Infection with Dirofilaria immitis<\/i>, more commonly known as heartworm, is not an issue limited to dogs. Although the observed prevalence of heartworm infection is lower in cats than in dogs, infected cats can be subject to serious and even fatal disease. Accurate diagnosis persists as a major obstacle in tackling feline heartworm infection and estimating its prevalence. Multimodal diagnostics can enhance detection and help to differentiate between the clinical manifestations of feline heartworm infection. This article briefly reviews the life cycle and clinical aspects of feline heartworm infection and focuses on optimizing diagnosis and updating prevalence information.<\/p>\n
Take-Home Points<\/strong><\/p>\n <\/div><\/div><\/p>\n Within the past few decades, it has become apparent that infection with Dirofilaria immitis<\/i>, more commonly known as heartworm, can cause serious and even fatal disease in cats as well as dogs. Although the prevalence of heartworm infection in cats is admittedly lower than that observed in dogs<\/a>, its occurrence is likely underestimated, in part due to difficulty in making an accurate diagnosis in cats. <\/span><\/p>\n Multimodal diagnostics can enhance detection and help to differentiate between the clinical manifestations of feline heartworm infection: heartworm-associated respiratory disease (HARD) and adult heartworm infection.<\/span><\/p>\n Heartworm infection begins when third-stage D\u00a0immitis<\/i> larvae are deposited by a mosquito onto the host\u2019s skin during blood feeding and invade the host through the bite wound. During their subsequent migration to the pulmonary arteries, they molt twice, to the fourth larval stage and then the immature adult stage. Immature adults arrive in the pulmonary arteries 70 to 90 days after infection; here, they complete sexual maturation and begin to reproduce.1<\/sup> <\/span><\/p>\n Dogs are usually permissive hosts and tolerate the initial arrival to the pulmonary arteries relatively well, thus allowing the heartworms to persist and complete development into mature adults. Clinical heartworm disease in dogs is most often due to mature heartworms and cumulative, chronic damage within the pulmonary arteries.1<\/sup><\/span><\/p>\n In cats, however, the arrival of immature adult worms in the pulmonary system induces a sizable inflammatory response. While the feline immune system is often capable of eliminating the immature worms without therapeutic intervention (sometimes called \u201cself-curing\u201d), the death of the parasites and resulting inflammation can cause heartworm-associated respiratory disease (HARD), a serious condition.2<\/sup> The most common pathological findings associated with HARD are eosinophilic infiltration of the lungs, villous endarteritis (often worst within the caudal lung lobes), and thoracic radiograph abnormalities.3-5<\/sup> Clinically, HARD most often manifests with coughing, dyspnea, and vomiting,6<\/sup> with an onset approximately 3 months postinfection, coinciding with arrival and death of immature parasites in the pulmonary system.<\/span><\/p>\n Less frequently, immature parasites survive the cat\u2019s immune response and adult heartworm infection develops. Presence of adult worms induces pathology in the lungs and pulmonary vessels similar to that observed with HARD. Interestingly, it has been demonstrated that mature, living heartworms are immunosuppressive in cats and that they downregulate pulmonary intravascular macrophage activity.7<\/sup> It is unclear why immature stages are more immunogenic than adult heartworms in cats.<\/span><\/p>\n It is not uncommon for a cat infected with adult heartworms to suffer acute collapse and death (<\/span>FIGURE 1<\/b><\/span>), regardless of whether it previously exhibited signs associated with heartworm infection.8,9<\/sup> This scenario is likely attributable to death of an adult heartworm and a sudden inflammatory response or thromboembolism caused by adult worms.8<\/sup> Many canine and feline heartworm infections exist in the absence of clinical signs, although failure to develop overt clinical signs does not necessarily indicate less severe pathology.<\/span><\/p>\n Figure 1. Photo taken at necropsy demonstrating natural heartworm infection in a cat that died suddenly after sharing a household with a dog with heartworm disease. Courtesy Dr. Sarah Morar Schneider, University of Georgia<\/p><\/div>\n Feline heartworm infections rarely become microfilaremic, and when microfilaremia has been observed in cats, it has been only transiently detectable, with low numbers of microfilariae.10<\/sup> This would indicate that cats do not routinely contribute to the transmission of heartworm to mosquitoes and subsequently other hosts.<\/span><\/p>\n Recommendations regarding diagnostic testing and screening for heartworm infection in dogs are straightforward: Perform an antigen test and a microfilaria test for every dog, every year.11<\/sup> The tendency for heartworm-infected cats to develop clinical disease in the absence of detectable antigen or microfilariae (or despite elimination of the parasite altogether) makes diagnosis and formulating feline diagnostic guidelines difficult. No sole test performs sufficiently for diagnosing feline heartworm infection. Utilizing multiple diagnostic tests both dramatically improves the likelihood of diagnosis and aids in differentiating the type of heartworm infection\u2014HARD or adult heartworms (FIGURE<\/b><\/span><\/span>\u00a02<\/b><\/span>).<\/span><\/p>\n Cats initially infected with third-stage larvae can seroconvert as early as 1 to 2 months postinfection, and it was demonstrated under experimental conditions that all cats seroconverted by 5 months postinfection.12<\/sup> In the same study, at least 90% of cats seroconverted by 3 months postinfection, coinciding with the typical time for the onset of HARD symptoms.12<\/sup><\/span><\/p>\n The primary limitation of antibody testing is that it cannot delineate between historical and current infection, and the longevity of antibody titer<\/a> persistence is not well established and likely varies between individuals. However, antibody positivity in the presence of consistent clinical signs and history should strongly increase the suspicion for HARD or adult heartworm infection. A positive antibody test result in a clinically healthy cat indicates both exposure risk and the possibility of current infection. In these cases, the cat should be started on heartworm prevention and the owner should be educated regarding not only the importance of continued year-round prevention but also the possibility that clinical signs could develop and what to look for. Conversely, a negative test drastically lowers, but does not completely eliminate, the index of suspicion and helps to lessen the probability of heartworm as the cause of respiratory signs.<\/span><\/p>\n\n
Clinical Aspects of Feline Heartworm Infection<\/h2>\n
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<\/a>Optimizing Diagnosis: A Multimodal Approach<\/h2>\n
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<\/a><\/p>\nAntibody Testing<\/h3>\n
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