Spatial and temporal variation in the seroprevalence of canine heartworm antigen in the island fox.

Island foxes (Urocyon littoralis) are endemic to six of the eight California Channel Islands (USA). The island fox is classified as a threatened species by the State of California, and recently three of the six subspecies have experienced abrupt population declines. As part of a continuing effort to determine the cause of the declines, we tested island fox serum samples collected in 1988 (n = 176) and 1997-98 (n = 156) over the entire geographic range of the species for seroprevalence of canine heartworm (Dirofilaria immitis) antigen. Using a commercially available enzyme-linked immunosorbent assay (PetChek, Idexx Laboratories, Westbrook, Maine, USA) we detected heartworm antigen in four of the six populations of island foxes. On San Miguel and Santa Rosa Islands, seroprevalence in adult foxes was >85% (n = 62) in 1988 and increased to 100% (n = 24) in 1997-98. On Santa Cruz Island, seroprevalence in adult foxes decreased from 83% (n = 30) to 58% (n = 26), whereas on San Nicolas Island, seroprevalence increased from 25% (n = 32) to 77% (n = 30) during the same period. All of the pups assayed (n = 33) were seronegative. The seroprevalences of heartworm reported herein for the four populations of island foxes are the highest yet reported for a fox species. However, additional demographic data reported elsewhere suggests that heartworm has not been a major factor in the recent declines of island fox populations.

The use of microsatellite variation to infer population structure and demographic history in a natural model system.

To assess the reliability of genetic markers it is important to compare inferences that are based on them to a priori expectations. In this article we present an analysis of microsatellite variation within and among populations of island foxes (Urocyon littoralis) on California's Channel Islands. We first show that microsatellite variation at a moderate number of loci (19) can provide an essentially perfect description of the boundaries between populations and an accurate representation of their historical relationships. We also show that the pattern of variation across unlinked microsatellite loci can be used to test whether population size has been constant or increasing. Application of these approaches to the island fox system indicates that microsatellite variation may carry considerably more information about population history than is currently being used.