Pig-hunting dogs are an at-risk population for canine heartworm (Dirofilaria immitis) infection in eastern Australia

BackgroundCanine heartworm disease, caused by Dirofilaria immitis, has global veterinary importance. In Australia, the prevalence of canine heartworm infection decreased markedly following the introduction of over-the-counter macrocyclic lactones. We aimed to estimate the prevalence of canine heartworm infection in at-risk populations of dogs in eastern Australia and analyse published prevalence data from Australia.MethodsIn total, 566 dogs from eastern Australia were tested for the presence of D. immitis antigen. Four cohorts were studied: pig-hunting dogs from Queensland (Cohort 1, n=104), dogs from remote New South Wales (NSW) (Cohort 2, n=332), urban pets from rural NSW (Cohort 3, n=45) and ex-racing Greyhounds from Sydney, NSW (Cohort 4, n=85). Serum samples were screened for D. immitis antigen using a reference laboratory microwell-based assay (DiroChek((R))) or a point-of-care immunochromatography test kit (Anigen Rapid((R))). Risk factors associated with the odds of D. immitis antigen seropositivity were identified using binary logistic regression models. Seropositive blood samples were tested for the presence and quantity of D. immitis DNA using a species specific real-time (q)PCR assay. A metanalysis of the Australian canine heartworm literature was conducted.ResultsThe prevalence of dirofilariasis in pig-hunting dogs from Queensland (Cohort 1) was 12.5% (95% CI: 6.5-18.9%), with a subpopulation of dogs from Central Queensland having a prevalence of 21% (95% CI: 12.3-33.4%). Age was significantly associated with D. immitis antigen seropositivity (increased risk with increased age). The odds of being >5 years versus <= 5 years was 3.7-times (95% CI: 1.1-12.5) greater in antigen positive versus antigen negative dogs. No D. immitis antigen positive dogs were detected in dogs from NSW (Cohorts 2-4). The Australian canine heartworm disease literature includes 98 peer-reviewed publications (1901-2019) with 30 studies reporting on D. immitis prevalence in dogs. Throughout the publication peak period (1980s), the primary antemortem diagnostic test was detection of microfilariae.ConclusionsCanine heartworm infection in dogs used for pig hunting is a previously unexplored topic in Australia. Pig-hunting dogs are infected with canine heartworm in Queensland, Australia, placing pet dogs and cats at increased risk of infection.