The importance of disease management programmes for wildlife conservation

Abstract

The unprecedented rate of disease emergence across the globe has raised concern among conservationists. Within the last decades there have been many examples of infectious disease-driven mortality of endangered populations, including the chytridiomycosis panzootic in amphibians and morbillivirus infections in marine mammals. These and other events have provided evidence that disease may pose a significant threat to biodiversity (reviewed in Smith, Acevedo-Whitehouse & Pedersen, 2009) and have highlighted the need for control programmes at different levels, from preventing a disease from being introduced or controlling an already-present disease, to completely eradicating the disease (Wobeser, 2002). Programmes aimed at lowering transmission by vaccination or by limiting contact with domestic animals might considerably reduce the risk of disease-related mortality for endangered populations (Pedersen et al., 2007). Lamentably, few such programmes have been fully implemented as part of conservation plans, partly because there is still limited data available to assess disease risks for many wildlife populations; and partly because the required interventions are costly and, in many cases, logistically impractical. As a result, it is difficult to assess the efficacy of disease management measures and determine whether it is necessary, feasible and affordable to develop and implement disease control strategies. In this context, the featured study by Lopez et al. (2009) provides a good example of the importance of a disease management strategy for a critically endangered species and highlights some points that might be relevant to other wildlife disease scenarios. The authors report the outcome of emergency measures to control a feline leukaemia virus (FeLV) epizootic among Iberian lynx. The disease management plan that was implemented to stop the FeLV outbreak from spreading used a multi-pronged approach which included serological surveys, isolation of infected individuals and vaccination of non-infected individuals, as well as selective culling/removal of feral and domestic cats. The authors managed to capture more than 80% of the lynx population and conducted sensitive diagnostic tests to determine the prevalence of the disease and identify suitable candidates for vaccination. This gargantuan effort was sustained for eight months and resulted in the vaccination of a relatively large number of lynxes across the species distribution. The combined measures were reported to be successful in confining the outbreak and preventing a potentially catastrophic outcome for the lynx population in south-western Spain. Lopez and colleagues highlight two issues that must be taken into account for conservation plans. First, it could be argued that as the disease drove the host population below the density threshold needed for persistence the outbreak would have been self-limiting and, thus, may not have needed any control strategy. However, transmission of FeLV is not dependent on population size; rather, it increases with frequency of contacts between infected and susceptible individuals (Fromont et al., 1997). The extremely low population size and small range of the Iberian lynx makes it possible for FeLV to propagate quickly during mating season and potentially kill the entire population. This is further complicated by the fact that, as occurs for most small and isolated populations, the Iberian lynx suffers from low genetic variability and inbreeding (Johnson et al., 2004), which can lead to lower immunocompetence, susceptibility to infection and higher disease severity (Spielman et al., 2004). Moreover, because the lynx appear to have been relatively unexposed to FeLV in the past, the population would expectedly have little, if any, acquired immunity against the infection. Therefore, the timely and concerted disease management strategy reported by Lopez and colleagues is likely to have been essential to contain the epidemic and avoid a catastrophe for this endangered species. The second issue that arises from the featured study relates to the financial cost and intense labour which were obviously needed to implement the disease control programme during the active epidemic. Lopez and colleagues’