Assessing the significance of endemic disease in conservation – koalas, chlamydia, and koala retrovirus as a case study
McCallum, H, Kerlin, DH, Ellis, W & Carrick, F 2017, Conservation Letters, e12425.
The value of disease management as a tool for biological conservation is often contested. The koala presents an ideal case study for examining this debate. Endemic diseases, most notably chlamydiosis and koala retrovirus (KoRV), are highly prevalent throughout koala populations. The extent to which disease contributes to population decline, however, is not agreed upon.
Although koala populations have experienced declines in recent decades and endemic diseases are widespread throughout its range, disease is not necessarily a key driver of decline. Disease almost certainly affects koala population dynamics; however, this effect may not be direct. In addition to disease, koalas are threatened by drought, habitat destruction, and other anthropogenic factors such as vehicle strikes and dog attacks. It is likely that these threats interact synergistically to cause population declines, thus to tease out the individual contribution of disease to population decline is not a straightforward process. For instance, the physical stress associated with a threatening event is thought to lead to reduced tolerance of chlamydial infection, which can, in turn, cause reduced fecundity or even life-threatening symptoms. In this regard, disease is correlated with, but not necessarily the primary cause, of mortality. There is considerable pressure upon decision makers to intervene in the decline of koala populations, and one of the most popular suggestions for such an intervention is a Chlamydia vaccination program. While significant developments in this field have been made, the implementation of a vaccination program is not yet viable as trialled vaccines have not yet offered protection against a live challenge. Of additional concern is the impracticality of coordinating multiple capture events of koalas for administering the vaccine and the resources involved in undertaking a large-scale program. A more important consideration than cost, however, is cost-effectiveness. If a Chlamydia vaccination program was, in fact, successful in reducing the impacts of disease and consequently halting population decline, then the high cost is justified by its high effectiveness. Conservation scientists do not necessarily agree, however, that disease levels in koala populations will improve population viability. In this instance, a vaccination program would not be cost-effective. It is more likely that an effective conservation intervention for declining koala populations will target multiple drivers of decline simultaneously, including Chlamydia and KoRV.
The lesson of this case study is not that disease management will not play an important role in koala conservation, but rather that the threat of endemic disease should be considered in the context of its interactions with other threats. Furthermore, the cost-effectiveness of a management intervention for population decline that targets one of several synergistic threats should be carefully evaluated before resources are committed to the initiative. The authors recommend long-term, longitudinal studies of koala populations to monitor population size, structure, disease burden and survival rates. Such studies are necessary to inform models with which the role of infectious disease in population decline can be determined, and suitable management strategies developed. Only at this stage can we demonstrate the feasibility of a large-scale vaccination program of a demonstrably effective vaccine.
Summarised by Joanna Horsfall
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