Perfectly, predictable storms - factors that enhance the vulnerability of endangered species to spill-over infections

How many pathogens should a host species or a subpopulation maintain? Somewhat paradoxically it is the absence of infectious disease in a population that is a marker of poor population health. Once a host population falls to below a minimum population size or density capable of maintaining endemic infection flock immunity fades away and the remaining individuals become susceptible to disease transmission. Reservoirs of pathogens, in the environment or closely related hosts increase the vulnerability to spill-over infection. As an endangered population dwindles in size there is an increased vulnerability to stochastic extinction events and it has been shown that the proportion of species threatened by disease increases relative to their (IUCN) red list status. A broader assessment of all the potential health threats in the environment needs to be considered for managing critically endangered species. The Orange-bellied Parrot (Neophema chrysogaster) was free from beak and feather disease virus (BFDV) infection between 2000 and 2006 but in 2007 more than half of the birds in the Tasmanian captive flock had laboratory evidence of infection. Despite routine diagnostic surveillance BFDV infection continues to hinder the overall effectiveness of the captive recovery program. BFDV is the dominant pathogen of wild Psittaciformes in Australasia where it has been present for at least 10 million years. The richness of psittacine avifauna in this region has produced a mixture of potential hosts for the pathogen resulting in competing forces of virus co-evolution, spill-over infection and virus host-switches within parrots, cockatoos and lorikeets. Recent evidence has shown that all threatened and endangered Australian psittacine bird species can be infected by BFDV genotypes from any other close or distantly related host reservoir species. The potential threat to endangered psittacine bird populations especially captive breeding flocks is reviewed.