Heartworm Disease in Dogs Symptoms, Cause, Treatment and Prevention

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A dog at the beach
A Dog At The Beach

Heartworm is one of the most well known diseases contracted by man's best friend. In the past, vets prescribed preventative medication only to owners residing in areas where mosquitoes were prevalent. Today, heartworm disease exists in all 50 states and globally around the world. Some dogs may test positive for heartworm disease but show no signs of being ill. It's important to have your dog tested regularly because heartworm disease, when untreated, almost always leads to death.

What Is Heartworm Disease?

Heartworm is a potentially deadly parasitic disease of dogs transmitted by mosquitoes. Female mosquitoes ingest heartworm larvae when they bite an infected dog. The larvae mature within the mosquito for up to one month. Larvae are innoculated into different dog when the female mosquito bites again. The larvae mature further within the skin. Eventually, after several months, they enter the bloodstream where they are carried to the pulmonary arteries, the large blood vessels exiting the heart, where they mature into adults. Approximately 6-7 months after initial innoculation, adult worms reproduce and begin shedding larvae into the bloodstream. The cycle repeats itself when a female mosquito bites the infected dog and ingests the newly formed larvae.

Complications of heartworm disease occur secondary to the worms causing a blockage within blood vessels surrounding the heart. Caval syndrome is a particularly severe form of heartworm disease where the heartworms fill the entire right side of the heart and back up into the vena cava.

Signs of Heartworm Disease in Dogs

There are 4 stages of heartworm disease in dogs. During the first stage, dogs show no signs or experience only a mild cough. Dogs with stage 1 disease test may or may not test positive for heartworm. The show no abnormal clinical, radiographic, or laboratory signs. In stage two, the dog's cough becomes persistent and it experiences fatigue and exercise intolerance. By stage three, a dog's health is severely compromised. Cough and fatigue continue. Your dog may refuse to exercise and it may even cough up blood. X-rays performed during stage three should show significant signs of heartworm infection. By stage four, if heartworm is left untreated, the disease can lead to death due to heart failure or other organ damage.

Causes of Heartworm Disease

Dogs contract heartworm through the bite of an infected mosquito. As the larvae mature into adult heartworms they cause progressive blockage of the pulmonary artery, leading to pulmonary hypertension and inflammation within the lungs. Progression of these changes leads to enlargement of the heart and heart failure. Heartworm infestation can also cause liver and kidney damage.

In addition to the worm itself, a rickettsial organism known as Wolbachia coexists as a parasite of the heartworm. It is believed that Wolbachia may provide a type of protection for the adult heartworm and may contribute to the inflammation in a dog's lungs that results when heartworms die off.


Treating canine heartworm disease involves killing the adult worms that live in the heart and pulmonary arteries, as well as those in the larval stages (called microfilaria) that circulate in the bloodstream. As with any other disease, formulating a treatment plan begins with a thorough examination of the dog's overall health and condition.

Treatment for heartworm involves the administration of two medications. Melarsomine—administered through a series of injections—eradicates adult worms, and ivermectin kills the microfilaria (larval stage). Additionally, a vet may prescribe antibiotics or steroids, depending on the stage and severity of infection. As melarsomine is injected deep into the muscles of the back, pain medications are often given concurrently with the injections to reduce the level of discomfort for the dog.

close-up photography of tricolor dog
close-up photography of tricolor dog

There are two different melarsomine injection protocols. The first protocol is reserved for dogs that are otherwise healthy and void of significant signs related to heartworm disease. In this protocol, two injections of melarsomine are administered 24 hours apart. The second protocol entails three injections of melarsomine—an initial injection followed one month later by two additional injections given 24 hours apart. The three-dose protocol has decreased complication rates and increased safety compared with the two-dose protocol.

The slow kill method of heartworm treatment involves administering monthly heartworm preventive medications (usually ivermectin-based) and waiting for the adult worms in the heart to die a naturally. This is recommended in cases where the dog is not a candidate for treatment with melarsomine or where financial constraints come into play. Adult worms can take up to two years to die and are still capable of causing damage to the heart and lungs during that period.

Dogs with caval syndrome are generally very ill and the only successful treatment is the physical removal of the worms from the heart.

Home Care

The most important thing you can provide for your dog during heartworm treatment is complete rest. During the recovery period, embolism (blockage of blood vessels) by dead or dying worms is a major concern. Exercise increases the risk of embolism, thereby increasing the risk of serious side effects or death. So, strict confinement is essential during the treatment period and for at least one month following the last injection of medicine.

black and white boston terrier
black and white boston terrier
white and brown bulldog
white and brown bulldog
dog staring left
dog staring left

In addition to inducing a sedentary state, dog owners treating their pet for infection should provide high-quality food and water and be sure to carefully follow the directions on any prescribed medication. After the treatment course is over, heartworm preventive medicine is continued.

How to Prevent Heartworm Disease