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Animals In Print
The On-Line Newsletter

From 28 May 2002 Issue

Heartworm Disease

Heartworms are large worms that live in the hearts of dogs, cats and in other species, including ferrets, foxes, wolves, sea lions, and horses.  Dogs are the common host for this parasite.  This worm is also known as Dirofilaria Immitis. It is a long, spaghetti-like worm that can be anywhere from 6 to 10 inches in length (~17 - 27 cm).

How are heartworms transmitted?
In addition to the animal 'host', heartworms need a mosquito to complete their life cycle.

1) A mosquito bites a heartworm-infected animal.

2) The mosquito is then carrying microscopic versions of the heartworm, called microfilariae.

3) When the mosquito bites another dog or cat, that animal is now infected with the heartworm microfilariae.

4) Within 70 to 90 days, the microfilariae have made it through the tissues to the animal's heart, where they reproduce (providing both male and female worms are present) and live for several years. If both sexes of worms are present, they will be producing their own little microfilariae within 6 - 7 months after that mosquito bite.

5) The cycle continues.

What are the signs of heartworm disease?
The signs vary according to number of worms present, stage of life cycle, age and species of host.  The heartworms live primarily in the right side of the heart and lung, and can cause significant damage and even death.  Here are some general signs for the most common hosts, dogs and cats.

DOG - possible heartworm signs.
Acute disease - usually no clinical signs (the dog just acquired the disease)
Mild to moderate - cough, reluctance/inability to exercise
Severe - marked shortness of breath, coughing, fainting episodes, weight loss, fever, abdominal swelling (ascites), death.

CAT - possible heartworm signs
The signs of heartworm disease are different in the cat than the dog.  Cats can present with sudden death (no other signs) or can live with the disease free of clinical signs.  Most commonly, heartworm disease in the cat mimics feline asthma - coughing and difficulty breathing.  Vomiting can be another sign of feline heartworm disease (vomiting is a common sign in many feline diseases).

Could my dog or cat be at risk?
Yes, depending on your geographic location.
Heartworm disease is now worldwide, and mosquitoes are too.

Diagnosis is most commonly done by a blood test in your Veterinarian's office. Additionally, x-ray, ultrasounds, or other tests may be performed.

Treatment for heartworm is not without some risk. Bloods tests are used to assess kidney and liver function before initiating treatment. The worms are killed slowly, so as not to cause a sudden blockage in the heart or lungs, and the patient must be kept quiet. The next phase is medication to kill the remaining microfilariae.

Your veterinarian must first test your pet and find the heartworm status to be negative. You can then begin heartworm prevention. Prevention is in the form of a chewable daily or monthly tablet, given in the summer months or year round, depending on the climate where you live. The most commonly prescribed oral heartworm preventatives are: Heartgard®, Interceptor®, and Sentinel®. See the "Related Links" at the end of this article for links to more information about each product.

A new topical treatment, Revolution™ (Selamectin), by Pfizer is effective against heartworms, fleas, ear mites, sarcoptic mange (dogs), hookworms (cats), roundworms (cats), and the American Dog Tick (Dermacentor variabilis) (dogs). The American Dog Tick is the principal vector for a Rickettsial disease, Rocky Mountain Spotted Fever, in dogs and humans. Revolution™ is typically applied once a month for parasite control.

Revolution™ works by absorbing through the skin to the bloodstream, where it prevents heartworms and treats intestinal parasites. Revolution™ also disperses from the blood to the sebaceous glands (microscopic oil glands in the skin) to act as a reservoir of drug for protection against fleas, ticks, and mites.

Drug precautions. This drug should not be used in animals that are sick, malnourished, debilitated, or underweight.

source: Janet Tobiassen Crosby

Return to Animals in Print 28 May 2002 Issue

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