Heartworm Disease


Heartworms (Dirofilaria immitis) are fairly large worms (can grow up to 14 inches long) that, in adulthood, live in the heart and pulmonary arteries of an infected animal. Pets acquire these worms through mosquito bites as mosquitoes readily pick up larval heartworms from infected animals and carry them to new animals.
These parasitic worms have 5 larval stages and are termed L1, L2, L3, L4, & L5. Heartworm microfilariae are first stage larvae: L1. Heartworms do not lay eggs like other worm parasites; instead they give live birth and the baby worms are called microfilariae. Microfilariae are released into the circulatory system in hope that they will be taken up by a mosquito taking a blood meal and carried to a new host. Microfilariae may also be transmitted across the placental barrier to unborn offspring of a parent host. However, in order for a heartworm to reach adulthood, it must be passed through a mosquito.
Within the mosquito’s body, the microfilariae will develop to L2s and finally to L3s, the stage capable of infecting a new dog. This can take a few weeks. The process goes faster in warmer weather.
Adult heartworms (stage L5) are several inches in length, and prefer to live in the pulmonary arteries leaving the heart. They swim through the blood into an artery, where they are massaged and nourished by blood flowing past them. In the pulmonary arteries of an infected animal, the worm’s presence generates a strong inflammatory response and a tendency for blood to inappropriately clot. If enough worms are present, the heart must work extra hard to pump blood through the obstructed arteries. Over time, heartworm infection can be fatal if left untreated.

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. Sudden death may occur just as it may occur in infected dogs.
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 dog, 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.


Monthly Heartworm Prevention 

Heartworm infection is easily and affordably prevented through the use of a regular heartworm prevention product, such as Heartgard, Trifexis or Revolution. These products are administered orally or topically (on-the-skin) and will prevent heartworm, as well as other internal parasites and occasionally fleas and ticks, for an entire month. Find our heartworm preventative products in the shop, or call 1.888.729.7758 to discuss these products with a live PawsPlus representative.

Recommended protocol:

Administer a heartworm preventative product to your pet every month for the duration of its life. Dogs and cats are both at risk for heartworm infection at any age and in most areas of the U.S.

Recommended Lifestyle:

All dogs and cats


Heartworm/Lyme Combo Test  

The Heartworm/Lyme Combo test is a blood screening for Heartworm disease, Lyme disease, Ehrlichiosis, and Anaplasmosis. A PawsPlus representative will draw a small amount of blood from your dog and submit it to a national laboratory the following week. You will receive your test results, as well as recommendations for follow up care (if needed) the week following the blood draw. PawsPlus recommends the Heartworm/Lyme Combo test over the Heartworm-only test for more comprehensive disease and infection screening.
Heartworm disease can be prevented with a monthly heartworm prevention product, like Heartgard or Trifexis. While there is no full-proof method of preventing Lyme disease, Ehrlichiosis and Anaplasmosis, these can be discouraged by keeping your pet on a regular anti-tick preventative, such as Revolution.