A Pet's View / June 2007
Matters of the Heart
Once upon a time, heartworms were a parasite of southern climates, but today heartworm disease is found in every state and on every continent except Antarctica. Although the highest infection rates occur in dogs within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries, areas with large mosquito populations also have a high rate of infestation.
Heartworm infestation is dangerous. The parasitic roundworms cause heart and lung damage and masses of twisted, intertwined worms create blockage to the normal flow of blood. Untreated dogs die. Treated dogs endure weeks of discomfort while the worms are killed and expelled from their bodies. Severely affected dogs may not survive the treatment.
In spring, veterinarians began checking dogs and cats for heartworm exposure from the previous mosquito season. The heartworm goes through four molts to mature into an adult worm, two inside the mosquito and two inside the pet. Worms takes 6-7 months to complete the last two molts and mature enough to be detected by a heartworm test. But, larva works through tissues to reach the heart as early as 70 days after entering the host. They grow rapidly in length and size until death 5-7 years later.
Adult female worms may grow up to 14 inches and produce thousands of live young, microfilaria, daily.
More and more worms congregate in the right ventricle. The smaller the pet, the fewer worms it takes to cause problems. The artery lining is damaged within days; worms cause damage faster than the body can heal. The arteries become tortuous and dilated. Blood clots and aneurysms are a common side effect, and complete blockage of small blood vessels can occur. The immune system goes into overdrive, causing inflammation, more tissue damage and pain. Blood re-routes to arteries unoccupied by worms but, blocked blood vessels cause fluid accumulation in the lungs. Sometimes surgery to remove worms from the heart can help reestablish blood flow. Congestive heart failure, kidney failure or liver failure are the common causes of death.
Early diagnosis and treatment means a better chance the pet will recover. Elimination of the parasite is a two-step process. Adults are treated first, then a different treatment is given to kill microscopic and migrating larvae. As worms die, they lodge in pulmonary arteries, blocking even more blood vessels. The usual inflammation caused by worm presence is amplified by decomposing worms in the blood vessels. When heartworms break down, they release foreign substances into the circulation, causing more inflammation and swelling. Though adult heartworms are gone, thousands of the microfilaria, microscopic baby heartworms, remain in the bloodstream. After a month of adult treatment (and its side effects), the microfilaria must be eliminated with a heartworm preventive. Four months later, since very immature hear tworms and young female adults are resistant to treatment, pets are retested to see if another course of treatment is necessary.
Early symptoms include a cough, especially after exercise, and early exhaustion upon exercise. More advanced cases progress to severe weight loss, fainting, coughing up blood, and, finally, congestive heart failure.
Cats acquire heartworms at just 5-20% the rate of dogs in the same area and a cat’s immune system is readily activated against heartworms. Cats seem to harbor fewer adult worms, the worms are shorter, usually only males, and they die sooner in cats than dogs, in 2-3 years. However, since the cat’s heart and blood vessels are much smaller, one worm can cause a medical emergency and infected cats can die suddenly from the effects of just a few worms.
The most common heartworm preventives are given once a month. Highly effective, preventives kill immature heartworm larvae before they molt. When regularly administered, they protect more than 99 percent of dogs from infection. No “homeopathic,” “natural” or “organic” products have been proved effective for heartworm disease. Animals must be tested before the administration of a heartworm preventive since giving it to a dog with heartworms can lead to severe, even fatal. reactions.
Monthly heartworm prevention should begin a month before the local mosquito season and continue for a month after. However, many West Virginia and Ohio veterinarians recommend year round administration since mosquito activity may occur during unseasonable warm spells and, owners travel.
Worms in the heart have devastating consequences. The odds are high that unprotected dogs, inside, outside or puppies, will be bitten by an infected mosquito. Prevention is simple and affordable. Treatment is expensive and complicated. Pet owners can’t afford the gamble. PL
For more information, visit heartwormsociety.org or talk with a veterinarian.
Copyright © 2007 A Woman's View. All rights reserved.
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