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Health Topic: Heartworm

What are Heartworms?
Heartworms are roundworms that, as the name suggests, live in the heart and also the lungs of mammals. But this is only at two stages of their lifecycle. Heartworms begin their life in the pulmonary (lung) arteries of a mammal, typically a dog, as microfilaria ("little worms", the L1 stage) and quickly make their way into the circulatory system of their host. A mosquito takes a bloodmeal from this infected animal ingesting microfilaria at the same time. In the dog, microfilaria not ingested by a mosquito will, over a 2-3 year period, die; they will not mature past this stage of life. In the mosquito the microfilaria go through two molts, into the larval (L3) stage, and then migrate to the mouthparts of the mosquito. The mosquito takes a bloodmeal, depositing the larvae onto the animal. The larvae make their way into the bite wound left by the mosquito and into the tissue of the animal. Over the next 190 days the larvae go through two more molts, becoming immature adults, the L5 stage, at which point they migrate into the venous system and to the heart and pulmonary artery. Once they reach the heart they mature fully and mate (if both sexes are present), producing microfilaria, and the cycle begins again. Microfilaria are not typically seen in infected mammals not in the canine family.

How do I know if my pet has Heartworms?
Dogs with heartworms will typically have a cough that is especially noted after exercise. They may also become listless, lose weight and cough up phlegm that may or may not contain blood. Cats with heartworms may have asthma-like symptoms, but are most often diagnosed on a post-mortem exam. Symptoms alone, however, are not enough to determine whether you pet is infected or not. Your veterinarian will draw a blood sample from your pet to run one or two tests. One test examines the blood for microfilaria directly under the microscope. Microfilaria will not be seen in blood from dogs taking a heartworm preventive, and may be missed if there is a small number in the circulatory system. Another test detects antigen, a protein produced by adult worms, in the blood. Because cats will have very few adult worms there is a blood test specifically designed for cats, which detects antibody, the cat's response to heartworms of all stages, in the blood. Antigen and antibody tests may be performed either at the clinic or your veterinarian may choose to submit the blood to a laboratory for testing.

How are Heartworms prevented?
Your veterinarian can prescribe a preventive designed specifically for the weight of your animal and whether he or she is a cat or a dog. Depending on the part of the country in which you live, your veterinarian may prescribe year round preventive or may recommend preventive only between the first thaw and the last freeze of the year. Dogs and cats on a preventive should be tested annually.

You can minimize mosquito populations by ensuring your property is well drained and cleared of stagnant water.

How is Heartworm infection treated?
After the dog is diagnosed as heartworm positive, your veterinarian will typically take a chest x-ray to determine the severity of the infection. The severity will determine the course treatment. Your dog will then be admitted into the hospital to have the heartworm treatment medication administered. If after 4 months an antigen test is negative, your dog is assumed to be free of worms. Dogs that are infected can be safely put on a preventive to prevent additional infection.

Currently, there is not an approved treatment for heartworm infection in cats. They are typically treated symptomatically and put on preventive to prevent additional infections.