Feline heartworm disease is present anywhere in the world where canine heartworm exists. The true prevalence is likely understated. Significant differences exist between canine and feline heartworm disease; cats are more resistant to adult worm infections than dogs, and most feline infections are comparatively light, consisting of less than 6 adult worms. Still, even a small number of heartworms is potentially life-threatening to cats.
Signs, when present, usually develop upon arrival of immature worms in the pulmonary vasculature, part of a syndrome known as heartworm-associated respiratory disease (HARD), or upon death of adult heartworms. The most common clinical signs are those of chronic respiratory disease, although many cats tolerate infections with inapparent or transient signs. Diagnosis in cats is more elusive than in dogs; interpretation of antibody and antigen tests is complicated, and cats are infrequently microfilaremic when presented. While antigen testing is the gold standard for diagnosis in dogs, none of the antigen tests currently available can be relied upon to rule out feline heartworm infections. Antibody tests have the advantage of detecting both sexes of heartworms, but they do not indicate whether an infection is ongoing. When used together, however, both types of tests help increase the probability of making appropriate diagnostic decisions. Thoracic radiography and echocardiography can also aid in confirming a diagnosis, although radiographs do not always provide evidence of an infection. When feline heartworm disease is suspected of causing death, a diagnosis should be confirmed during necropsy.
Treatment for feline heartworm cases may include tapered doses of prednisolone or adulticide treatment using ivermectin. Surgical removal may also be a reasonable alternative. Monitoring infection status via serologic testing every 6 to 12 months is recommended for infected cats. In cats living in heartworm endemic areas, oral or topical monthly chemoprophylaxis is a safe and effective option.
COMMENTARY: The difference in how Dirofilaria immitis causes disease in dogs and cats has probably led to the hesitancy to recognize heartworm disease in cats. However, the clinical importance of heartworms is amplified in cats because even a small number of heartworms is potentially life-threatening. The guidelines of the American Heartworm Society can be found at heartwormsociety.org.