Reviewed and fact checked by Bruce Whittle, DVM
Just the name heartworm disease sounds like a bad thing. Canine heartworm disease is caused by a parasite named Dirofilaria immitis that is spread from dog to dog by mosquitoes. Dogs that live in southern climates usually have a greater exposure to heartworms because mosquitoes may be present throughout the year. However, if you travel with your dog, you may be exposing them to this nefarious parasite if you travel to an area that has a higher incidence of heartworms even if you live in an area where they are typically not found.
What is the life cycle of the canine heartworm?
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Adult heartworms in a dog produce immature worms called microfilaria
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A mosquito feeding on the blood of the infected dog consumes these microfilaria
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Microfilaria molt twice within the digestive tract of the mosquito before becoming infective to other dogs (10 to 14 days)
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The mosquito regurgitates infective microfilaria into the tissues of their next victim
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The microfilaria mature into adult worms up to 12 inches in length in about 6 months and are then capable of producing microfilaria and can serve as a source of infection at that time
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Adult heartworms can live in a dog for at least 5 years
What are some signs my dog may be infected with heartworms?
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No signs in early cases
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Persistent coughing
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Exercise intolerance – can’t run as far or play as long as normal
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Decreased appetite and/or weight loss
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Signs of heart failure (swollen abdomen, pale gums, labored breathing) in advanced or severe cases
How will my veterinarian diagnose if my dog is infected with heartworms?
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Direct blood smear – Examination of blood under microscope for microfilaria. This test will miss some infections when only adult heartworms are present
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Antigen test – Blood test that looks for proteins from adult female heartworms. This test will miss infections where all the worms are males or there are not yet adult female heartworms
How do heartworms cause disease in my dog?
Though this parasite is called the heartworm, it usually causes more damage in the lungs. Heartworm infection results in localized inflammation in the lungs and one of the common presenting signs in infected dogs is coughing. That is why veterinarians will recommend a heartworm test, in this case an antigen test that looks for adult heartworms, in dogs that come in with a complaint of cough to make sure this is not the cause. The severity of clinical signs often depends upon how many adult heartworms are present in the dog. If there are fewer than 5 adult heartworms present in a dog they primarily live in the large blood vessels of the lungs. When there are up to 40 or more adults, they end up living in the chambers of the heart, causing problems with heart valves. If worms die due to normal aging or during treatment, the dead worms can block off blood supply to segments of the lung similar to blood clots causing heart attacks in people.
How is heartworm disease treated in my dog?
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The only licensed treatment for adult heartworm infection is 2 or 3 injections of an arsenic-containing compound in the muscles of the back.
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Additional testing is often performed prior to treatment to determine severity as that will change some of the other treatments used to help prevent side effects.
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Treatment is often delayed for a couple months to first treat with a heartworm preventative medication that removes most of the microfilaria.
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Some dogs may have serious side effects from the treatment.
How can heartworm disease be prevented in my dog?
Heartworm disease can fortunately be more easily prevented than in the past. Several options are available for preventing a dog from become infected with heartworms:
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Monthly products given by mouth
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Monthly topical products
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Injection given every 6 months or once per year by a veterinarian
Heartworm preventative medications are available in a wide array of combinations and forms. Because all heartworm preventatives are only available on a prescription basis, it is important to discuss your individual dog’s needs with your veterinarian to determine which product will be best for him or her. Missing a dose of heartworm prevention can lead to a lapse in protection so it is crucial to make sure doses are given on schedule. You may not know if your dog vomits up her preventative medication and therefore misses a dose, so it is a good idea to periodically test for the disease as directed by your veterinarian.
Summary
Heartworm disease is a serious disease of dogs that kills many dogs annually. It was estimated that there were 1.1 million heartworm positive dogs in 2019. This number increases as many infected dogs from the south are relocated as a result of hurricanes thus serving as infection sources in areas of the country that typically have a lower incidence of disease. It is important to discuss heartworm prevention strategies with your veterinarian based on the risk of infection in your area or areas you travel to. Though canine heartworm disease is a very serious disease in dogs, we now have a variety of good tests and preventative medications with which to help keep our canine companions better protected.
