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Miserable Little Parasites, Part 3

We conclude our review of miserable little parasites with everyone’s favorite, the heartworm!

Dirofilaria immitis is a mosquito-borne nematode that lives inside the heart and pulmonary arteries of dogs and other animals, including people. These little nastys are quite large (think angel hair spaghetti) and become so numerous that they impede circulation, ultimately causing death similar to heart failure. It is often unappreciated that a dog infected with heartworms may have BAZILLIONS of microscopic larvae in their bloodstream, just waiting for a mosquito to bite and transport them to the next victim.

All dog-like animals, including foxes, coyotes, and wolves can be infected with heartworms. Non-canids, such as raccoons, ferrets, muskrats, sea lions, and California gray seals may also be parasitized. Heartworm disease in these animals is very similar to that seen in dogs.

Cats are often infected by heartworms but do not support the worm’s entire life cycle. A cat will generally have only two to five worms in the heart and few if any larvae circulating in the bloodstream. Feline heartworm-associated respiratory disease (HARD) is an inflammatory lung disease similar to severe asthma. These kitties may be lethargic, lose weight and often cough so severely that they vomit – or look perfectly normal. HARD is a frequent cause of sudden death in young, apparently healthy cats.

Humans can also be infected by heartworms, but rarely develop problems. The worms die early in their development to adults, but dead worm tissue may cause enough irritation to generate a small granuloma (an inflammatory “tumor”) in the pulmonary artery. These granulomas typically disappear in a matter of weeks.

Treatment of established heartworm infections is both difficult and dangerous, requiring the administration of arsenic-based worm-killing drugs. Careful pre-treatment evaluation is necessary to determine a dog’s ability to handle the treatment, and patients with advanced heartworm disease may not survive even with the most cautious management. Some dogs that undergo treatment still suffer permanent lung damage from heartworm infection.

Prevention of heartworm infection is simple; just place your dog on monthly heartworm preventive medication (also available for cats). There are dozens of products available; tablets, chew tabs, yummy treat-like wafers, and even drops that are applied to the skin. Most of these products also eliminate common intestinal parasites and some also kill fleas. The choices are myriad, so discuss the options with your veterinarian to decide which best serves your pets’ needs.

All FDA-approved heartworm preventive products are available by prescription only. These are powerful medications; federal law requires these drugs be used only under the supervision of a licensed veterinarian. Bad things can happen when a heartworm-positive dog is given a heartworm preventive drug. Remember those bazillion heartworm larvae in the dog’s bloodstream? A dose of heartworm prevention kills them in a matter of minutes. All that worm protein, which was bad enough when locked inside the worms, is suddenly released into the animal’s blood. Amazingly, some dogs tolerate this kind of thing, but when they don’t the reactions can be deadly. Dogs who are free of heartworms tolerate heartworm preventive medications extremely well with only rare exceptions.

How much should a pet owner worry about heartworms? Ask a mosquito! If your home microenvironment - or those your pets visit - includes a mosquito species capable of transmitting heartworms, then your pets are at risk. That risk increases with every mosquito bite your pets receive. Infected dogs and wildlife serve as a reservoir for infective larvae, so there are always plenty of heartworms available to start a new infection.

Here in Sonoma County, land of 10,000 microenvironments, the risk of heartworm infection varies (almost) from one house to the next. If you live at higher elevation or in a forested area, chances are your local mosquitos are capable of transmitting heartworms. If you live at sea level, your pets will be safer, but not completely safe. Animals that live indoors have less exposure to mosquitos, but still get bitten. If you travel with your pets to a heartworm area, they may become infected there and bring the infection home when you return. This is how heartworms were transported into our community in the first place.

Prevention, which is cheap, safe, and easy, is always the best cure for heartworm disease. While some animals have little exposure and live their entire lives without problems, I’m not willing to bet my pet’s life on it. The only truly safe choice is to place every dog (and cat) on regular preventive medication.

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