How Do Dogs Get Heartworms?

By Jane Meggitt

Dogs get heartworm disease via a bite from an infected mosquito. The microfilariae, or baby worms, spread and mature within the dog. If your dog is diagnosed with heartworm disease, don't despair. Treatment is available, although it requires a good deal of supportive care. Dogs with untreated heartworm often succumb to the infestation.

Mosquito Transmission

Dirofilaria immitis, a worm parasite, is responsible for canine heartworm disease. Transmission begins when a female mosquito consumes a blood meal from an infected canid -- including dogs, foxes, coyotes and wolves -- ingesting the baby worms, or microfilariae. These microfilariae continue developing for the next 10 to 30 days within the insect, eventually entering its mouth. When the infected mosquito bites a dog, the microfilariae enter the animal's bloodstream. It takes microfilariae approximately six to seven months to reach adulthood and start producing their own offspring within the dog's body.

Heartworm Growth

Grown heartworms migrate to the animal's heart and nearby blood vessels. A badly infected dog might have hundreds of heartworms in his body, with an individual female worm maturing between 6 and 15 inches long and 1/8 inch wide. Male worms are smaller. The female offspring circulate throughout the bloodstream, congregating in the small vessels.

Heartworm Symptoms

Many heartworm infested dogs appear perfectly fine until the disease is in its latter stages. In such dogs, sudden death might be the first sign of any problem. Other dogs with heartworm infestation might cough, vomit, experience breathing difficulties or become tired easily. An infected dog might lose his appetite, subsequently dropping weight. As the infection worsens, a dog might develop congestive heart failure, with the tell-tale sign of abdominal swelling due to fluid accumulation.

Heartworm Prevention and Treatment

Your vet tests a blood sample from your dog for the presence of microfilariae. If there's no sign of infection, your vet prescribes a monthly, oral heartworm treatment for your dog. You must bring your dog in for an annual visit to ensure he hasn't picked up heartworms, often the result of a missed dose. If your dog is heartworm-positive, your vet can treat your dog through a series of injections designed to kill the worms. During his treatment, it's important to confine your dog and keep him as quiet as possible -- not always an easy task, especially with a young, active canine. That's because the dead and dying worms can set off serious reactions, which are more likely to occur after exercise or any sustained activity.