Spring Brings Heartworm

The District Vet – May 2017


Spring is in the air. Winter has turned from a persistent gray to an emerging green. As the temperatures warm, we all begin to stir, as do mosquitoes and their parasites. We veterinarians stress the importance of using heartworm preventives, but few clients know the lifecycle and basics about this preventable, potentially deadly disease.

Heartworm disease is caused by foot-long worms that live within the right side of the heart, lungs, and associated blood vessels of affected pets, leading to lung disease, heart failure, and damage to other organs. The disease primarily affects dogs but can be carried by other mammals such as coyotes, foxes, and wolves. Cats can be infected with heartworms but do not enable the heartworm to continue its lifecycle (more about this in a bit).

Dogs are the natural hosts for heartworms. Adult female heartworms living within a dog, fox, coyote, or wolf produce immature heartworms called microfilariae, which are released into the bloodstream of the host animal. The microfilariae are then picked up by a mosquito when it feeds off the infected animal, and over a period of nearly two weeks the heartworms mature and become infective. When the mosquito feeds again on a susceptible animal, the infective larvae are injected into the skin through the bite wound. After entering their new host, the larvae migrate through the skin, slowly mature into adults, and migrate into the right side of the heart and the main pulmonary arteries. This process of maturation takes approximately six months. Once in the heart, the worms begin to produce new microfilariae and can live as long as seven years.

Most dogs do not immediately show signs of heartworm disease. This is why so many rescue dogs from the South arrive with active heartworm infections: they look normal. Clinical problems such as a persistent cough, decreased ability to exercise, weight loss, poor haircoat, and lack of appetite may emerge. In more severe cases, heart failure occurs, leading to fluid accumulating in the abdomen or a condition where the worms block flow within the heart, leading to cardiovascular collapse, a condition called caval syndrome. Few dogs survive once heart failure or caval syndrome are present.

Heartworm infects cats, but the disease progression is different. Heartworms migrate within the cat and may mature, but not necessarily. Heartworms are not capable of reproducing within cats but they still may cause significant heart and lung disease.

Cats usually harbor only a few adult worms, but any heartworm infection may lead to coughing, asthma-like attacks, heart failure, or even sudden death. Cats may sometimes eliminate the infection on their own, but usually there will be lasting effects on the lungs. Since cats are not the definitive host for the parasite, sometimes the worms become confused when migrating through cats and can end up in the eye, brain, or even spinal cord.

Remember that heartworm is transmitted by mosquitoes. Therefore any pet is susceptible, even indoor dogs and cats. As the global climate warms and people bring more dogs up from the deep South, heartworm has been increasing its range and prevalence across the country. While the immediate area of the city does not have a large stray dog population, it does have Rock Creek Park and areas of woodlands that support foxes and coyotes. Yes, coyotes have been seen within Capitol Hill and other areas of the city.

We test dogs annually for heartworm disease. It is simple, requires only a few drops of blood, and is a highly accurate, affordable test. In dogs the test looks for proteins of adult heartworms, so immature infections may go undetected. This is why many veterinarians test rescue dogs again six months after adoption: the initial test may be negative, but the followup may be positive since the worms had time to mature.

Testing in cats is performed when there is a suspicion of heartworm disease, but the specific tests are a bit different. Remember that cats may have few or no adult worms within them but still have wayward immature worms.

Annual testing is necessary in dogs even if a preventive is given year-round. This is done to be certain that the preventive is working and that no doses were missed or vomited or spit out. One missed dose can leave your dog susceptible.

Preventing heartworm in dogs and cats is safe, simple, and effective. A once-monthly preventive, either by mouth or topically, can be used to stop heartworm. The topical formulation most popularly used is called Revolution. It also prevents fleas and is our preferred method of control in cats. For dogs we use an oral preventive such as Interceptor Plus or HeartGard Plus. These also prevent a number of intestinal parasites. We recommend keeping dogs and cats on a preventive year-round.

If you have any questions or concerns about heartworm disease, please feel free to talk to us or your regular veterinarian. Have a happy, healthy, heartworm-free spring.

Hill resident Dan Teich, DVM, practices at District Veterinary Hospital, 3748 10th St. NE, www.districtvet.com.