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The bane of man’s best friend

July 1, 1993

The bane of man’s best friend

By Dr. Robert A. Hedeen, Naturalist

Now that warm weather has finally arrived, many of our thoughts have turned to the mosquito-transmitted West Nile virus threat to humans. Most of us do not realize, however, the renewal of mosquito activity also brings a threat to “man’s best friend.” The same mosquito that transmits West Nile to humans transmits heartworm disease to dogs. Though dog heartworm is found occasionally in a few other animals such as raccoons, cats, and foxes, its physiological make-up is such that it is almost an exclusive invader of dog tissues.

The adult worms may be several inches in length and are found primarily in the right side of the dog’s heart and in the pulmonary artery leading to the lungs. In place, they produce millions of small larvae called microfilariae, and these minute beasts gain access to the dog’s bloodstream and are distributed throughout the body.

Microfilariae will not develop into adult worms unless they are first ingested by a female mosquito when she sucks the canine’s blood. Once in the mosquito, the larval worms undergo development in the digestive tract, and then they migrate to the salivary glands. When the insect takes another blood meal, saliva containing the tiny worms is injected into the puncture to inhibit the clotting of the victim’s blood during the time required for feeding. Heartworm disease cannot be transmitted directly from one dog to another. The causative agents must undergo a part of their developmental cycle in the mosquito before they can infect a new host. In a dog, the worms undergo additional development and finally the adolescent forms move to the heart to mature sexually.

Dog heartworm infections are often without symptoms at first and may go undetected for a considerable time. When the worm’s numbers increase, they often form tangled knots within the heart, and this leads to heart failure, embolisms, and respiratory failure. Dog heartworm disease is widespread in the United States and Canada, with the entire state of Florida being a veritable hotbed for it. Veterinarians diagnosed 7,339 cases of dog heartworm disease in Illinois during the year 2001, with 132 in Winnebago County. Without question, many more cases went undiagnosed. Symptoms of a heartworm problem in a dog include shortness of breath, coughing, and lack of energy and appetite. By the time these symptoms become apparent, the dog may be doomed if the infection is of long duration.

Dr. Beth Christensen, a Rockford veterinarian, suggests all dog owners who care about the welfare of their pets have them tested for this malady once a year. She recommends they be placed on a prophylatic regimen that consists of one pill a month of an anti-filarial drug. Detrimental side effects are rare and the modern, therapeutic drugs have the extra benefit of eliminating dog hookworms and other intestinal roundworm parasites. Treatment is different and more difficult after an animal has been diagnosed with an abundance of adult worms. Prevention of infection is the key!

Dr. Christensen emphasizes that preventive treatment be on a year-round basis and not be confined to the so-called mosquito season. An owner may think because his animal is kept in the house most of the time it is immune to mosquito bites and safe from heartworm disease. But, most housedogs are permitted outside at least once or twice a day to perform their physiological functions and may be bitten and infected by a mosquito.

I have witnessed two autopsies of dogs that died of heartworm disease, and I have been awed at the destruction caused by heartworms. Believe me, a dog’s heart infested with these monsters is not a pretty sight. I used to have one of these infested hearts in a specimen jar and would show it to students in an advanced parasitology course I taught. This teaching aid always proved to be effective.

I strongly believe that when we take on the responsibility of a pet it is our duty to see that the animal’s health is maintained. A preventive program for heartworm disease should be on every dog owner’s agenda.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

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