Heartworm Disease in Dogs

A dog at the beach

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Heartworm is one of the most talked-about illnesses contracted by man's best friend. For years, vets have prescribed preventative medication to owners residing in areas where mosquito infection is prevalent. Today, heartworm has been diagnosed in all 50 states. While some dogs may appear asymptomatic (testing positive for heartworm, but not sick), it's important to get them tested if you suspect infection. Heartworm, when left untreated, almost always leads to death.

What Is Heartworm Disease?

Heartworm is a parasitic disease that can be deadly, as, seven months after infection, adult worms lodge themselves into a dog's heart, lungs, and blood vessels.

Caval syndrome is a particularly severe form of heartworm disease where the heartworms fill the entire right side of the heart and spill out into the surrounding blood vessels.

Symptoms of Heartworm Disease in Dogs

The symptoms of heartworm present themselves in four stages. During the first stage, dogs can be asymptomatic or experience only a mild cough. Tests conducted at the vet can show negative results. In stage two, the dog's cough becomes persistent and it experiences fatigue, usually after exercise. By stage three, a dog's health is severely compromised. Cough and fatigue will 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 infection. By stage four, if heartworm is left untreated, the disease can lead to death.

Causes of Heartworm Disease

Dogs contract heartworm, usually seasonally, through the bite of an infected mosquito. These "roundworms" go through a series of developmental stages while inside their host, which include a larval stage where baby worms (or microfilaria) enter the dog's bloodstream. Throughout a six-months, microfilaria mature into adult heartworms, lodging themselves into the heart and lung and reproducing rapidly. At this point, the dog should show symptoms of the disease.

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 heartworms 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 the infection. And since 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.

Two protocols can be used for treating heartworm-infected dogs with melarsomine injections. The first protocol is reserved for dogs that are relatively healthy and void of significant heartworm symptoms. In this protocol, two injections of melarsomine are administered 24 hours apart. The second protocol is often recommended by veterinarians for use on all dogs, regardless of the stage of the disease. This treatment course involves three injections of melarsomine—an initial injection followed one month later by two additional injections given 24 hours apart.

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 natural death. This is only recommended in cases where the dog is not a candidate for treatment with melarsomine or where financial constraints come into play. In this scenario, 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 dying worms poses a major concern. Exercise increases the risk of embolism, thereby increasing the risk of serious side effects. So, strict confinement is essential during the treatment period and for at least one month following the last injection of medicine.

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 should be continued.

How to Prevent Heartworm Disease

Today's flea and tick collars also repel mosquitos. But since it's next to impossible to protect your pet from getting bitten, heartworm medications containing ivermectin are prescribed seasonally by vets to prevent the larva from completing their life cycle inside the dog. In addition to administering medication, regular blood tests are also recommended. And in some warm, humid climates, like the southeastern portion of the United States, heartworm medicine is given year-round, as the risk of infection is more widespread.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.