Disease ecologists must consider health of individual animals and populations in research and conservation plans

NEW YORK (October 30, 2009)—Whereas healthy animals are able to fend off diseases and infections, individuals in poor condition become susceptible to a “vicious circle” in which animals in poor health are more prone to becoming infected, triggering a negative loop where they become weaker in the process, according to recent work by health professionals from the Wildlife Conservation Society and the University of Liverpool.

In the review appearing in the most recent edition of Trends in Ecology and Evolution, the authors focus on the largely unexamined role of underlying animal fitness as a cause for susceptibility rather than an effect, as is most often the case in disease ecology studies.

The study authors include Dr. Pablo Beldomenico of the Wildlife Conservation Society and Michael Begon of the University of Liverpool.

 “The susceptibility to infectious and parasitic disease of both individual animals and whole populations might be highly variable,” said Dr. Beldomenico, a WCS epidemiologist and the lead author of the study. “Animals in poor condition are more prone to infection, leading to even poorer condition and becoming a source for the proliferation of infectious pathogens, with the potential of spreading disease to other individuals in a population.”

What many disease studies lack, the authors point out, is an acknowledgement of the range of host susceptibility that influences the rate and severity of infection in both individual animals and populations. This also might result in cause and effect being misinterpreted. For example, in studies focusing on wild field voles, individuals with cowpox virus infections were less likely to survive in comparison to uninfected voles, but voles in poor condition to begin with were more likely to become infected. The common assumption is that infected voles become less fit as a result of the virus, but it is also possible that the voles that become infected are the ones in poorer condition, and therefore they would have poorer survival even in the absence of cowpox.

Many factors, such as acquired immunity to pathogens, nutrient availability, competing physiological demands, and other factors determine the variability of host susceptibility in individuals and populations. In a world undergoing a rapid global change as a result of human activity (pollution, habitat destruction, introduction of invasive species, climate change, etc.), environmental stressors may become important drivers of increased host susceptibility to pathogens. If such stressors cause a large proportion of a wildlife population to be vulnerable, native parasites and diseases that would otherwise be tolerated could become a health threat for wildlife. The “vicious circle” might become a vicious spiral, down towards population extinction, according to the study.

According to Beldomenico, this new perspective brings forward two concepts that are very important to wildlife conservation. On the one hand, the fate of a wild animal population depends on the dynamics of health in that population. If a large proportion of individuals in a population are in poor health, vicious circles would be triggered and consequently the population would decline.

On the other hand, while disease is increasingly acknowledged as a factor contributing to biodiversity loss, emerging and spreading health threats are not the only conservation concern; rather, it is the complex interaction of a number of factors which determines the health dynamics of a particular population.

“Understanding the interactions between dynamics of health, dynamics of infection, and animal population dynamics can play a big role in effective conservation plans for many wildlife species,” added Beldomenico. “Effective species conservation requires healthy populations.”