Heartworm: History and Prevention

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 that the microfilariae can mature into larvae.

If it’s warm enough, it takes just two weeks for the larvae to be ready for the next mammalian host. As the mosquito sucks blood, larvae charge down the salivary duct into the host’s body. The host could be a dog, 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 heartworms 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 scruffy coat) to suggest 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
Advances have been made considerably in the last 20 years in testing for heartworms in dogs. Previously, veterinarians looked for microfilariae on blood smears under a microscope. Writhing worms magnified 400 times look impressive, and it is certainly a positive diagnosis of the disease, but 20 percent of heartworm-infested dogs do not have circulating microfilariae . This means the infestation can be overlooked.

The modern screen test, which is done in-house at Animal & Bird Medical Center during a dog’s annual exam with the doctor, 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. It takes about seven months for the tiny, microscopic larvae to develop into adults. This means the test won’t be effective until at least seven months after the last period of heartworm transmission. Testing puppies less than seven months of age is not done because no adult heartworms will be found.

Factors that Make Heartworms a Threat to Your Dog
Three factors are necessary for heartworm to become a threat to your dog: mosquitoes carrying the heartworm, the right temperature, and other infested dogs in the area. Domestic dogs and wild canids (coyotes, for example) are all hosts for heartworm. There are also many mosquito strains capable of transmitting heartworm. 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. Florida has such a mild climate that dogs, cats, and their human families must be on guard year round against mosquitoes and heartworms and continue using heartworm prevention even when their northern U.S. cousins can stop for the winter.

Treatment of Heartworm Infestation: Costly and Complicated
Dogs infested with heartworms will have to endure a complex treatment, which is expensive. The veterinarian administers an insecticidal drug that will 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. There are follow up visits for additional treatments, plus testing after treatment is completed to confirm no microfilarae survive. By comparison, using a monthly prevention, such as Revolution or Interceptor, is much easier for the dog and less costly to the owner.

Why a Test is Necessary Even When Your Dog is on Prevention
Once-a-year testing is necessary to make sure no heartworms (or microfilariae) are present in your family dog. The medications used to prevent heartworm development are excellent; however, no drug is 100 percent effective every time. Think of it like the human flu vaccine. Despite getting vaccinated, you could still potentially come down with the flu. For your dog’s protection, a yearly test is necessary so we can continue your dog’s heartworm preventive without causing harm.