VETgirl December 2024 BEAT e-Magazine

QUARTERLY BEAT / DECEMBER 2024

FELINE HEARTWORM DISEASE DIFFERS FROM ITS CANINE COUNTERPART Dogs and cats are both considered susceptible hosts of Dirofilaria immitis , the heartworm parasite. However, because feline patients today are not routinely screened for heartworms, the prevalence of heartworm in cats is not well understood. In a study conducted at a central Florida animal shelter, sets of 100 unprotected dogs and 100 unprotected cats were screened for heartworm infection and/or history of infection using several different diagnostic modalities (antigen and microfilaria tests in dogs; antigen, antibody and microfilaria tests in cats). When comparing the percentage of animals testing positive on any one modality, there was no statistical difference between the two species. 1 Because the feline immune system mounts a more robust response against immature stages, fewer heartworms reach adulthood in cats. This likely contributes to the misconception that heartworm disease is uncommon in cats, when heartworm disease simply follows a different disease process in cats than dogs. In cats, clinical disease first manifests when immature worms reach the lungs—notably before an antigen test can detect infection. From there, potent inflammatory mediators within the lungs mount a response that may create clinical signs like dyspnea, coughing, vomiting, lethargy, anorexia, etc. In fact, it is likely that many cases diagnosed as feline asthma are actually heartworm disease. If the immune system fails to kill all the larval stage worms—and if the cat survives the process—the cat will go on to develop a patent adult heartworm infection. During this phase of infection, the pulmonary damage created by the arrival, and death, of larval stages can lead to long-lasting asthma-like clinical signs. Adult worms also put additional stress on the heart and pulmonary vasculature. When the adult heartworm eventually dies (typically 2-4 years post-infection, compared to 5-7 years in dogs), the risk of sudden respiratory distress, embolism, and death is renewed.

ROUTINE DIAGNOSTIC HEARTWORM SCREENING NOW RECOMMENDED IN CATS Annual heartworm testing in dogs is standard practice in veterinary hospitals, but most cats are not routinely tested. In dogs, annual antigen testing provides a quick, affordable and accurate means of pinpointing heartworm infections. However, because antigen tests detect adult, female worms versus the male-only or immature heartworm infections that are much more common in cats, antigen screening by itself is unlikely to be a sufficient diagnostic strategy. Heartworm antibody testing detects the presence of antibodies produced by cats in response to a previous or existing heartworm infection at any stage from larvae to adults. The limitation of antibody testing is that it cannot distinguish between present and past infections—only that an infection has occurred. The accuracy of antibody tests is also variable and false-negative results are common. Given the inherent shortcomings of antigen and antibody tests described above, tailoring the diagnostic strategy to achieve accurate detection of feline heartworm is critical. After reviewing testing studies from recent years, the AHS recently identified several strategies that can improve the accuracy of heartworm testing in cats. For the first time, the AHS now strongly recommends annual heartworm screening in cats using the following protocol: • A Heska (an Antech company) antibody test , which has been found to be up to 8 times more sensitive than other tests • An antigen test conducted with heat-treated serum . Antigen-antibody immune complexes can produce false-negative heartworm test results in both dogs and cats. Data accrued in recent years have revealed that these immune complexes occur at much higher rates in feline infections than canine infections.

FEATURED STORY

In this VETgirl feature article sponsored* by the American Heartworm Society (AHS), Dr. Jenni Rizzo, reviews what’s new with treating feline heartworm disease. Feline heartworm disease is misunderstood, underdiagnosed, and undermanaged. So, what do you need to know about the revised AHS Feline Heartworm Disease Guidelines? Read on to learn more! * Please note the opinions of this blog are the expressed opinion of the author and not directly endorsed by VETgirl.

THE REVISED AMERICAN HEARTWORM SOCIETY FELINE HEARTWORM DISEASE GUIDELINES: CAN WE DETECT, PREVENT, AND BETTER MANAGE THESE CASES?

BY DR. JENNI RIZZO DVM, President, American Heartworm Society (AHS)

Conventional wisdom among veterinarians has long held that heartworm disease is a serious, ubiquitous issue in dogs. At the same time, while many practitioners also believe that heartworm disease in cats is a serious disease, they view it as relatively uncommon. This assumption has led many practitioners to look for heartworms in feline patients only when overt clinical signs point to the problem, and to refrain from recommending routine heartworm testing and prevention in cats. The result: feline heartworm disease today is misunderstood, underdiagnosed and undermanaged. A recent revision of the American Heartworm Society (AHS) Guidelines on Feline Heartworm Management, which includes updates to the organization’s recommendations for prevention, diagnosis and treatment in cats, is designed to challenge these misperceptions about heartworms in cats and to encourage practitioners to consider a more proactive approach to this serious disease.

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