The low prevalence of D. immitis infections in cats in this region was unexpected, but supported by a very low rate of clinically reported feline heartworm cases (Adagra, personal observations). The prevalence of D. immitis infection in cats in a particular geographic location is typically much lower than the prevalence in dogs, but the differences are quite variable by region2–6. This variability is thought to depend on the variable willingness of local mosquito species to feed on cats versus dogs2–9. When the mosquito species that are considered to be heartworm vectors in the Townsville region are compared to the mosquito species that are identified to be natural hosts of D. immitis in Florida in the United States, only one is endemic to both regions; Culex quinquefasciatus22 –24. This species is thought to have a preference to feed on poultry rather than humans23 and it is quite possible that other feeding preferences exist. However, there are over 300 species of mosquito present in Australia23 .The role of different species of mosquito as vectors in HWD in Australia has not been adequately studied and is also liable to change over time.
Whilst climate change is likely to alter population densities of mosquitos worldwide, human manipulation of mosquito populations may also alter heartworm prevalences. Recently there was a planned release of large numbers of Wolbachia pipientis-infected Aedes aegypti mosquitos in Townsville25. This successfully established endemic Wolbachia infection into the local A. aegypti population. This led to a marked decrease in human cases of Dengue fever25. Any effect on heartworm prevalence from such interventions is unknown. Wolbachia is an important symbiote for D. immitis, which is usually transmitted vertically26. Whether Wolbachia presence within the mosquitos could aid transmission of D. immitis or possibly enable the worm to use atypical mosquito species as vectors is yet to be researched.
We have estimated the maximum antibody prevalence in cats in this region to be 2.1% and the maximum antigen and PCR prevalence rates to be 1.27% and 2.1%. The true rates could be lower than this. Not having a single positive sample raises questions about how low the true prevalence is, as well as the validity of the diagnostic tests being used. Ideally we would have continued testing cats until we obtained at least a single positive sample to address these concerns but were unable to do this due to financial constraints. The manufacturers of the commercial kits were able to provide data supporting their validity. We did attempt independently to validate the kits by contacting four feline specialist centres across eastern Australia to obtain blood from a D. immitis infected cat. None had seen a cat that had tested positive to heartworm infection in recent years which, in itself supports our low prevalence findings.
Maximum antigen prevalence in this study was calculated at 1.27%. Antigen testing for D. immitis in cats often produces false negative results leading to an artificially low maximum prevalence14. Whilst the specificity of commercial test kits is good, false negatives can be obtained from infections with juvenile worms(less than 5.6 months old), infections where only male worms are present, if antigen-antibody binding occurs and possibly after the administration of heartworm preventatives or doxycycline14.
The phenomenon of antibody-antigen binding has been studied in dogs14. Heat treatment of sera increases the sensitivity of antigen detection test kits and can resolve discordant results in dogs that are microfilaria positive- but antigen negative14. We opted not to perform heat treatment on our samples. Heat treatment has been reported to increase the number of false positive results in one study27 and was not shown to increase the number of positive feline heartworm antigen test results in a second study6. Theoretically increasing the sensitivity of our test from 94.1–99% would have decreased the calculated maximum true prevalence only from 1.27–1.21%. However if heat treating sera generated a single false positive, this would have significantly increased our calculated maximum true prevalence from 1.27–2.22%. The use of concurrent PCR testing was intended to help reveal more about any false-negative results and confirm the species involved, of course no relevant data were obtained.
Climate across the world is usually classified by the Kőppen-Geiger climate classification system which assigns climate types a code based on seasonal precipitation and temperature patterns28. The climate of the Townsville region is classified Aw (Tropical savanna climate with dry-winter characteristics)29. Most regions where heartworm prevalence in cats has been studied are classified Csa or Cfa (Mediterranean hot summer climate or humid subtropical climate)29. However, Florida (USA) is a location where a large zone of the Aw climate type can be found, as well as Cfa, Am (Tropical monsoon climate) and Af (Tropical rainforest climate). In a recent study of HWD prevalence in Florida, animals were recruited from three locations6. One of these was Miami-Dade county where Aw,Am and Af climate types are found. The prevalence of HWD was not significantly different in this county to the more northern locations studied where only Cfa climate is found.(Jessica Rodriguez, Personal communication). Miami-Dade county is much wetter than the Townsville region30,31 and we considered the possibility that sparse winter rainfall could be reducing the number of mosquito vectors and therefore the overall heartworm prevalence in this region. However, the prevalence of D. immitis in dogs on entry to a local rescue shelter was determined by conventional antigen testing to be 22.1% (Constantinoiu C. and Coleman, G., personal communication). This canine HWD rate in the Townsville region would suggest that localised climate conditions are not the cause for the lack of feline cases in this study.
Our sample population consisted primarily of pet cats. In Florida in 2019, 100 cats from animal shelters were tested for heartworm antibody, 17% were positive6. However in the same location and year, the results of 32,067 feline heartworm antibody tests were reported to the Companion Animal Parasite Council. The overall positive rate was 0.45% (1 in 225 cats)32. The heartworm prophylaxis status of the cats on which the 32,067 tests were performed is unknown, but presumably the data would be skewed towards cats that were considered to be on inadequate heartworm prophylaxis. This would suggest that maybe another factor such as poor nutrition or other parasite burden leads stray cats to be more susceptible to developing heartworm antibodies and presumably disease. Of course, we would have required a much larger sample size to detect a prevalence this low.
The current research has been limited by sample numbers and our lack of positive test results. This makes it difficult to be confident when calculating the prevalence of HWD. However, the prevalence of D. immitis infection in cats in this study is much lower than expected considering the prevalence in dogs from this region. The authors still support the routine use of heartworm prophylaxis in Australian cats. The consequences of infection in this species are severe, if not fatal, with occasional cases still being reported in the country.