Heartworm Infections In Dogs – Causes, Symptoms, Treatment

What is a heartworm?

The heartworm is a parasite called Dirofilaria immitis that is spread via mosquito bites. It can infect dogs, cats, other hairy animals, and, in rare cases, humans. The parasite is a small, thread-like worm (roundworm) that enters the bloodstream and resides in the heart & lungs, causing a disease called filariasis.

Heartworm infections, once common only in the southern United States, has been reported all 50 states. The highest rates of infection occur along the coast from Texas to New Jersey, and along the Mississippi River – hot, humid areas where the mosquito thrives. The infection is also prevalent in the warmer regions of Canada, southern Europe, the Middle East and most of Asia.

Heartworm Incidence; from American Heartworm Society

Important note: humans cannot get heartworm infections from dogs. A human must be bitten by an infected mosquito for this to happen, and, even then, it’s very rare for humans to develop this infection.

Signs & symptoms of heartworm infection

The severity of symptoms correlates directly with the number of heartworms in a dog’s body, along with the duration of the infection and the dog’s immune response to the infection. From the time a dog is infected (bitten by the mosquito), it takes about 6 months for the worms to mature and and start causing signs & symptoms of disease. A mature heartworm can measure up to 12 inches, and will soon start reproducing.

The first symptoms to appear are coughing and easy fatigability on heavy exertion, these symptoms will be mild and easy to miss.  If untreated, the coughing and fatigability will progress and will start to manifest with even mild activity. Weight loss, difficulty of breathing and an enlarging, fluid-filled abdomen (from heart failure) will follow. End-stage disease involves a large mass of worms blocking the blood vessels to and from the organs, ultimately causing death from blocked blood vessels in the heart.

Uncommon symptoms may occur if the worms migrate to other parts of a dog’s body, such as the eye, brain or leg, causing blindness, seizures or limping.


Diagnosis of & screening for heartworm infection

Diagnosis is confirmed only by blood test. A blood sample will have to be drawn by your vet and checked under a microscope for presence of worms or their byproducts. Once the diagnosis is confirmed, other tests can be done for prognosis and monitoring the severity of the infection, including X-rays, ultrasounds and more blood tests for kidney & liver function.

Screening is recommended for all dogs once every 12 months. Although heartworm infections are more common in certain areas than in others, the fact is that the risk for infection is present in all areas, and all dogs are at risk.

Heartworm Testing Algorithm; from American Heartworm Society

Treatment of heartworm infection

Heartworm infections are treatable. The treatment is somewhat long (about 3 months), and may cost $300-1000 (includes testing), but the treatment options are available and effective. A dog with mild disease has excellent chances of survival; dogs with more advanced disease can still be treated successfully, but the chances of treatment complications are also higher.

The treatment occurs in two phases. The first phase is to kill the adult heartworms with 3 injections over the course of a month. The second phase starts a few weeks afterwards, to kill the smaller, immature worms.

Activity restriction during and up to several weeks after treatment will be absolutely necessary in most cases. During this time, the dead worms (and the by-products they release) are still in the dog’s body, occupying his heart & lungs and blocking his blood vessels. This will cause a strong immune response in the dog’s body, and any significant exertion may cause catastrophic complications for the dog. Your vet will be able to tell you what activities are allowed during this period (short walks, potty breaks) and when he can resume normal activities. You should also be prepared for the following possible recommendations from your vet:

  • preparatory treatment with antibiotics or steroids prior to the two treatment phases
  • remaining in the hospital for short observation periods (during the first phase of treatment)
  • surgery for advanced disease

Re-testing blood samples should be done 6 months after treatment is completed. If the tests turn up negative, the treatment is successful. If the tests show the presence of worms, repeat treatment may be necessary.


How to prevent heartworm infection in your dog

Heartworm infections are preventable. The drugs available to prevent heartworm infections in dogs are highly effective. They may come in the form of chewable pills, injections or topical forms. All at-risk dogs will need these preventive treatments regularly, especially those living in warmer areas or any area with high rates of heartworm infections. Even dogs who have previously been treated for heartworm infections will need preventative treatment, as they can get re-infected.

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