and a natural gradient in pathogen prevalence to investigate if: (i) at the individual level, BTB infection reduces reproduction; (ii) BTB infection increases vulnerability to predation; and (iii) at the population level, increased BTB prevalence causes reduced population growth. 3. There was only a marginal reduction in calving success associated with BTB infection, as indexed by the probability of sighting a known adult female with or without a calf ( P = 0·065). 4. Since 1991, BTB prevalence increased from 27 to 45% in the southern region and from 4 to 28% in the central region of Kruger National Park. The prevalence in the northern regions was only 1·5% in 1998. Buffalo population growth rates, however, were neither statistically different among regions nor declining over time. 5. Lions Panthera leo did not appear to preferentially kill test-positive buffalo. The best (Akaike's Information Criterion corrected for small sample size) AIC c model with BTB as a covariate [exp( β ) = 0·49; 95% CI = (0·24-1·02)] suggested that the mortality hazard for positive individuals was no greater than for test-negative individuals. 6. Synthesis and applications . Test accuracy, time-varying disease status, and movement among populations are some of the issues that make the detection of chronic disease impacts challenging. For these reasons, the demographic impacts of bovine tuberculosis in the Kruger National Park remain undetectable despite 6 years of study on known individuals and 40 years of population counts. However, the rainfall and forage conditions during this study were relatively good and the impacts of many chronic diseases may be a non-linear function of environmental conditions such that they are only detectable in stressful periods.

Rift Valley Fever Virus Infection in African Buffalo (Syncerus caffer) Herds in Rural South Africa: Evidence of Interepidemic Transmission

“…Although analysis of crude mortality suggested that RVFV-seropositive animals were less likely to survive than seronegative animals, seropositive animals were also more likely to be older and female, and these characteristics were associated with differences in mortality risk. 17 After multivariable adjustment of mortality hazard for age, sex, and age-sex interaction with the use of Cox proportional hazards technique, mortality rates were not significantly different between RVFV-seronegative and RVFV-seropositive buffalo ( Figure 6 ). The multiply-adjusted hazard ratio for RVFV seropositivity was 1.09 (95% confidence interval = 0.57-2.08) for seronegative versus seropositive animals (χ 2 = 0.074, degrees of freedom = 1, P = 0.78, not significant).…”

“…Description of area habitat and the details of animal capture, clinical assessment, specimen sampling, and tracking have been presented in detail elsewhere. [16][17][18] ( Figure 1 ). Briefly, from a defined sub-region of Kruger National Park having a buffalo population of approximately 3,000, 593 buffalo were captured at 302 georeferenced locations by using aerial or ground darting techniques, working in close collaboration with veterinarians in Kruger National Park.…”

“…19 Age of animals ≥ 5 years of age at the time of first examination was determined by visual comparison of horn development and body size to a calibrated photographic reference collection. 17,20 Four age groups were considered: youngest = 1-3 years, mid-low = 3-5 years, mid-high = 5-8 years, highest = more than 8 years.…”

“…17,20 Four age groups were considered: youngest = 1-3 years, mid-low = 3-5 years, mid-high = 5-8 years, highest = more than 8 years.…”

Evaluating the protection of wildlife in parks: the case of African buffalo in Serengeti

“…Rinderpest virus originally caused major declines in buffalo numbers after 1890 but the virus has not caused declines since the 1960s (Dobson 1995;Dublin et al 1990a;Rossiter et al 1983;Sinclair et al 2008), and indeed it is now globally extinct (Normille 2008). Bovine tuberculosis (Myobacterium bovis), although prevalent in South Africa (Cross et al 2009), has not been found in Serengeti buffalo (Cleaveland et al 2008;Sinclair 1977). Drought can be a major controlling factor and drought induced mortality occurred in 1993 causing approximately 40% mortality in the buffalo population.…”