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David Adams
Department of Health Sciences, AASU


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Heartworm Disease: What You Need to Know

by David Adams

January 02, 2014



Heartworm Disease is a significant cause of morbidity and premature mortality in dogs. Among canine populations that do not receive appropriate prophylaxis, infection rates may reach nearly 50%. As a rule of thumb, Heartworm Disease is most prevalent in regions within 150 miles of the eastern seaboard—especially in the temperate areas such as the Southeastern United States.


A famous parasitologist once proclaimed, ‘It’s a wormy world!’ This saying is certainly true for dogs. Parasitic worms can cause significant disease in all sorts of creatures, big and small. As for the canine heartworm, the wily worm goes by the name Dirofilaria immitis—we’ll call it ‘D.I.’ for short—and it’s transmitted by mosquitoes.

The story goes like this: Ms. Mossy takes a blood meal from a dog that is infected with D.I. Down goes D.I. deep into her (only female mosquitoes are blood-feeders) GI tract where it develops into 3rd-stage, adult, larvae, all dressed up and ready to infect a passing pooch. When Ms. Mossy, teeming with 3rd stagers, takes a subsequent blood meal, it transmits D.I. to a previously uninfected dog. As the illustration below shows, this cycle is repeated continuously as long as there as D.I.-positive canines and mosquitoes to transmit it. (By the way, that is a human being in the picture. D.I. is ‘zoonotic’ which means people can get it too.)

Now there’s real trouble afoot. The male and female adult, 3rd stager, D.I.’s (males and females alike) now have entered the canine’s circulatory system and proceed to cause clinical disease. Some dogs may end up with hundreds of larvae, any of which initially sequester in the lungs, the heart, and, ultimately in the large vein of the lower body, the caudal vena cava. Inflammation of the pulmonary and cardiac tissue, causing enlargement of the heart, are common.



Heartworm Disease can produce a variety of medical problems for the infected canine. These include lungs, heart, liver, and kidney problems. The malady may present acutely, however onset often is subtle, a function of chronic infection. The canine couch potato may never show clinical signs of disease. Most dogs, however, will show various degrees of clinical signs, the most common of which occur in the heart and lungs.

Early infections are often asymptomatic. Neither hound nor handler is aware of a problem. As the disease progresses, a cough may develop, leading to dyspnea and abnormal lung sounds. Severe disease is characterized by worsening respiratory symptoms, enlarged liver, fainting, abnormal heart sounds, and death.


Diagnosis of canine heartworm disease depends upon several factors. An accurate history is the first start. What is the general health status of the dog? How long have the suspicious symptoms been going on? Given the vet’s index of suspicion, he or she may order various diagnostics such as blood tests, x-rays, angiogram, and/or ultrasound. The definitive diagnostic, however, remains the necropsy—upon the death of the dog.


There currently is but one FDA-approved drug for the eradication of adult heartworms. An arsenical (yes, as in arsenic) compound, its administration requires careful administration in an in-patient setting. Melarsomine dihydrochloride (trade name, Immiticide®, Merial, administered by deep intramuscular injection into the muscles of the back, has demonstrated better safety and efficacy than previous therapies. Your veterinarian can give you complete details on this drug.


Many years ago Benjamin Franklin observed that a paucity of prevention often yielded a bunch of benefit, an axiom no less applicable to canine heartworm disease. Happily, dog owners can select from several regimens available from veterinarians. These formulations come in a variety of forms, chewables, tablets, topical, and injectable. Dosing ranges from daily to bi-annually. All have proven extremely effective when properly administered according to schedule. Moreover, some compounds are effective against a host of canine parasites—some of which are zoonotic, i.e., people can catch them. It is important, however, to test canines for possible adult heartworm infestation before the initiation of a heartworm prevention program.

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