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Canine Heartworm Disease


How does a dog get heartworms?

Heartworms are spread to dogs by mosquitoes. A mosquito carrying infective heartworm larvae bites your dog and within 6-8 months, if your dog is not on heartworm preventative, he or she is very likely to have adult heartworms.

Adult heartworms reproduce and release their offspring (microfilariae) into the dog’s bloodstream. A mosquito bites a heartworm positive dog and picks up some microfilariae in its blood meal. In the mosquito host, these microfilariae mature into infective larvae which can enter the bloodstream of the next dog bitten by this mosquito and the cycle continues.

The number of worms that develop after infection depends upon the number of larvae that enter the dogs bloodstream with the mosquito bite. Dogs may have one worm or in very severe cases up to 250. If untreated, most dogs will eventually suffer heart failure and death.

Administration of monthly heartworm preventative (obtained through a licensed veterinarian) prevents the maturation of larvae to adult heartworms and is very effective at preventing canine heartworm disease.

The most commonly used test is a blood test that measures the level of a specific antigen or worm protein (from female worms) in the dog’s bloodstream. The test takes about 15 minutes to perform and is very sensitive. Since it takes 6-8 months for adult heartworms to mature after a dog is infected through a mosquito bite, this test is performed on adoptable dogs over 8-9 months old at the Cuyahoga County Animal Shelter.

How is canine heartworm disease treated?

Heartworm-positive dogs can be successfully treated if they show minimal signs of cardiac damage at the time of diagnosis. Dogs with advanced heartworm disease as evidenced by pronounced coughing and congestive heart failure are not candidates for treatment.

Heartworm treatment is a two-step process: (1) an adulticide is administered to kill the adult heartworms and (2) a microfilaricide is administered to kill the immature heartworms. The adult worms are killed by injection of an arsenical compound directly into the back muscle. The only drug approved by the FDA to kill adult heartworms is Immiticide (Merial). Depending upon the perceived severity of the infection, a dog may receive one Immiticide injection and then after a 30-day waiting period, receive two more Immiticide injections 24 hours apart. Less severe infections may be treated with the two injection protocol. It is crucial that the dog be kept as quiet as possible during the 30-days after the Immiticide injection to minimize the risk of pulmonary embolism from the dislodged and dying worms. A microfilaricide such as Heartguard (ivermectin) or Interceptor (milbemycin) is administered to kill the immature heartworms. It takes about 6 months for the worm proteins to clear the bloodstream so a retest 6 months after the last Immiticide injection should be done to confirm eradication of adult heartworms. Once the heartworms have been eliminated, treated dogs MUST remain on heartworm preventative their entire lives. These dogs go on to live normal, active lives.

Our experience at the Cuyahoga County Animal Shelter

The incidence of heartworm positive dogs has increased dramatically in the last several years. In 2007, about one dog per month tested positive for heartworms. This increased to about one dog every week or two in 2008 and already in 2009, there have been two or more dogs testing positive per week. It is imperative that all dogs in the community be on heartworm preventative to help stop the spread of infection.

Treating dogs for heartworm is a very expensive and lengthy process. Given the increase in heartworm positive dogs, this has become a major focus of the shelter’s Best Friends’ Medical Relief Fund. You can help by making a donation to the fund or providing a foster home for a dog going through heartworm treatment.

If you’d like to learn more about canine heartworm disease, please visit the website of the American Heartworm Society at

Heartworm Survivors