Heartworm: History and Prevention

Jeff Grognet is a veterinarian with a practice in British Columbia (Canada) and is vice president of the British Columbia Veterinary Medical Association. He is a frequent contributor to the AKC GAZETTE.

Copyright the American Kennel Club, Inc., 2008. No portion of this article may be reproduced without permission of the copyright holder. Reprinted with permission from AKC Family Dog, July/August 2008. To subscribe:http://www.akc.org

A century ago, when veterinarians
found 12-inch-long, spaghetti-like worms in dogs’ hearts, they couldn’t help but call them heartworm.That’s all they knew about the condition: They
didn’t know where it came from and certainly had no way to treat it.

Today’s veterinarians are much better educated about this disease. Most importantly, they now know how to prevent it. But they still find dogs with the
parasite. The reason: Some dog owners have a poor appreciation of the seriousness of heartworm and don’t put their dogs on preventives. Their dogs
become infested and, besides the threat to their own lives, put other dogs at risk.

Contrary to what the name suggests, heartworms prefer to live in the pulmonary arteries leading from the heart to the lungs rather than in the heart itself.
The worms bathe in and feed on the blood, reproduce, and release thousands of microscopic young (microfilariae) into the bloodstream. These drift along,
waiting for a mosquito to take a blood meal and pick them up. It’s only inside a mosquito that the microfilariae can mature into larvae.

If it’s warm enough, it takes just two weeks for the larvae to be ready to infest the next mammalian host. As the mosquito sucks blood, the larvae charge
down the salivary duct into the host’s body. The host could be a dog, a cat, or a human. (A heartworm larva in a person usually ends up in the lungs,
where the body creates a cyst around the invader. This does not cause a medical problem, but it can be mistaken for lung cancer.) In dogs and cats,
the larvae begin a six-to-seven month migration as they change into adult heartworms. Adult worms can live up to seven years.

Heartworms in dogs don’t physically block the flow of blood. Instead, they create inflammation in the arterial wall, which thickens and thereby disrupts
blood flow, making the heart work harder. Once blood flow slows sufficiently, a heartworm-infested dog develops a mild, persistent cough, may become
fatigued after only mild exercise, and suffers from a reduced appetite. The result can be heart failure.

Though veterinarians look for typical signs (such as weight loss, poor muscling, and a scurfy coat) to suggest a heartworm infestation, most dogs harboring
this parasite do not have clinical symptoms before the worms are detected by screening tests conducted prior to administering preventives.


Heartworm testing has advanced considerably in the last 20 years. Back then, veterinarians used to look for microfilariae on blood smears under the microscope.
Writhing worms magnified 400 times look impressive, and it is certainly a positive diagnosis of the disease, but 20 percent of the heartworm-infested
dogs did not have circulating microfilariae. This means the infestation can he overlooked.

The modern screening test, which is often done in-office during routine veterinary check-ups, detects heartworm antigen (minute amounts of cuticle from
the surface of the adult parasite). It is so sensitive that it can detect a single worm in a dog’s body. But because the test can only detect the presence
of adult heartworms, the timing of the test is extremely important. Remember that it takes up to seven months for the minute larvae to develop into
adults. This means the test won’t he effective until at least seven months after the last period of heartworm transmission. There is also no point
in testing puppies less than 7 months old, even if they were born in heartworm season.

Three factors are necessary for heart, worm to become a threat to your dog: other infested dogs, mosquitoes to carry the parasite, and the right temperature.
If any one of these three is missing, heartworm is not a problem.

Domestic canines and wild Lids (coyotes, foxes, wolves) are all hosts for heartworm. There are also many mosquito strains capable transmitting heartworm.
But even when the first two requirements are satisfied, the third temperature-can stop transmission.

For heartworm larvae to mature in a mosquito, the temperature must remain above 57 degrees F. The period for a microfilaria to mature into an infective
larva inside a mosquito is about 29 days at 64 degrees, and 8 days at 86 degrees. Thus the occasional emergence of overwintering mosquitoes on warm
winter days does not represent a risk of infection.

Most areas of the United States are too cold in the winter for heartworm larvae to develop. In most of the country, heartworm transmission is probably
limited to no more than six to eight months.

For dogs infested with heartworm, treatment is complicated and expensive. Veterinarians administer an insecticidal drug to kill the worms. The dog must
be kept quiet during treatment to decrease the risk of dying worms blocking blood flow to the lungs and triggering heart or breathing problems.

Fortunately, there are many extremely effective options for heartworm prevention (see sidebar). These include daily and monthly tablets and chewables,
and monthly topicals. All these medications interrupt development of the larvae and prevent them from maturing into adult worms. It is essential that
any of the preventives be administered according to the prescribed schedule, but otherwise it is a fairly simple matter nowadays to keep your dog healthy
and safe from heartworm.

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