What Are the Causes of Heartworms in Dogs?

"Keep those mosquitoes away from me."

"Keep those mosquitoes away from me."

Nasty parasites, heartworms take up residence in your dog's heart and lungs. They're spread by mosquitoes that enjoy feasting on your dog. If you live somewhere you hear the buzz of a mosquito, your pup's at risk. You can prevent heartworm infection in Chip, although if he's a collie, his prevention plan may be different from other breeds.

From Mosquito to Heartworm

It's pretty easy for a dog to get infected with heartworm; all it takes is a bite from a mosquito infected with heartworm larvae. If Chip's bitten by an infected mosquito, larvae will find their way into Chip through the tiny bite wound. Chip's now playing an unknowing host, where the larvae continue to develop. It takes anywhere from two to three months for the larvae to work their way to your dog's heart.


Heartworms continue to grow and live in Chip's heart, lungs and nearby blood vessels, where they mate and release their offspring, known as microfilaria, into his bloodstream. It takes about seven months after infection for a heartworm test to detect their presence. Symptoms that Chip may be infected range from a light cough in mild cases of infection, to coughing, difficulty exercising and abnormal lung sounds in moderate cases. Signs of a severe case of infection include coughing, difficulty breathing and exercising, temporary loss of consciousness, abnormal heart sounds and an enlarged liver. The worms live their lives in Chip's heart, usually for around five to seven years.


Blood tests, sometimes supplemented with X-rays and ultrasounds, confirm the presence of heartworms. Treatment tends to be successful, with the goal to kill the adult worms and microfilaria. The more severe the infection, though, the higher the possibility of mortality. Other physical conditions can affect the course of treatment for a heartworm-infected pup. When the adult worms are killed with an adulticide, it's important to keep Chip very quiet. After the adult heartworms are eliminated, the microfilaria are killed. About four months after the adulticide treatment, an infected dog is re-tested to ensure a second treatment isn't necessary.

The Best Medicine

The best plan is to keep Chip from being infected by heartworm. It's not possible to guarantee a mosquito won't find him alluring -- particularly in the Deep South, where the insects are a year-round problem. The best thing to do is to use a preventive prescribed by your vet. Chip will have to have a simple antigen test first to ensure he's hasn't been infected, but after a negative result, it's a simple matter of giving him his medicine once a month.


Choices including Heartgard, Heartgard Plus, Interceptor and Revolution, available in tablet form or as a liquid applied to his skin. In addition to preventing heartworm, some brands also control other parasites such as roundworm and hookworm. If you live in a place where mosquitoes are a year-round problem, Chip should get his medication year-round. Regardless, he'll have to have an antigen test annually to ensure he's negative.

The Collie Difference

If Chip is a border collie, rough or bearded collie, or sheltie, he may have a bad response to some preventives. According to the American Working Collie Association, up to one in three collies has a genetic defect resulting in a fatal reaction to ivermectin, the medication used in common heartworm preventives such as Heartgard. Don't despair -- there's a test to determine if Chip will be vulnerable to ivermectin. It's as simple as swabbing his cheek. Milbemycin oxime, used in Interceptor, is a safe alternative for collies and related dogs. Discuss the best option with your vet.

About the Author

Betty Lewis has been writing professionally since 2000, specializing in animal care and issues, business analysis and homeland security. Lewis holds a bachelor’s degree in journalism from West Virginia University as well as master’s degrees from Old Dominion University and Tulane University.

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