Exotic disease and an insular endemic carnivore, the island fox

Diseases pose serious threats to rare species, and mammalian carnivores on islands may be especially susceptible. The objective of our study was to assess the prevalence of an exotic pathogen, heartworm, in an insular endemic carnivore, the island fox. Because the island fox is an insular endemic and presumably vulnerable to exotic parasites, we expected that the prevalence of heartworm would be higher in island foxes than in their closest relative, the mainland gray fox. We also determined the age structure of island foxes; because heartworm is a progressive disease that may ultimately be fatal, we suspected that heartworm infection might contribute to a shift to a relatively young age structure, reflecting increased mortality of older individuals. We found that 78% of the island foxes tested positive for heartworm antigen, in contrast to a 5% prevalence of antigen-positive results in gray foxes on mainland California. Few mature or old foxes existed in the island fox population compared with two decades ago. Our results expose heartworm as a possible threat to island foxes, and underscore the vulnerability of insular endemic species to exotic parasites.