Prime detection of Dirofilaria immitis: understanding the influence of blocked antigen on heartworm test performance

Abstract

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.

Changes in canine serum N-glycosylation as a result of infection with the heartworm parasite Dirofilaria immitis

“…In clinical practice nowadays, a combination of antigen and microfilaria testing is recommended for diagnosis 6 . Detection of circulating antigens is mainly specific for the fecund female adult worm, which makes infection detectable only about half a year post-infection 7 , 8 . However, it is theoretically also possible to detect antibodies raised to parasite antigens present in earlier (pre-patent) phases of infection 9 , 10 .…”

Development of a multiplex qPCR-based approach for the diagnosis of Dirofilaria immitis, D. repens and Acanthocheilonema reconditum

“…Commercially available diagnostic kits for the detection of D. immitis antigens may also cross-react with filarial and non-filarial nematodes, such as D. repens, A. reconditum and Onchocerca spp. [14], Spirocerca lupi and Angiostrongylus vasorum, especially the latter which can cause a cross-reaction without prior heat pre-treatment of the sera [15][16][17]. Angiostrongylus vasorum, the agent of French heartworm disease, should also be taken into account in the differential diagnosis of pulmonary disease [17].…”

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

“…In areas of recent heartworm introduction, newly infected dogs will have no Mff due to the prepatent period of at least six months, single sex infections or burdens with less than two female heartworms. Additionally, the formation of immune complexes can further complicate diagnosis and detection of D. immitis antigen [30,31].…”