As a follow-up to the February 1997 Antech News which focused on canine heartworm disease, we present a summary of current views on the clinical, diagnostic, prophylactic and treatment aspects of this disease in cats.

Cats are susceptible but resistant natural hosts to heartworms. Dirofilaria immitus is the causative agent for feline heartworm disease and has the highest incidence rate in the south- eastern United States. The prevalence of feline heartworm disease is generally stated to be 10 percent (10%) of that seen in dogs in a particular region. A vigorous immune response to all stages of the parasite is mounted in host species other than dogs, with that of cats being intermediate between that of humans and dogs. A relatively small percentage of migrating heartworm larvae ever reach the heart in cats. Many migrating worms end up in various other body sites where they die and elicit local inflammatory reactions. If adult worms do reach the heart, microfilariae usually appear in the cat’s blood for just a few weeks. Thus, most cats have occult heartworm infections.

Pulmonary vascular lesions are usually more severe on radiographic and histopathologic examinations of affected cats in comparison to dogs. Radiographic signs are usually lobar, with pulmonary artery enlargement and a mixed bronchial interstitial pattern. Mild to moderate cardiac enlargement is seen in about 50% of affected cats. Aberrant migration of worms to other body tissues produces associated tissue reactions and disease more commonly in cats.

The clinical syndromes are seen in feline dirafilariasis:

• Asymptomatic, self-limiting infections;
  
• Clinically apparent pulmonary disease, and
  
• Miscellaneous symptoms associated with aberrant or ectopic heartworm migration, such as neurologic disease.

In experimentally infected cats, 40% of infections are asymptomatic and self-limiting. They are likely to be more common in naturally exposed cats. This type of infection occurs when larvae never migrate beyond the subcutaneous tissues or when adults reach the heart but are subsequently destroyed by the strong immunologic response they elicit. Even though pronounced abnormalities may be seen on radiographs, cats rarely show outward signs of heartworm disease.

Pulmonary dirofilariasis in cats is usually associated with a small number of adult worms (1-7) located in the right ventricle, right atrium, or main pulmonary arteries. When cats are experimentally infected with large numbers of third-stage larvae, 60% will develop pulmonary signs. The incidence of pulmonary disease reported for 156 heartworm positive cats was about 50% with signs of coughing, gagging, dyspnea and tachypnea. In 80-90% of cases, infection is occult due to lack of mating pairs or immune-mediated microfilarial destruction. Up to 1/3 of these infected cats can die suddenly or develop acute, often fatal, respiratory distress. Most cats with the pulmonary form of heartworm disease have a more chronic disorder which can last up to a year or more. Vomiting, weight loss, listless- ness, gagging and a chronic cough can be seen. Vomiting without diarrhea has actually been a more consistent sign than coughing, and has a ready explanation. One study reported that 70% of infected cats were males. Aberrant migration of heartworms (also called "spurious" migration or ectopic heartworms) in cats occurs most commonly in the brain, and to a lesser extent, the spinal cord.