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Heartworm Disease

General Information

Heartworm disease is very common in all parts of the country where mosquitoes are common. In this part of South Carolina we estimate an incidence of about 30-40% in a middle age outdoor dog. It is less common in dogs that have little exposure to mosquitoes but we have diagnosed it many times in dogs in which you would not expect infection. Since prevention is safe and effective we ALWAYS recommend heartworm prevention in dogs living in the upstate of South Carolina. We recommend that preventative be given year round though an alternative is to use a preventative between March and November. However, if this is done a heartworm test must be done each March before the preventative is started. There are many other advantages to using heartworm preventatives. We always recommend that FDA approved heartworm preventives be used and try our best to make them affordable.

Heartworm disease is caused by the heartworm, Dirofilaria immitis. This parasite lives in the right side of the dog's heart and the nearby large vessels (pulmonary arteries). The female worm produces large numbers of microscopic, immature heartworms that circulate in the blood. These immature worms (microfilariae) are taken up with the blood by a mosquito feeding on an infected dog. After living in the mosquito for 10-14 days, the microfilariae can then infect another dog that the mosquito feeds on. The feeding mosquito deposits infective microfilariae on the skin of another dog, and these enter the body through the mosquito bite wound. The microfilariae eventually travel to the heart where they develop into adult heartworms. The adult heartworms produce new microfilariae within 3 months. It takes at least 190 days from the time the dog is bitten by an infected mosquito until the dog becomes a new source of infective microfilariae. This is why we usually assume that we can start a dog less than 6 months of age on heartworm prevention without having to do a heartworm test first. If a dog is older than 6 months of age it is possible that it may have developed heartworm disease and have microfilaria (baby heartworms) in circulation. Placing dogs on heartworm preventative that are currently infected can cause serious, sometimes fatal reactions with any heartworm preventative. We have seen several fatalities when this was done. This is why we have to be sticklers for a certain set of "rules" about heartworm prevention. This is also why we do not think that heartworm preventative should ever be used except under the direction of a veterinarian and discourage the sale of heartworm preventatives from feedstores and over the internet. This is illegal without a prescription but is done frequently (which accounts for ALL of the fatal reactions that I have seen).

Many cases of heartworm are diagnosed by finding the microfilariae in the blood. Sometimes, however, no microfilariae are found in the blood (occult heartworm disease). Almost 1/3 of dogs infected with heartworm disease do NOT have microfilaria in circulation. These cases are diagnosed by a combination of blood (serology) tests and chest radiographs (x-rays). The tests that are available to diagnose these "occult" infections have become quite sophisticated and not really expensive.

Failure to treat heartworm disease may result in heart failure and/or serious disease of the liver and kidneys. Untreated heartworm disease is usually fatal. It is true that some dogs will outlive the heartworms and recover. However, it is also true that some cases will decline rapidly and die despite our efforts to treat the disease. Most cases lie somewhere in between these two extremes. They will live lives shortened by the infection. There are fatalities during treatment but most cases recover with treatment . . . and most of the damage is reversed if treatment is successful.

Important Points in Treatment

1. Treatment for heartworms consists of 2 phases: destruction of the adult heartworms, followed by elimination of microfilariae from the blood. In severe cases we may even recommend that the adults be killed in 2 steps. This method is always desirable but is more expensive. We usually reserve this for the more severely affected animals.

2. Before treatment, a thorough physical examination, including blood tests, chest radiographs (x-rays) and an electrocardiogram may be advised. An underlying liver or kidney disease may be treated first. It is my personal opinion that if my clinical instinct is that the patient can survive treatment we should proceed since most of the damage to the organs can be reversed with successful treatment.

3. Heartworm disease is very serious, and complications may arise during and after treatment. The doctor will discuss what to watch for after the initial phase of treatment.

4. Give all medication as directed. Call the doctor if you cannot give the medication.

5. Activity: Your pet's activity must be severely restricted for 4 weeks after treatment. Do your very best not to allow unrestricted running. There are some cases where confinement actually makes the dogs activity level increase. You must keep in mind that the heartworms are dying slowly during the weeks after the adult killing drugs are given. Since the worms are in the heart anything that increases cardiac output increase the chance of rapid release of dying worms and "parts" of worms being release out into the lung circulation.

6. We recomend aspirin therapy before (2 weeks) and after (2 weeks) the adult heartworms are treated. Some veterinarians, and some "experts" do not feel that this is important. However, we do feel that it protects against the likelyhood of clots being formed as the worms release from the heart. The dosage of your pet for two weeks before treatment will be __________ regular strength or ___________ baby aspirin once daily for two weeks before and after treatment. It is best to give aspirin with food and when we say aspirin we MEAN aspirin (not any other pain releiver). Improper dosage of aspirin can cause serious problems.

7. Special instructions: _________________________________________________________


Notify the Doctor if Any of the Following Occur:

* Your pet refuses to eat.
* Your pet has diarrhea or vomiting, or seems in pain.
* Your pet coughs frequently or has trouble breathing.
* There is a change in your pet's general health.