Abstract: Dirofilaria immitis (D. immitis), a filarioid nematode that causes dog heartworm infection, is transmitted by the bite of an infected mosquito. The number of heartworm cases in dogs and cats continues to increase nationally and this increase has been observed in both areas historically considered to be endemic for dog heartworm, as well as in areas of low endemicity like in Orange County, California. Increased disease cases can be attributed to the increase in mosquito populations and with the introduction of the vector competent Aedes mosquito species in Orange County, the purpose of this study was to test the prevalence of D. immitis positive mosquitoes in Ae. aegypti, Ae. albopictus, and Ae. notoscriptus as well as the native Culiseta incidens. Various traps were used to collect and test mosquitoes for 2020-2022 and D. immitis was detected in all four mosquito species. Positive dog heartworm infection rates in mosquito samples tested increased every year at 0.55% (2020), 1.28% (2021), and 1.75% (2022). All infected samples were collected starting in late June through November. Real-time PCR probes were used to detect the mitochondria cytochrome I (CO1) gene and D. immitis positive mosquito samples were confirmed by DNA sequencing. Our results reveal the presence of D. immitis in wild mosquitoes in Orange County, and based on the geographical distribution of the positives, the results demonstrate the need to continue to survey for this pathogen throughout the county as well as inform and educate veterinarians, animal care agencies, and the affected communities.
