Connectivity and the spread of infectious diseases in wildlife

Background/Question/Methods

To connect or to isolate wildlife populations suffering from infectious disease is a common management conundrum.  We compare two disease-host systems: 1) Hendra virus in fruit bats, an acute, fast-moving fatal zoonotic disease that sporadically spills over from Australian fruit bats into horses and humans; and 2) pneumonia in bighorn sheep, an acute to chronic disease that is a major threat to bighorn sheep recovery across the west.  We use a spatially structured metapopulation model to examine the impact of declining migratory behavior in fruit bats, and use the model to make inferences about the impact of migration on a spectrum of host-pathogen systems.  To understand the impact of connectivity on bighorn sheep pneumonia dynamics, we analyze a 15 year data set on pneumonia incidence in 14 interconnected populations of bighorn sheep, and use a model selection procedure to look at how factors such as acquired immunity and movement patterns regulate dynamics.

Results/Conclusions

We predict that decreased migratory behavior of urbanized fruit bats leads to declining population immunity and more intense outbreaks within bat populations after viral reintroduction.  Decreasing connectivity is a potential contributory mechanism explaining the pattern of Hendra virus emergence in Australia.  Bighorn sheep pneumonia is an excellent contrast to Hendra virus, with slow moving pathogen dynamics and a spectrum of acute to chronic manifestation of disease.  We show preliminary results from our analysis, and predict that movement is frequent relative to recovery rate and that only drastic reductions in connectivity could prevent disease spread.  We conclude that fast-moving acute immunizing pathogens are very sensitive to degrees of connectivity, and counter-intuitively, connectivity can prevent explosive epidemics by maintaining population immunity.  In contrast, slow-moving chronic diseases such as pneumonia in bighorn sheep are spread and maintained even when connectivity is limited.  We show that the impact of wildlife connectivity on infectious diseases can have important public health and conservation implications.