The worm burden in cats is usually low, as most have fewer than 10 worms, although there have been documented cases of worm burdens as high as 45 and 65 in two different male cats. Although heartworms reach a substantial size in cats, they are somewhat smaller than those of similar age in dogs. The life span of the worms in cats appears to be about 2 to 3 years, compared to 5 to 7 years in dogs. As such, cats are poor reservoirs of infection, as heartworms are less likely to mature to adults in cats than in dogs and are short-lived when present. There is no age predilection to D. immitis infection in cats, and cases have been reported in cats from 9 months to 17 years of age, with a median age at diagnosis or death of approximately 4 years of age.
Some cats appear to be able to rid themselves of the infection spontaneously.
It is assumed that the cat develops a strong immune response to the worms which causes their death, or perhaps the parasite dies as a result of physiologic maladaptation and the intense immune response occurs only after worm death. Single sex heartworm infections are also more common in cats. Indoor and outdoor cats are equally susceptible to infection, with outdoor cats commonly believed to be at increased risk. The distribution of feline D. immitis infection in the U.S. seems to parallel that in dogs but at a decreased prevalence. The clinical signs of heartworm infection in cats can be very non-specific, and may mimic many other feline diseases.