Heartworm-Associated Respiratory Disease in Client-Owned Cats

Feline heartworm disease is diagnosed less frequently than its canine counterpart. Several factors likely contribute to this discrepancy, including the inherent resistance of cats to heart worm infection, the limitations of diagnostic testing for cats, and the nonspecific (and frequently absent) clinical signs associated with feline heartworm disease.

Although cats infected with heartworms are less likely than dogs to develop adult worms, studies have shown that the arrival of immature heartworms at the pulmonary vasculature approximately 70 to 90 days postinfection can be associated with severe parenchymal, vascular, and airway disease in cats. These changes, which can be evaluated radiographically and histologically, have been defined as heartworm-associated respiratory disease (HARD). Associated clinical signs may include dyspnea, coughing, and tachypnea resembling feline asthma. However, differentiating HARD from asthma or other diseases can be difficult—another factor that complicates determination of prevalence rates for feline heartworm disease, even in areas where the disease is endemic in dogs.

Taiwan is considered a heartworm-endemic region, with canine infection rates ranging from approximately 13% to 57%. A study recently published in the Journal of Feline Medicine and Surgery evaluated the numbers of cats at risk for HARD in Taipei, Taiwan. The study also looked at correlations between clinical signs of heartworm-associated lung disease and actual heartworm exposure among cats to determine if cats with suspicious clinical signs were more likely to be heartworm positive.*

Study Design

One hundred eighty-six client-owned cats completed the study. Of these, 83 cats had signs of lower airway or lung disease (LA/L; coughing, open-mouth breathing, or labored breathing), and 103 had no clinical signs. Investigators also included previously collected blood test results from a cohort of 40 stray cats to help estimate seropositivity among stray cats in the region. Combining results from both groups of cats, 226 blood samples were included in the final calculations. Feline heartworm antibody testing was used to determine heartworm exposure.

Patients in the study were enrolled prospectively over a period of approximately 1 year from a veterinary university teaching hospital and 4 private practices in Taipei. Cats younger than 6 months of age were excluded, as were cats that had received heartworm prophylaxis within the 6 months prior to study enrollment.

Results

Among cats with clinical signs, the most common signs re- ported by owners were coughing (89%), open-mouth breathing (23%), and labored breathing (22%). Vomiting, diarrhea, and syncopal episodes were also reported. The researchers found that cats with signs of LA/L disease were 1.8 times more likely to be heartworm-antibody positive than cats that didn’t have clinical signs, but the difference between the groups was not statistically significant. Similarly, overall seropositivity among client-owned cats was 10.2%, and seropositivity among stray cats was 7.5%, but the difference in seropositivity between these two groups was not significantly different (TABLE).

Although living exclusively indoors did not eliminate heartworm exposure risk, having access to the outdoors was found to be a significant risk factor among client-owned cats, as cats that spent time outside were 3.7 times more likely to be seropositive. In the study, 11.9% of the client-owned cats had access to the outdoors.

Discussion

The investigators noted that the presence of asymptomatic positive cats may have contributed to the lack of correlation between seropositivity and LA/L signs. The authors also acknowledged that confirming heartworm exposure (via antibody testing) in a cat with clinical signs of LA/L does not confirm that those signs are due to heartworm disease or HARD.

According to the authors, the small number of cats enrolled in the study may have limited the ability of the investigators to observe a significant difference in heartworm seropositivity between cats with and without LA/L signs.

Additionally, the researchers noted that the inherent shortcomings associated with feline heartworm antibody tests—including the recognition of false-negative results during the early stages of infection (<60 days), when antibody levels are too low to be detected, and when antibody levels wane over time&mdash;may have been a factor in determining heartworm status for cats in the study.

Confirming a diagnosis of HARD in client-owned cats is difficult, the authors commented, “There are diagnostic challenges because of the lack of a gold standard,” they said. Routine tests such as radiographs and bronchoalveolar lavage may offer supportive evidence but not conclusive proof. Even if eosinophilic inflammation of the lower airways can be confirmed via bronchoalveolar lavage, this does not rule out feline asthma or other differentials besides HARD. Further, the investigators caution that pet owners may be reluctant or unwilling to pursue more specific testing, such as lung biopsies. As such, definitive diagnosis of HARD is not always practical or possible.

The authors recommend that when seropositive cats living in heartworm-endemic areas present with suspicious clinical signs, a diagnosis of HARD should be considered. They also suggest that cats in endemic areas receive heartworm prophylaxis, even if they live exclusively indoors, because of the potential risk for developing HARD and the challenges inherent in confirming that diagnosis.

^{*Four of the study’s authors have received lecture fees and/or royalties from Zoetis Taiwan. However, Zoetis Taiwan was not involved in this study and did not provide financial support.}

Dr. Todd-Jenkins received her VMD degree from the University of Pennsylvania School of Veterinary Medicine. She is a medical writer and has remained in clinical practice for over 20 years. She is a member of the American Medical Writers Association and One Health Initiative.