HEARTWORM DISEASE

HEARTWORM DISEASE

What causes heartworm disease?

Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs. It is caused by a worm called Dirofilaria immitis.

Heartworms are found in the heart and large adjacent vessels of infected dogs. The female worm is 6 to 14 inches (2.3 to 5.5 cm) long and 1/8 inch (5 mm) wide; the male is about half the size of the female. One dog may have as many as 300 worms.

How do heartworms get into the heart?

Adult heartworms live in the heart and pulmonary arteries of infected dogs. They have been found in other areas of the body, but this is unusual. They survive up to 5 years and, during this time, the female produces millions of young (microfilaria). These microfilaria live in the bloodstream, mainly in the small blood vessels. The immature heartworms cannot complete the entire life cycle in the dog; the mosquito is required for some stages of the heartworm life cycle. The microfilaria are therefore not infective (cannot grow to adulthood) in the dog, although they do cause problems.

As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae during a blood meal. The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouth parts of the mosquito. The microfilariae are now called infective larvae because at this stage of development, they will grow to adulthood when they enter a dog. The mosquito bites the dog where the haircoat is thinnest. However, having long hair does not prevent a dog from getting heartworms.

When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels, where they grow to maturity in 2 to 3 months and start reproducing, thereby completing the full life cycle.

Where are heartworms found?

Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. However, the disease is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent.

How do dogs get infected with them?

The disease is not spread directly from dog to dog. An intermediate host, the mosquito, is required for transmission. Spread of the disease therefore coincides with the mosquito season. The number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area.

It takes a number of years before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in 4 to 8 year old dogs. The disease is seldom diagnosed in a dog under 1 year of age because the young worms (larvae) take up to 7 months to mature following establishment of infection in a dog.

What do heartworms do to the dog?

Adult worms: Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.

Most dogs infected with heartworms do not show any signs of disease for as long as two years. Unfortunately, by the time signs are seen, the disease is well advanced. The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys from the adult worms and the microfilariae.

The most obvious signs are: a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint.

Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation. There may also be evidence of weight loss, poor condition, and anemia.

Severely infected dogs may die suddenly during exercise or excitement.

Microfilariae (Young worms): Microfilariae circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow in these vessels. The body cells being supplied by these vessels are deprived of the nutrients and oxygen normally supplied by the blood. The lungs and liver are primarily affected.

Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and general weakness because this organ is essential in maintaining a healthy animal. The kidneys may also be affected and allow poisons to accumulate in the body.

How is heartworm infection diagnosed?

In most cases, diagnosis of heartworm disease can be made by a blood test that can be run in the veterinary hospital. Further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog can tolerate heartworm treatment. Depending on the case, we will recommend some or all of the following procedures before treatment is started.

Serological test for antigens to adult heartworms: This is a test performed on a blood sample. It is the most widely used test because it detects antigens (proteins) produced by adult heartworms. It will be positive even if the dog does not have any microfilaria in the blood; this occurs about 20% of the time. Dogs with less than five adult heartworms will not have enough antigen to turn the test positive, so there may be some false negative results in early infections. Because the antigen detected is produced only by the female worm, a pure population of male heartworms will give a false negative, also. Therefore, there must be at least 5 female worms present for the most common test to be positive.

Blood test for microfilariae: A blood sample is examined under the microscope for the presence of microfilariae. If microfilariae are seen, the test is positive. The number of microfilariae seen gives us a general indication of the severity of the infection. However, the microfilariae are seen in greater numbers in the summer months and in the evening, so these variations must be considered. Approximately 20% of dogs do not test positive even though they have heartworms because of an acquired immunity to this stage of the heartworm. Because of this, the antigen test is the preferred test. Also, there is another microfilarial parasite which is fairly common in dogs; on the blood smear, these can be hard to distinguish from heartworm microfilariae.

Blood chemistries: Complete blood counts and blood tests for kidney and liver function may give an indirect indication of the presence of heartworm disease. These tests are also performed on dogs diagnosed as heartworm-infected to determine the function of the dog's organs prior to treatment.

Radiographs (X-rays): A radiograph of a dog with heartworms will usually show heart enlargement and swelling of the large artery leading to the lungs from the heart. These signs are considered presumptive evidence of heartworm disease. Radiographs may also reveal the condition of the heart, lungs, and vessels. This information allows us to predict an increased possibility of complications related to treatment.

Electrocardiogram: An electrocardiogram (EKG) is a tracing of the electric currents generated by the heart. It is most useful to determine the presence of abnormal heart rhythms.

Echocardiography (Sonogram): An echocardiogram allows us to see into the heart chambers and even visualize the heartworms themselves. Although somewhat expensive, this procedure can diagnose heartworms when other tests fail.

How are dogs treated for heartworms?

Adulticidal Treatment

Drug treatment involves a three pronged approach, in order to account for varying drug susceptibility and drug safety. First, the adults must be killed, and there are two main classes of adulticides used. Melarsomine dihydrochloride, "Immiticide", is a new arsenical adulticide which is given intramuscularly into the lumbar muscles. The standard protocol is two doses of the drug twenty-four hours apart, however there is a modified protocol where a single dose is given, followed by paired doses 4-6 weeks later. The latter regimen is especially useful in treating very sick dogs, or dogs with high worm burdens at risk of thromboembolic complications.

The other adulticide used is thiacetarsemide ('Caparsolate'), the predecessor to Immiticide. Caparsolate must be administered intravenously, and tissue sloughing can occur if the drug goes perivascularly. This drug is contraindicated when some liver and kidney diseases are present. With either of these drugs, if treatment with adulticides is successful, antigenemia and microfilaremia should be gone by sixteen weeks post-treatment.

Microfilaricidal Treatment

Microfilaricides are the next major type of drugs used when treating for heartworm. Ivermectin, not approved as a microfilaricide, has been used at fairly low doses, although microfilaricides are usually not given until after recovery from any side effects due to the arsenicals, not earlier than 3-4 weeks after this treatment had been administered. Milbemycin oxime at 500-999mcg/kg is an effective microfilaricide, and ivermectin given on an off-label basis, at 50 mcg/kg, will kill circulating microlfilariae. Careful observation is required after an initial dose in order to treat potential systemic side effects. There have been some CNS side effects in rough coated collies (and other pure breeds) at high doses of Ivermectin outside the recommended dosage range.

Chemoprophylactic Treatment

Finally, prophylaxis is the major component of heartworm chemotherapy for any animal that could potentially become infected. Three main drugs exist for this purpose. All dogs should be tested for microfilariae before medication begins, particularly when using DEC, because microfilaremic dogs may progress to hypovolemic shock and death if DEC is administered.
Macrolide endectocides, such as ivermectin (6-12 mcg/kg) and milbemycin oxime (500-999 mcg/kg) are completely effective in preventing adult maturation. Ivermectin (Heartguard) is administered orally, and has a low potential for shock reaction relative to DEC. Milbemycin oxime (Interceptor) is also administered orally, and acts as both a microfilaricide and a larvicide. CNS effects have not been shown with this drug. Either of these drugs should be given once monthly per os from within one month following the onset of the transmission season to one month after transmission is considered unlikely. If a dose is missed, there is some afforded protection from residual amounts of drug remaining in the circulation. As a result, delayed administration may not lead to an adult infection.
Diethylcarbamazine (DEC) is given orally once per day, and is highly effective, but is only safe in amicrofilaremic dogs. However, missed doses are significant if the elapsed time is longer than 2 months. If the omission time is under six weeks, readministration should establish full protection in the dog again. Other than the potential for shock reaction in microfilaremic dogs, no significant direct side effects have been seen with DEC, even at elevated doses

How can I prevent this from happening again?

When a dog has been successfully treated for heartworms, you cannot sit back and relax because dogs can be reinfected. Therefore, it is essential to begin a heartworm prevention program. There are three drugs which can be used to prevent heartworm infection. One is a daily, chewable tablet; the others are chewable tablets that are given only once monthly. All these products are very safe and very effective. Their costs are essentially identical. One of these should be started immediately after the treatment is completed.

BOXER BUDDIES RESCUE INC.
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Penny Harris ~ Sue Stephens ~ Adelle Condon

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