Canine Heartworm Disease

What is heartworm disease?

Heartworm disease is a serious and potentially fatal disease in dogs. It is caused by a worm called Dirofilaria immitis. The heartworms are found in the right ventricle of the heart and the pulmonary arteries. The female worm is 6 to 14 inches long; the male is about half that size. One dog may have dozens of worms. Heartworms live up to five years and during this time the females produce millions of larvae called microfilaria. These microfilaria live in the bloodstream, and may be concentrated in the spleen. The larvae go through a series of molts on their way to becoming an adult heart worm. One of the molts occurs in the salivary gland of the mosquito. Larvae cannot complete their entire life cycle in the dog; they must pass through a mosquito on their way to becoming an adult heartworm. 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 for 10 to 30 days in the mosquito and then enter the mouth parts of the mosquito. The microfilariae are now called infective larvae. At this stage of development they will grow to adulthood when they enter a dog.The mosquito bites the dog where the hair coat is thinnest. However, having long hair does not prevent a dog from getting heartworms. When fully developed, the infective larvae enter the bloodstream of the dog when the mosquito bites it. They grow to maturity in two to three months and start reproducing, thereby completing the life cycle. It only takes one mosquito carrying larvae to infect a dog with this potentially fatal disease.

Where are heartworms found?

Canine heartworm disease occurs worldwide. In the United States, it was once limited to the South and southeast regions. The disease has spread and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent.

My dog is never around other dogs, can it still get heartworms?

Yes, the disease is not spread directly from one dog to another. An intermediate host, the mosquito, is required for transmission. The mosquito bites an infected dog and picks up the larva. The larva molts into its infective stage and is then spread to the next dog the mosquito bites.Spread of the disease therefore coincides with the mosquito season. The number of dogs infected and the length of the mosquito season greatly influence the incidence of heartworm disease in any given area.

What do heartworms do to the dog?

Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They also interfere with the heart valves. 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 right away. 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.

What are the signs of heartworm disease?

The most common signs are a dry, chronic cough, shortness of breath, weakness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even pass out. The diagnosis of heartworm disease needs to be made by your veterinarian.

How can my veterinarian tell if my dog has heartworm disease?

After examining your pet your veterinarian may find symptoms that are suggestive of heartworm disease. Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In most cases diagnosis of heartworm disease can be made by a blood test that can be run in the veterinary hospital or by a veterinary laboratory. One type of test looks for antigens (proteins) produced by the adult heartworm. This is the most accurate type of test. There can be false negative results though. This occurs when there are very few worms present.Because the antigen detected is produced only by the female worm, a pure population of male heartworms will also give a false negative. There must be at least five female worms present for the most common test to be positive.

Another type of blood test looks for microfilariae. These are the larval offspring of the adult worms.

A blood sample is examined under the microscope for the presence of microfilariae. If microfilariae are seen, the test is positive. Approximately 20% of the dogs test negative even though they have heartworms because no larvae are present.

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.

X-rays of a dog with heartworms will usually show heart enlargement and swelling of the pulmonary artery leading to the lungs from the heart. These signs are suggestive of heartworm disease. X- rays may also reveal the condition of the heart, lungs, and vessels. This information allows your veterinarian to predict the possibility of complications related to treatment.

How are dogs treated for heartworms?

Heartworm disease is treated in two phases. After examining your dog and evaluating its lab work and X-rays your veterinarian can choose from several treatment options. In dogs with severe heartworm disease, it may be necessary to treat them with antibiotics, prescription diets, and drugs to improve cardiac function prior to treatment for the heartworms. The first phase involves eliminating the adult heartworms. A drug called Immiticide is now used.  We see some dogs with advanced heartworm disease. This means that the heartworms have been present long enough to cause damage to the heart, lungs, blood vessels, kidneys, and liver. A few of these cases will be so far advanced that it will be safer to just treat the organ damage rather than risk treatment to kill the worms. Dogs in this condition are not likely to live more than a few weeks or months. The second phase involves riding the dog of the heartworm larvae. This is usually done six weeks after the pet has been treated for adult heartworms. The drugs used to kill the larvae are not nearly as dangerous as the compounds used to treat for the adult worms. The second phase is very important because a dog with heartworm larvae is the reservoir mosquitos draw from to infect other dogs.

After the second phase of treatment your veterinarian will check your dog to be sure all the adults and larvae have been eliminated.

What happens after treatment?

Complete rest is essential after treatment: The adult worms begin to die in a few days and start to decompose. As they break up, they are pumped into to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This can be a critical time so it is absolutely essential that the dog be kept quiet and not be allowed to exercise for one month following treatment. A cough is noticeable for seven to eight weeks after treatment in many heavily infected dogs. Careful monitoring and prompt treatment are essential if the dog has a significant reaction in the weeks following the initial treatment, although such reactions are not common. If a dog shows loss of appetite, shortness of breath, severe coughing, fever, or depression, you should contact your veterinarian.

After the treatment can my dog ever get heartworm disease again?

Yes, the treatment eliminates the current problem, but does nothing to prevent it from happening again. Your veterinarian can offer several very effective products to prevent heartworm disease. There are medications that can be given daily, and others that are given monthly to prevent canine heartworm disease. There is even a product that is applied topically that prevents many different parasites, including heartworms, from infecting your dog. Consult with your veterinarian to determine which is best for your pet.

by William M. Fraser, D.V.M.

petEncyclopedia | Contact Us