Dirofilaria immitis is a parasitic worm that usually develops within the right side of the heart or the adjoining large vessels. Infection is diagnosed more commonly in dogs, which appear to be more susceptible to infection than cats.
Infection is spread to cats when a mosquito bites the skin, injecting infective heartworm larvae into the bloodstream of the cat. Some of the heartworm larvae travel to the heart and develop into the adult stage. Unlike heartworm infections in the dog, cats have fewer infective offspring that mature into the adult stage, a lower number of adult worms at one time, and adults that live for a shorter period.
Heartworms can be a serious disease in cats and can result in sudden death, if for example, it leads to a blood clot that travels to the lungs. However, some cats will show minor symptoms, or no clinical signs at all. Symptoms, if present, are nonspecific for heartworm disease and require a thorough physical exam and testing to rule out other possible causes.
Heartworm infections are difficult to suspect, diagnose and treat in cats due to the relative absence of clinical signs in most cases, the unreliability of feline heartworm tests, and the high risk of serious side effects from treatment. A veterinarian in general practice may refer a cat suspected of having heartworms to a veterinary cardiologist for specialized testing and evaluation.
Treatment is controversial and generally reserved for cats suspected of having large worm infections with recurrent symptoms. Heartworm disease in some cats may be self-limiting, since the worms eventually die on their own, and thus the condition goes undetected. Heartworms should be considered in the list of diseases suspected of causing signs of respiratory distress, heart disease, and vomiting.
Preventive medication is available and recommended for cats living in geographic areas where the disease is prevalent, such as the eastern and Midwestern states, and California. Cats should be tested for heartworms prior to starting prevention. No adverse reactions to heartworm preventive treatments have been reported in cats.
