A dog sticking its tongue out.

Canine Heartworm Disease

Canine heartworm disease is a serious parasitic disease caused by the worm Dirofilaria immitis. Heartworm disease is prevalent worldwide, specifically regions with humid temperatures and dense mosquito populations.

Life Cycle of Heartworm Disease

Canine heartworm needs an intermediate host, like the mosquito, to help in replication and transmission. The life cycle of canine heartworm disease starts with a mosquito feasting on a blood meal from an heartworm-infected canid. Unfortunately, domestic and wild canines can become infected and serve as hosts for heartworm disease as well. When dogs are infected, they have circulating microfilariae (early stages of adult worms) in their blood streams, making them microfilaremic hosts. When a mosquito bites these hosts, they ingest some of the microfilaria. These microfilaria now replicate and advance in the intermediate host, and canine heartworm disease cannot replicate or transmit without the mosquito.

While in the mosquito, the heartworm develops into larval stage 1 (L1), larval stage 2 (L2), and finally into larval stage 3 (L3). Once at L3, the heartworm moves from the body cavity of the mosquito to the mouthparts, and is set up to transmit to a definitive host, which is usually a canine species. The time frame for the maturation from microfilaria is dependent on the temperature and humidity of the environment. At 80 °F with 80% humidity, the process can take 10-14 days. This process takes longer in cooler and less humid environments.

Once the mosquito bites a new host, the L3 lives in the skin until it molts to larval stage 4 (L4). The L3- L4 time frame can be as quick as 3 days, but usually lasts 9-12. L4 is then able to travel through the subcutaneous tissues and muscle in the host.

The Final Stage

Per American Heartworm Society guidelines, the final molt from L4 to juvenile/immature adults happens between 50-70 days post infection, as they are migrating through the body. Once they are juvenile, they migrate via the circulatory system to the heart and lungs, with the immature adults being noted in the pulmonary system between 67-120 days post infection. When they reach the lungs, they are usually 1-1.5”, but the females will mature to closer to 10-12” at full maturity. They are sexually mature at 120 days post infection and dogs can have documented infection (microfiliaremic) between 6-9 months post infected mosquito bite. The adult worms primarily reside in the pulmonary arteries and, in dogs with large amounts of worms, they can be found in the right ventricle and atrium.

Dogs are the definitive host of heartworm disease. On average, 56% of L3 larva will advance to adult heartworms in dogs. Ferrets and cats can also be susceptible hosts, but the rate of L3 developing into adults is lower in these species, noted to be 6% in cats and 40% in ferrets.

Clinical Signs of Heartworm Infection

Most dogs will have no clinical signs during the L3-adult life stages of infection. Once the worms have reached their final location in the heart and lungs, clinical signs can be noted. Clinical signs are highly dependent on the worm burden (amount of worms), location of the worms, and the dog’s immune system. Clinical signs are usually noted late in the disease when a large number of worms are present.
Common signs include:

Dogs with severe disease will present with caval syndrome. Caval syndrome occurs when a large amount of worms reside in the right atrium, ventricle, and vena cava. These worms stop the tricuspid valve in the heart from functioning properly and stop normal blood flow out of the right side of the heart. This eventually leads to cardiovascular collapse.

Dogs are graded into classes based on the severity of clinical signs.

These classes are I-IV in escalating order:

Diagnosis of Heartworm Disease

Antigen testing is the most readily available testing for veterinary professionals. This test has a quick turn around and a high sensitivity and specificity for the disease. Dogs should not be tested when less than 7 months of age, as the lifespan of the heartworm has not had time to complete. Dogs should be tested annually for heartworm disease, even if on monthly heartworm preventatives.

Other testing options include looking for microfilaria on a microscope slide of whole blood. 20% of infected dogs will not have circulating microfilariae, therefore antigen testing should still be completed in all dogs.

Echocardiogram can be used to visualize adult worms in the heart and to evaluate for evidence of pulmonary hypertension. Pulmonary hypertension is increased blood pressure in the lungs and can be seen in severe cases of canine heartworm disease.

Chest radiographs are recommended in all dogs showing clinical signs of heartworm disease. These radiographs are used to evaluate the size of the heart and pulmonary artery.

Blood work can be useful to evaluate for secondary organ dysfunction and used to monitor for trends during treatment.

Treatment of Canine Heartworm Disease

Treatment of canine heartworm disease is based on severity of clinical signs. If a patient is showing signs of stage III or severe caval syndrome, stabilization with oxygen therapy, supplemental oxygen support, blood transfusions if anemic (low red blood cell count), and administration of corticosteroids to reduce inflammation may be used to treat the condition. In severe cases where caval syndrome is present, surgical removal of the heartworms is recommended. Care must be taken to minimize damage to the worms during removal, as an allergic reaction can ensue.

Any treatment runs a risk of pulmonary thromboembolism of worm parts. One of the extremely important parts of the treatment plan is strict exercise restriction. This decreases the chances of a pulmonary thromboembolism.

Caval Syndrome Treatment

Treatment for dogs that are not showing signs of caval syndrome include:

This is a treatment plan outlined by the American Heartworm Society, but you should always follow the advice of your veterinarian, since they may recommend modifications based on your individual pet. Not all dogs may be recommended to follow this exact plan, but all dogs should practice strict exercise restriction during treatment.


Prevention of heartworm disease is simple. In most cases, a once-monthly prescription tablet or topical treatment is all that is needed to effectively protect your pet. Some heartworm preventatives also control intestinal or external parasites. The wide range of excellent and safe heartworm prescription products can be explained by your veterinarian. Recently, there has also been an FDA injection that can be given every six months or once yearly. Using these injections minimizes need for monthly medications. Flea and tick preventatives still need to be given monthly, since they are not covered with these injections.

These preventatives are only available from your veterinarian, who must first make certain that your dog is not heartworm positive. These “preventatives” kill microscopic larvae that are left behind by mosquitoes when they bite a dog. Before beginning heartworm prevention, any dog over 7 months of age should first have a heartworm test. Preventatives in heartworm positive dogs can cause severe reactions. Repeated heartworm blood testing every year is recommended, even for dogs taking heartworm preventative year round.

Previous recommendations were for testing every 1 – 3 years, but the American Heartworm Society (AHS) changed their recommendation to once yearly in 2005. This is due to concerns of pets contracting heartworms during breaks from their preventative medication. By testing annually, you can guarantee that an infection is caught before it becomes unmanageable.


The AHS recommends that all dogs in areas endemic for heartworms should take a year-round preventative. If you are not certain about the danger of heartworms in your area, call your veterinarian. Most veterinarians follow the guidelines published by the American Heartworm Society, a group of concerned veterinarians and scientists. As noted above, dogs over 7 months of age should first have a heartworm test.
For more information about the most recent recommendations on heartworm prevention, visit the guidelines posted on the Society’s website.