The domestic dog and some wild canids are the normal definitive hosts for heartworms and thus serve as the main reservoir of infection. Even less suitable hosts, such as cats and ferrets, occasionally have low-level, transient microfilaremia and therefore, theoretically, may serve as a limited source of infection for mosquitoes during these short periods of microfilaremia. The Culex spp mosquito, which is the most common species in many urban areas, feeds on both cats and dogs without preference. Heartworm infection is preventable despite the dog’s inherently high susceptibility.
Because all dogs living in heartworm-endemic areas are at risk, chemoprophylaxis is a high priority. Puppies should be started on chemoprophylaxis as early as possible, no later than 8 weeks of age.

Even though continuous, year-round transmission may not occur throughout the country, the administration of broad-spectrum chemoprophylactic products with endoparasitic and/or ectoparasitic activity for 12 months each year likely enhances compliance and may assist in preventing pathogenic and/or zoonotic parasitic infections. Continuous, year-round administration of heartworm preventive is critical in most, if not all, areas of the United States.

Annual testing is an integral part of ensuring that prophylaxis is achieved and maintained. Should an infection be diagnosed, more timely treatment can be provided to minimize pathology and the potential selection of resistant subpopulations.

Some Collies and other P-glycoprotein–deficient dogs are unusually sensitive to a variety of commonly used veterinary drugs, including some antidepressants, antimicrobial agents, opioids, immunosuppressants, and cardiac drugs. These intoxications have occurred most often when concentrated livestock preparations of macrocyclic lactones are either accidentally ingested or overdosed because of human error in dosage calculation. The standard chemoprophylactic dosages of all macrocyclic lactones have been shown to be safe in all breeds.


Significant differences exist between feline heartworm disease and its classical canine counterpart. Although cats are susceptible hosts, they are more resistant to infection with adult Dirofilaria immitis than are dogs. Most heartworm infections in cats are comparatively light and consist of less than six adult worms. Typically only one or two worms are present and worms in approximately one third of infections are single sex. Cats with only a few worms are still considered to be heavily infected in terms of parasite biomass, however, because of their relatively small body size.

The clinical importance of heartworms is amplified in cats because even a small number of heartworms are potentially life threatening. Although live adult worms in the pulmonary arteries cause a local arteritis, some cats never manifest clinical signs. When signs are evident, they usually develop during two stages of the disease: 1) arrival of heartworms in the pulmonary vasculature and 2) death of adult heartworms.

The most characteristic radiographic features of heartworm disease in cats, as in dogs, are a sometimes subtle enlargement of the main lobar and peripheral pulmonary arteries, characterized by loss of taper, and sometimes tortuosity and truncation in the caudal lobar branches. These vascular features are visualized best in the ventrodorsal view and are sometimes visible only in the right caudal lobar artery, where heartworms are found most often.

Since both juvenile and adult worms are capable of causing clinical disease in the cat, both antibody and antigen tests are useful tools and when used together increase the probability of making appropriate diagnostic decisions.

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