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Don’t Break My Heart – Heartworms

Happy Valentine’s Day! In honor of this holiday, we are going to write about protecting your dog’s heart from heartworms. Heartworms are parasitic worms that live in the lungs and heart of dogs. Dogs get infected with heartworms by getting bitten by an infected mosquito. Indoor dogs are at risk because mosquitoes can come indoors. When a mosquito becomes trapped in doors, your dog becomes one of the only sources for a blood meal.

Heartworm Life Cycle

Female heartworms living inside an infected dog release microfilariae (heartworm babies) into the dog’s bloodstream. When mosquitoes feed on this infected dog, they pick up the microfilariae. The microfilariae continue to mature inside the mosquito and become infective larvae. When the mosquito bites another dog, the infective larvae enter through the mosquito bite wound. Once in the dog, the worms take 6 months to mature into adult worms. Adult worms can live up to 7 years. Since the immature worms are only susceptible to the medicines used for prevention for a limited amount of time, heartworm preventatives must be given monthly. Monthly preventatives actually work by killing the immature heartworms already in the dog to prevent the adult heartworms from reaching and damaging the dog’s heart.

Clinical Signs

Signs of heartworm disease depend on the number of worms present and how long they’ve been in the dog. Signs include a persistent cough, reluctance to move or exercise, tiring easily during exercise, poor appetite, and weight loss. Heartworm disease can cause death.


Heartworm disease can be prevented by giving a monthly heartworm preventative. These medications kill the immature worms before they reach the heart and lungs.


Once a dog has adult heartworms inside its heart and lungs, treatment (performed by a veterinarian) involves injecting the dog with an adulticide (a medication that kills adult worms). These dogs must be kept calm for 6 months as they recover since the worms can dislodge and block blood flow. The dying worms can also cause a life threatening allergic reaction (anaphylaxis). This is why patients are usually hospitalized for adulticide treatment and treated with other medications to prevent complications.

In order to keep your dog’s heart safe from these parasites, the best thing to do is to use monthly preventative. Prevention is the best medicine! We will blog about heartworm disease in cats in a future post.

Happy Valentine’s Day!

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