How Does Heartworm Affect Your Dog or Your Cat?

Heartworm affects dogs and cats differently. Read more to understand how heartworm is contracted and signs of heartworm.

Heartworm in Dogs:

When a dog is bitten by an infected mosquito, the microfilariae is deposited in a tiny drop of mosquito saliva adjacent to the mosquito bite. Once in the bloodstream of the new host, the microfilariae will spend the next week or two developing into the next stage of development within the host’s skin. It will live in the skin at this stage for three months or so until it develops to the final stage of development and is ready to enter the host’s circulatory system. In this final stage, the microfilariae migrates to the heart and out into the pulmonary arteries (if there is room) where it will mate, approximately 5-7 months after first entering the new host.

NOTE: All commercially available heartworm preventives act by wiping out the microfilariae in the middle stages. When a dog tests positive for an adult heartworm infection, treatment at a full-service veterinary facility is necessary.

Most dogs infected with heartworm can be successfully treated. It is important to try to accomplish this with a minimum of harmful effects from drugs and a tolerable degree of complications created by the dying heartworms. Heartworm infected dogs showing no signs or mild signs have a high success rate with treatment. Patients with evidence of more severe heartworm disease can be successfully treated, but the possibility of complications and mortality are greater. The presence of severe heartworm disease within a patient in addition to the presence of other life-threatening diseases may decrease the efficacy of treatment.

Heartworm in Cats:

Contrary to common public perception, cats CAN and DO get heartworms, although the situation is vastly different from canine heartworm disease. The cat is not a natural host for the heartworm which means the migrating larval heartworm is not likely to find its way to the heart when passed from a mosquito. Mosquitoes that carry heartworm definitely prefer to feed on dogs, but cat infections will happen from time to time. While a moderate heartworm infection in a dog would involve 25-50 adult heartworms, infected cats typically have less than six adult worms. Because the feline heart and blood vessels are so small, these few worms can wreak havoc. Worms found in the canine heart can reach lengths up to 14 inches, but the average length of worms found in feline hearts is only 5-8 inches. An adult heartworm can live up to 5 years in a dog, but will only live 2-3 years in a cat probably due to the cat’s especially strong immune reaction.

The cat’s immune system is extremely reactive against heartworms. For this reason, it is virtually impossible to detect microfilariae in an infected cat. (The cat’s immune system removes them too quickly). Cats usually develop respiratory disease, complete with respiratory stress, and coughing or vomiting chronically. Feline heartworm disease is often misdiagnosed as feline asthma.

Prevalence of feline heartworm infection has been difficult to estimate because definitive diagnosis is difficult. Diagnostics typically includes clinical signs and tests developed for use in the cat, such as radiographic/angiographic findings, necropsy findings (after death), microfilaria tests and parasite antigen tests. Antigen testing may be less accurate in the cat as the worm burdens are typically very low.

In general, if a known heartworm positive cat does not have symptoms, the American Heartworm Society recommends attempting to wait out the worm’s 2-3 year life span and simply monitor via chest x-rays every 6 months or so. Since the major signs of disease in the cat are due to inflammation and immune stimulation, a medication such as prednisone can help control symptoms.

Keep your pets healthy happy and safe!