Prevalence of canine heartworm infection in Queensland, Australia: comparison of diagnostic methods and investigation of factors associated with reduction in antigen detection

Abstract

Background The prevalence of Dirofilaria immitis infection in dogs is increasing globally and spreading into new areas. Prevalence of dirofilariosis in the state of Queensland, Australia, was as high as 90% before the introduction of macrocyclic lactones. Limited research on prevalence of D. immitis infection in dogs in Queensland has been reported in the last 30 years. Antigen testing is the most common method for detection of dirofilariosis but its accuracy is reduced by antigen getting trapped (blocked antigen) in immune complexes (ICs). The objectives of this research were to determine the prevalence of D. immitis infection in dogs from two geographical areas (Brisbane and Townsville) in Queensland, to determine the extent to which blocked antigen affects the validity of antigen testing, and to explore whether this was associated with microfilaraemia, location, age or sex. Methods Blood samples from Brisbane (sub-tropical climate) and Townsville (tropical climate) shelter dogs were evaluated for the presence of D. immitis antigen before (conventional antigen testing, CAT) and after dissociation of ICs by heat treatment (antigen testing after heat treatment, ATHT), using a commercially available test. Microfilariae were detected using modified Knott’s test (MKT). Test proportions were compared with McNemar’s test and the association between antigen test-discordant results (positive for antigen after dissociation of ICs) and microfilaraemia, location, sex and age was modelled using logistic regression. Results Dirofilaria immitis prevalence in dogs from Townsville (22% by CAT, 32.1% by ATHT and 16.7% by MKT) was significantly higher than in dogs from Brisbane (1.1% by CAT and MKT and 1.7% by ATHT) $$(P< 0.001)$$ ( P < 0.001 ) . Dissociation of ICs allowed detection of significantly more D. immitis infected dogs than either conventional antigen testing or microfilariae detection, or the combined antigen and microfilariae detection $$(P< 0.001)$$ ( P < 0.001 ) . The odds of dogs being positive for antigen after dissociation of ICs were significantly higher for microfilaraemic, 3–4-year-old female dogs from Townsville. Conclusions The high prevalence of infection with D. immitis in dogs from Townsville poses a health risk for local susceptible host species, including humans. Dissociation of ICs increases antigen detection and should be considered in dogs suspected of D. immitis infection but negative on routine testing. Graphical Abstract

Human pulmonary dirofilariasis: an unexpected differential diagnosis for a solitary lung lesion

“…In Australia, the routine use of macrocyclic lactone prophylaxis for dogs was introduced in the 1990s 8 . Despite an overall reduction of heartworm disease in Australia, a recent study of D. immitis in Queensland dog populations revealed disproportionately high rates of canine infection in north Queensland compared with southeast Queensland 8 . This geographical difference is multifactorial, reflecting north Queensland's tropical climate, vector prevalence, wild dog populations, and possibly reduced dog owner education and compliance with preventive treatments 8 .…”

“…Despite an overall reduction of heartworm disease in Australia, a recent study of D. immitis in Queensland dog populations revealed disproportionately high rates of canine infection in north Queensland compared with southeast Queensland 8 . This geographical difference is multifactorial, reflecting north Queensland's tropical climate, vector prevalence, wild dog populations, and possibly reduced dog owner education and compliance with preventive treatments 8 . This highlights the ongoing challenges of D. immitis infection control in tropical regions of Australia and the possible implications for humans as dead‐end hosts.…”

“…The incidence of D. immitis is increasing in dog populations around the world, attributed in part due to climate change, prevalence of vector mosquitos, and affordability and accessibility of chemoprophylaxis 2,4,7‐8 . In Australia, the routine use of macrocyclic lactone prophylaxis for dogs was introduced in the 1990s 8 .…”

“…The incidence of D. immitis is increasing in dog populations around the world, attributed in part due to climate change, prevalence of vector mosquitos, and affordability and accessibility of chemoprophylaxis 2,4,7‐8 . In Australia, the routine use of macrocyclic lactone prophylaxis for dogs was introduced in the 1990s 8 . Despite an overall reduction of heartworm disease in Australia, a recent study of D. immitis in Queensland dog populations revealed disproportionately high rates of canine infection in north Queensland compared with southeast Queensland 8 .…”

Ocular Dirofilariasis in Migrant from Sri Lanka, Australia

“…In Australia, estimated prevalence of the D. immitis nematode in the canine population is 20% in some areas of the east coast, home to many species and genera of mosquitoes (e.g., Aedes , Anopheles , and Culex ) ( 4 , 5 , 6 ). Of the 21 human dirofilariasis cases reported from mainland Australia in the past 40 years, most are linked to D. immitis nematode infection, and others are linked to D. repens nematode infection in returned travelers ( 1 , 4 ).…”