Heartworms in Dogs

Heartworms in Dogs

Canine heartworms affect dogs throughout the world. These worms scientific name is Dirofilaria immitus. They are common in the hearts and major heart blood vesicles in pet and stray dogs in all 50 American States. The male worms are a few inches in length and female worms about double the size of male worms and cause most of the damage The worms are transferred from dog to dog through the bite of an infected mosquito. After a mosquito sucks larval heartworms with blood from an infected dog; it must rest for a period of time before these larva become infective. If that mosquito then bites another dog or the same dog, it transfers these microscopic larva as it bites. Then, during the next few months, these larva migrate through the dogs body arriving at the heart several months later. There they mature into adult worms. Damage to the dog’s heart is due to adult worms and depends on the number of worms present, the worm’s sexes, and the area of the heart in which the worm lodges as well as the length of time the worms are present. Genetic factors present in the dog also govern the severity of the disease.

Heartworm disease is commonest in warmer areas of the world where mosquitoes are active throughout the year. Statistically, male dogs are slightly more prone to the disease than females. Large-breed dogs also suffer more from the disease as do short hair dogs and dogs kept out-of-doors.

The first sign of heartworm disease is often premature aging. Dogs grey prematurely about the muzzle and forelegs. Their activity levels decrease and their coats lack luster. As the disease progresses, a chronic dry cough often begins. This cough is most noticeable at night when the dog is resting or in a sitting position. It is due to fluid accumulation in the lungs and bronchitis. Later, the dog’s tummy (abdomen) assumes a pear-shaped look as the dog’s liver enlarges and fluid accumulates in the abdomen. While these events are occurring, the dog’s heart and pulmonary arteries enlarge due to mechanical obstruction of the worms, inflammation and valvular damage to the heart.

Heartgard (ivermectin) is an extraordinarily effective prevention for heartworm infection. Not only does a single dose of the medicine (delivering approximately 8mcg/kg body weight) kill larval heart worms recently transferred by mosquitoes but it is known to “reach back” several month and kill larva that have not yet reached the heart.

Until recently, the only medicine available to cure infected dogs of heartworms contained arsenic. Recent research indicates that after a year on Heartgard, adult worms present in the hearts of infected dogs have withered and died even if the arsenic treatment is not administered. For this reason, I often give only a single injection of these arsenic-containing drugs (Imiticide, Caparsolate) and then put the dogs directly on Heartgard. Heartgard has an added ingredient, pyrantel pamoate, which prevents infestation with hookworms and roundworms as well. Heartworms only rarely affect human beings. When they do, they form nodules in the lungs which can be mistaken for tumors. A similar disease to heartworms in dogs is the tropical disease, Onchocerciasis or River Blindness. An older treatment for dog and human disease was diethylcarbamazine citrate. If that drug was given to animals or people who already had the disease, a severe reaction often occurred. No reaction is likely if Heartgard or any other ivermectin-containing product is given to heartworm-positive dogs. I still recommend that dogs be tested once a year for heartworms with a blood test that detects even slight infections.

Article submitted by: © Ron Hines DVM PhD


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