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UGA researchers find that hunting can increase the severity of wildlife disease epidemics

July 17, 2006

A new study by University of Georgia researchers shows that the common practice of killing wild animals to control disease outbreaks can actually make matters worse in some cases.

In a study published the August 7 edition of the journal Proceedings of the Royal Society B: Biological Sciences, post-doctoral researcher Marc Choisy and Pejman Rohani, associate professor of ecology and UGA Biomedical and Health Sciences Institute researcher, create a detailed mathematical model that demonstrates how the combination of hunting and factors such as birth season and mating season influence disease outbreaks. Their results suggest that wildlife managers and health officials use caution when considering hunting or culling as a means to manage diseases as diverse as rabies, tuberculosis and even avian influenza.

"One consequence of hunting that we show in this paper is that it can increase the probability of dying from the disease," Choisy said. "It can give you results that are contrary to what you expect."

The reasoning behind killing wild animals to control disease outbreaks is simple: fewer animals should result in reduced transmission of disease. Hunting has been used to control badger populations in England, rabies in European foxes and chronic wasting disease in deer and elk populations in the American West. The researchers note that in each instance, disease outbreaks have worsened in response to the hunting.

One reason the policies failed, Choisy and Rohani said, is that they didn't take into account an ecological principle known as compensation. When a portion of the animal population is reduced, those that survive are left with more resources such as food and shelter. As a result of the newly plentiful resources, the death rate decreases and the birth rate increases, compensating - and sometimes overcompensating - for the loss.

Killing wild animals can also increase the proportion of the population that's susceptible to disease by removing those individuals who have contracted a virus but have developed lifelong immunity as a result of their infection.

The researchers found that compensation and lifelong immunity conferred by a virus interact so that the timing of mating and birth seasons determine whether hunting increases or decreases disease prevalence. They found that an outbreak is barely affected when hunting occurs between mating and birth season while an outbreak lessens when hunting occurs during the birth season. An outbreak increases dramatically when hunting occurs between the birth season and the next mating season.

The effect can be so dramatic that in some cases hunting can increase not only the proportion of infections and deaths, but also the absolute numbers. For example, their model shows that in the case of swine fever, a highly infectious disease threatening boars and pigs in Europe, hunting can increase the number of infected individuals by twenty five percent.

"If we want to preserve the hunted population, we should be careful about when we schedule the hunting season compared to birth season because if it's too early or late, it can drive the population to extinction," Rohani said. "If we want to control a disease in the host population, the timing of the hunting season can be chosen to be optimal for that."

University of Georgia