Detection of circulating antigen of Dirofilaria immitis has been a mainstay of identifying heartworm infection in clinical practice for the past three decades. Several validated commercial antigen tests have very good sensitivity, specificity, and positive predictive values, especially when used in patients for which heartworm infection is likely. In some dogs and cats infected with heartworm, antigen may not be available for detection although present in the patient sample; heat pretreatment of these samples reveals the antigen, changing the false negative to positive. This phenomenon was documented in the literature in the 1980s but subsequently overlooked by the heartworm research community for many years. In this review, we provide a summary of the current understanding of the role of heat reversal in diagnosing heartworm infection. This additional diagnostic step is most important for patients in which heartworm infection is likely, such as dogs or cats in an endemic area with an inconsistent history of heartworm preventive use, or dogs with a prior diagnosis of heartworm infection that were recently treated. To illustrate the concept, we share a summary of results from canine samples tested at the state veterinary diagnostic laboratory in Oklahoma, USA in 2017 by modified Knott test and by commercial antigen test before and after heat treatment of samples; in this sample set, heat treatment changed all D. immitis microfilaria-positive but antigen-negative samples to antigen-positive. Pet dogs with a history of consistent preventive use are unlikely to become positive with heat pretreatment; for that reason, routine pretreatment of all samples tested in a veterinary practice is not recommended. We also review known causes of false negative and false positive results on heartworm antigen tests that, although uncommon, can complicate accurate diagnosis in individual patients. Together, this review provides a primer to aid understanding of strategies that can enhance accurate diagnosis of heartworm infection in veterinary practice and clinical research.

BackgroundSuccessful Cytauxzoon felis transmission studies have occurred using Amblyomma americanum adults acquisition-fed as nymphs on an experimentally infected domestic cat or Dermacentor variabilis adults fed as nymphs on a splenectomized bobcat. Here, we evaluated A. americanum and D. variabilis nymphs acquisition-fed as larvae on a C. felis-infected carrier domestic cat for competence to transmit the protozoan parasite as nymphs to naïve, healthy domestic cats.MethodsAmblyomma americanum and D. variabilis larvae were applied to a chronically infected, parasitemic C. felis donor cat (Felis catus) and allowed to feed to repletion. Engorged larvae were collected and held through ecdysis. Three cats were each infested with 66 A. americanum or 66 D. variabilis emerged nymphs. Cytauxzoon felis infections in principal cats were determined by clinical signs and detection of circulating parasite by blood smear and PCR evaluation.ResultsClinical signs of cytauxzoonosis were observed in cats infested with A. americanum nymphs beginning 12–15 days post-infestation (dpi). The same cats were PCR positive on 12–14 dpi; piroplasms were evident in blood smears at 16 dpi, and macrophage schizonts were observed in stained spleen impression smears in two animals at necropsy. Cats infested with acquisition-fed D. variabilis nymphs remained clinically normal and did not develop detectable parasitemia over the course of the study as determined by blood smear and PCR.ConclusionsCytauxzoon felis was successfully transmitted to domestic cats by A. americanum nymphs acquisition-fed as larvae on the donor cat. However, we were not able to transmit C. felis to healthy domestic cats with D. variabilis nymphs that were simultaneously acquisition-fed on the same donor cat. Results from this study suggest that larval and nymphal A. americanum likely play important roles in natural transmission cycles of C. felis.