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What is the cause of Heartworm?
Heartworm in Dogs
Heartworm in Cats
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Heartworm Disease

What is the cause of Heartworm Disease

Heartworms are a common parasite of dogs, wild dogs, bears, and sea lions. Recently, they have also been found in cats and ferrets. They can be deadly if left untreated. Luckily, there are medications available to prevent heartworm disease for both dogs and cats.

Heartworm disease is caused by a filarial nematode called Dirofilaria immitis. Heartworms have a complex lifecycle requiring both a mammal and an insect vector in order to survive. Larva, or microfilaria, is spread from an infected animal to a new one by mosquitoes. Once inside the animal, they migrate through the blood stream to the heart, where they develop into adult worms. In dogs, they mature in the right ventricle and main pulmonary arteries. In cats, the worms often migrate to the lungs.

Heartworm in Dogs

Dogs infected with heartworms often show signs of exercise intolerance, weight loss, coughing, fainting, or congestive heart failure. Diagnosis is made based on a blood test looking for antigen to the worms or detecting microfilaria. Radiographs of the chest are important to gauge the degree of damage done to the heart and lungs.

Once a dog has been diagnosed with heartworm disease, treatment is done in stages. First, the dog undergoes a complete medical work up with a physical examination, blood work, and chest radiographs. The blood work is necessary to ensure the dog is healthy enough for treatment. Next, the dog is treated with an injectable drug to kill the worms. During treatment and for a number of weeks after, the dog must be kept quiet and confined to reduce risk of sudden death. Finally, the dog is placed on prophylactic medications to prevent it from becoming infected again. Treatment is expensive and risky for the dog, but without treatment the disease will lead to heart failure.

Heartworm in Cats

Diagnosing heartworm disease in cats is much more difficult. Only a single worm is needed to cause disease and death in cats. The clinical signs in cats are usually related to respiratory disease, or difficulty breathing. Blood tests are not as effective for cats as they are for dogs. A CBC, or complete blood count, can increase suspicion of disease if certain white blood cells are elevated. Radiographs of the chest can be used to rule out other causes of respiratory disease. Occasionally, echocardiography, or an ultrasound of the heart, will show the worms inside the heart or vessels. Veterinarians use a combination of clinical signs, physical examination, radiographs, and blood work to diagnose this disease in cats.

Treatment for cats with heartworm disease is also different from dogs. Medications used in dogs to kill the adult worms are not approved for use in cats and can often be deadly. Also, the death of adult worms usually leads to thromboembolism or large blood clots that can block blood flow to the lungs, leading to death. Treatment is supportive for cats and is geared toward controlling the clinical signs.

Preventative Medicine

Medications are available to prevent heartworm disease for both dogs and cats. Dogs older than four months should be tested for heartworms before starting preventive medications and should be repeated on an annual basis. Some of the medications available for heartworm prevention will also prevent many parasites of the gastrointestinal tract. At Wildwood Veterinary Hospital, we recommend that all dogs are tested for heartworms and given Heartgard Plus or a similar preventative year round, and that all cats be given Heartgard for cats year round.

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