Longitudinal Study to Investigate the Role of Impala (<i>Aepyceros melampus</i>) in Foot-and-Mouth Disease Maintenance in the Kruger National Park, South Africa

A longitudinal study was performed in the Kruger National Park, South Africa to investigate the role of impala (Aepyceros melampus) in maintaining SAT serotypes of foot-and-mouth disease (FMD) virus. Three sampling sites with different histories of FMD outbreaks in impala and also of varying ecology were chosen. At three monthly intervals approximately 40 impala were bled and examined for clinical FMD at each of these sites for a period of 6 years, followed by 4 years of less frequent sampling. During the 10 years of the study, clinical disease was only observed once at a single sampling site, while at two of the three locations, serological evidence of infection was detected; in one locality this was a frequent occurrence. The discrepancy between clinically evident disease and serological evidence of infection indicated that sub-clinical infection with these viruses may be more regular than previously suspected. Furthermore, there was evidence that either SAT-serotype infection is maintained within local impala populations for prolonged periods or that re-infection of impala by buffalo occurs repetitively, sometimes at frequent intervals. A mixed-effects logistic regression model showed that females and older animals had a higher risk of seropositivity, while summer and autumn also represent periods when there is a heightened risk of seropositivity (as opposed to winter and spring which previous studies had shown to be associated with clinical disease). Comparison of impala and buffalo ratios in the three sampling regions indicated that the higher the impala density, the more likely disease transmission is from buffalo to impala, and that this is independent of buffalo numbers (presumably above an undetermined threshold). This study confirmed the potential role of impala for propagating FMD in southern Africa and this factor should therefore be considered when designing control strategies where wildlife and domestic animals interact.