Heartworm (Dirofilaria immitis) in carnivores kept in zoos in Texas, USA: risk perception, practices, and antigen detection

Background: Dirofilaria immitis is the causative agent of heartworm disease in wild and domestic canids, felids, and mustelids. Recent studies demonstrate that additional families in the order Carnivora are also susceptible to infection. Therefore, the objectives of this study were to (1) better understand current practices surrounding heartworm prevention and diagnostics in zoological facilities located in the state of Texas, USA, and (2) assess archival serum samples of carnivores kept in these facilities for the presence D. immitis antigen and/or antibody.

Methods: A questionnaire was completed by veterinarians or veterinary technicians representing 10 zoological facilities across Texas. This questionnaire was designed at the taxonomic family level, encompassing the 12 terrestrial carnivore families Ailuridae, Canidae, Eupleridae, Felidae, Herpestidae, Hyaenidae, Mephitidae, Mustelidae, Prionodontidae, Procyonidae, Ursidae, and Viverridae. The second objective was achieved with the use of archival serum samples made available by six zoo facilities.

Results: Risk perception varied across facilities for every family, including among species belonging to Canidae. All facilities used monthly heartworm prevention in canids and felids, with more variation existing in the other families. The use of diagnostic testing and type and route of administration of preventive varied by facility, with oral ivermectin the most commonly used preventive. A total of 217 archival serum samples, belonging to 211 individual animals encompassing 11 families and 39 species, were tested with a commercial heartworm antigen ELISA test, pre- and post-immune-complex dissociation. A subset of samples was also assessed for the presence of feline anti-heartworm antibodies using a commercial ELISA test. Two animals, both of which were Asian small-clawed otters from the same facility, had antigen detected (0.95%).

Conclusions: This study demonstrates that while the zoo veterinary community is aware of the risk and health impact of heartworm disease in canids and felids, there is still a great deal of uncertainty regarding the risks and ideal strategies for prevention in other carnivore families. The low proportion of antigen detection may serve as a baseline for future prevalence studies across the southern United States, where there is an emerging concern of macrocyclic lactone resistance in heartworm.