Why Should I Care About Heartworm?

For a variety of reasons, even in regions of the country where winters are cold or the climate is dry, the American Heartworm Society recommends year-round Heartworm prevention.

Heartworm has been found in all 50 states. Most states have “hot spots” where the heartworm infection rate is very high compared with other areas in the same state, however the incidence of heartworm disease in a particular area is not always predictable from year to year. Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas, and presence of animal reservoirs (infected dogs, foxes, coyotes).

Mosquito species constantly change and adapt to various climates, and many vectors are expanding their viable season and range as weather patterns change. Some mosquito species successfully overwinter indoors creating a potential year-round risk to pets. Moist microclimates (irrigated fields, backyard ponds, man-made golf courses) perpetuate mosquito populations, and affect the severity and duration of the mosquito season. Heartworm infection in wildlife with large ranges (eg. coyotes) adds to the spread of this parasite in domestic pets.

Heartworm Maps

VIP Petcare follows the guidance of The American Heartworm Society, and recommends year-round prevention for all areas of the US to help keep pets happy, healthy, and safe.

Heartworm disease maps

Heartworm in Dogs

When a dog is bitten by an infected mosquito, the L3 is not deposited directly into the dog’s bloodstream. Instead, it is deposited in a tiny drop of mosquito saliva adjacent to the mosquito bite. Once in the bloodstream of the new host, the L3 will spend the next week or two developing into an L4 within the host’s skin. The L4 will live in the skin for three months or so until it develops to the L5 stage and is ready to enter the host’s circulatory system. The L5, which is actually a young adult, 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 freshly delivered L3s and the L4s. 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.