By Amanda Poldoski, DVM
Senior Consulting Veterinarian, Clinical Toxicology
Here in Minnesota we often joke that our State Bird should actually be the mosquito with as prevalent as they are in the spring and summer. Heartworm infection, however, is a serious disease and should not be taken lightly. One of the most important, cost-effective investments you can make for your pet is year-round heartworm prevention.
According to the American Heartworm Society, heartworm disease has been reported in all 50 states though it is more prevalent in some regions of the United States than others. Heartworm disease is caused by parasitic worms living in the arteries of the lungs and right side of the heart in dogs, cats, and other mammals.
The disease is spread between animals by mosquitos (the vector). First, the mosquito bites a heartworm-infected animal and becomes infected itself with microfilariae (young worms) while taking a blood meal. Over the next 10-14 days, the microfilariae mature into infective larvae inside the mosquito. Then, when the mosquito bites a dog or cat the infective larvae enter the animal via the bite wound. These immature worms grow and migrate through the body tissues to the lungs, reaching the pulmonary blood vessels within 3-4 months. Initially, blood flow forces the juvenile worms into the small pulmonary arteries. As the worms grow they occupy larger and larger arteries until they are fully mature. If a large number of worms are present they may also migrate into the chambers of the right side of the heart. As early as six months post-infection, dogs can develop patent infections where sexually mature heartworms mate and release microfilariae into the blood stream.
Pets should be tested annually for the presence of heartworms (yes, even those that are on year-round preventative!).
Clinical signs of heartworm disease can vary depending on the stage of infection and the number of worms present in the animal. In dogs, there may be no signs at all in the early stages of infection. As worms develop and the disease progresses signs can range from cough, exercise intolerance, fatigue, difficulty breathing, abnormal lung sounds and heart rhythm, collapse, enlargement of the liver, ascites (accumulation of fluid in the abdomen), and death.
Cats are considered resistant hosts of heartworm, but infection can still certainly occur (even in indoor cats!). Clinical signs in cats may be more subtle than in dogs and mimic other common feline conditions, like asthma or bronchitis. Signs may include intermittent vomiting, lethargy, anorexia, weight loss, coughing, difficult or rapid breathing, panting, collapse, and sudden death.
The good news is that heartworm disease is easily preventable! Several monthly preventatives are available for both dogs and cats (either oral tablets/chews or topical treatments) and an injectable version that is given every 6 months (by your vet) is available for dogs. Heartworm prevention is safe, easy, and inexpensive when compared to the cost of treating heartworm disease in dogs (easily $1000 or more, takes months to complete, involves extreme exercise restriction to avoid complications). All versions are extremely effective when given consistently and properly as directed by your veterinarian. Currently, there is no safe and effective treatment for eliminating adult heartworms in cats so prevention is the key.
For the cost of about 2 fancy coffeehouse beverages each month you can prevent your pet against heartworm disease!
Also, many heartworm preventatives are combination products, meaning they include another medication(s) that prevent other parasites as well (e.g. roundworms, hookworms, whipworms, fleas and ticks- depending on the product). Many types of intestinal parasites are considered zoonotic*(can be spread directly from animals to humans) and capable of causing serious disease. Infants, young children, and immunocompromised adults are at highest risk for infection. This added benefit is another reason to keep your pet on preventive all year long. Consult with your veterinarian to decide which preventative is best for your pet.
*NOTE: Heartworm infection is not considered zoonotic as transmission must occur via mosquitos.
The American Heartworm Society is an excellent resource for up-to-date information regarding heartworm disease in dogs and cats, including current treatment recommendations. Visit http://www.heartwormsociety.org/ to learn more.