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An Overview of Heartworm Disease in Dogs

By:  Risa Roland, DVM, DACVIM (Cardiology)

Veterinarians often recommend preventative measures for heartworm disease in dogs. Heartworm disease can be detected by blood tests, as well as imaging such as ultrasound. Heartworm disease can be life threatening, and treatment can be difficult, uncomfortable for the dog, and costly.

How do dogs get heartworm?

Heartworm disease is caused by a parasitic organism called Dirofilaria immitis. An early or larval stage of this organism is transmitted to a dog by a mosquito bite. Once a mosquito infects a dog with the larval stage of the heartworm, it takes 6 to 7 months for heartworm larvae to grow and mature into adult worms. The adult worms mate, and then female worms release their offspring or microfilariae into the dog’s bloodstream, ready for another mosquito to ingest by biting the infected dog, and then the cycle starts again.

As a result, while heartworm is not directly contagious between dogs, it can spread from dog to dog by mosquito bites. Therefore, dogs everywhere are at risk of contracting this disease, although it tends to be more common in more humid and warm climates.

What are the symptoms of heartworm?

Early in the disease, dogs with heartworm are usually asymptomatic. This is why it’s important to follow guidelines for testing. Once a dog develops symptoms, the disease and treatment are more serious. Many heartworm positive dogs are detected on routine annual screening, allowing the veterinarian to treat the dog more safely before symptoms start.

Symptoms of heartworm disease include a cough, weakness, trouble breathing, decreased appetite, abdominal distension, and collapse. Severity of these symptoms are dependent how long a dog has had heartworm and on the number of adults worms they have.

How is heartworm treated and prevented?

Heartworm treatment is based on a dog’s condition at the time of treatment. A physical examination, bloodwork, and X-rays help a veterinarian determine the safest and most successful mode of treatment. In general, treatment includes a serious of injections in the back and oral medication to increase successful outcomes and decrease pain and complications. Treatment also includes use of topical and oral products to prevent transmission to other dogs by killing the early form of microfilariae. In general dogs need to be observed and possibly hospitalized during parts of treatment, to be monitored for life threatening reactions to the injections and dying worms. Once treatment is started, a dog’s activity must be strictly limited for an extended period time.

It’s important to note that in very advanced stage heartworm disease in dogs, the only possibility to save a dog is by manually removing adult heart worms by surgical means.

The best way to avoid heartworm disease and the painful, serious, and costly treatment is with year-round monthly preventatives and yearly testing as recommended by a veterinarian. By following the veterinary guidelines, a dog can be kept healthy and protected from this serious disease.