Canine heartworm (Dirofilaria immitis) in Fresno and Madera Counties, CA: prevalence differences between foothill and valley floor habitats

The purpose of this study was to evaluate presence of heartworm antigen in domestic dogs in Madera and, for the first time, Fresno Counties and test for the effects of habitat and other environmental variables on prevalence. Dogs were screened for heartworm via PetChek� ELISA from blood samples (N= 519) collected at seven sites during April-July 2009. Eighteen dogs were heartworm antigen positive. Pearson Chi-square analyses were run on the presence of heartworm versus the following variables: elevation range, percentage of time spent outdoors during the day, percentage of time spent outdoors during the night, pet coat length, weight class, prevention status, and sex. Dogs that spent at least 50% of their time outdoors during the day were significantly more likely to have heartworm that those who spent less time outside (N = 519, df = 1, p = 0.031). Overall prevalence (3.47%) was lower than expected with Madera County having 3.8% positive samples and Fresno County 3.5%; this prevalence is lower than many previous studies. The effect of time spent outdoors on heartworm prevalence was similar to previous studies. The effect of elevation, though not significant, requires further investigation, as does the prevalence of larval stages in mosquitoes.