Though recognized since 1921, heartworm infection (HWI) in the cat has received increasing interest over the last decade. Reasons for this are multiple, with an increasing diagnostic armamentarium, an increasingly aware and demanding public, and the development of safe, effective, and broad-spectrum preventative drugs. With this comes the responsibility of understanding the pathogenesis, clinical signs, diagnostic process, treatment, and most importantly, preventing HWI in cats.The domestic cat, though an atypical host, can be parasitized by Dirofilaria immitis (HW) with resultant heartworm disease (HWD). The clinical manifestations of the disease are different and more severe in this species, but the infection rate is only 5-20% of that of the dog. Experimental infection of the cat is more difficult than in the dog; <25% of L3 reach adulthood. This resistance is also reflected in natural infections, in which feline heartworm burdens are usually less than 6, and typically only1-3 worms. Other indications of the cat's inherent resistance to this parasite are a shortened period of worm patency, high frequency of amicrofilaremia or low microfilaria counts, and shortened life span of adult heartworms (2-3 years). Additionally, while some species of mosquito may feed on cats, most prefer the dog and, for a cat to become infected, the mosquito must first have fed on a dog. Nevertheless, studies have shown a prevalence as high as 10-14% in shelter cats and a study performed at NCSU revealed HWD in 9% of cats presented with cardiorespiratory signs. Furthermore, antibody testing showed 26% of 100 of these cats to have been exposed to HW. Similar to dogs, some studies have shown the male to be at higher risk for HWI than the female. Aberrant worm migration appears to be a greater problem in cats than in dogs.
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