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 as sess the prevalence of an exotic pathogen, heartworm, in an insular endemic carnivore, the island fox. Because he 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 relativ e, the mainland gray fox. We also determined the age structure of island fo xes; because heartworm is a progressive disease that may ultimately be fata l, we suspected that heartworm infection might contribute to a shift to a r elatively young age structure, reflecting increased mortality of older indi viduals. We found that 78% of the island foxes tested positive for heartwor m antigen, in contrast to a 5% prevalence of antigen-positive results in gr ay foxes on mainland California. Few mature or old foxes existed in the isl and fox population compared with two decades ago. Our results expose heartw orm as a possible threat to island foxes, and underscore the vulnerability of insular endemic species to exotic parasites. (C) 2001 Elsevier Science L td. All rights reserved.