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SOURCE: http://www.peteducation.com/ 

Heartworms, Dirofilaria immitis, are found in dogs, cats, and ferrets. They also occur in wild animals such as California sea lions, foxes and wolves. They have rarely been found in people.

Infection:
Adult heartworms living in the heart of an animal lay very tiny larvae called microfilariae, which then live in the bloodstream. These microfilariae enter a mosquito when it sucks blood from an infected animal (there are more than 60 different species of mosquitoes in the world that can transmit heartworms). In 2-3 weeks, the microfilariae develop into larger larvae in the mosquito and migrate to the mosquito's mouth. When the mosquito bites another animal, the larvae enter the animal's skin. The larvae grow and after about three months finish their migration to the heart, where they grow into adults, sometimes reaching a length of 14 inches. The time from when an animal was bitten by an infected mosquito until adult heartworms develop, mate, and lay microfilariae is about 6-7 months in dogs and 8 months in cats.

Severely infected dogs can have up to several hundred heartworms in their hearts and vessels. Adult worms in dogs usually live up to 5-7 years. 30-80% of infected dogs have microfilariae, and the microfilariae can live up to 2 years. Microfilariae cannot mature into adult heartworms unless they pass through a mosquito.

Diagnosis and symptoms:
In dogs, the adult worms can obstruct the various large blood vessels leading from the heart to the lungs. Worms may also enter smaller vessels in the lung and obstruct those vessels, as well. In severe cases, called "caval syndrome" worms start to fill the right ventricle of the heart.

Most dogs with heartworm infection do not show signs of disease. Some dogs may show decreased appetite, loss of weight, and listlessness. Often, the first sign of the disease is a cough. Animals with severe heartworm disease will start to show lack of endurance during exercise. Some will accumulate fluid in their abdomen (ascites) that makes them look pot-bellied. In rare situations in which animals have many adult worms, the animals may die of sudden heart failure.

Testing: There are several blood tests used to detect heartworm infection. Blood testing is performed to identify dogs infected with D. immitis. Because blood tests are not always accurate, we need to interpret test results in relation to the history and the symptoms the animal is showing. Radiographs (x-rays) and ultrasound (echocardiography) are often performed to look for typical changes in the heart and lungs caused by D. immitis, and determine the severity of the infection. Changes include enlargement of the pulmonary artery and the right ventricle. Certain types of cells may increase in the blood or secretions from the lungs in heartworm infections. These additional findings can all help support the diagnosis.

Prevention:
Medications used to prevent heartworm infections are called preventives. The first thing to remember is that preventives are NOT used to kill the adult worms. Some preventives can cause severe problems if given to animals with adult heartworms or microfilariae. This is why the animal should be first tested on the absence of Dirofilaria before taking preventive medication.

Preventive products should be used year-round, even in areas where mosquitoes only occur seasonally. There is a number of preventive products available on the market. One of them is Heartguard by Merial (pills, administered once a month, dose depending on weight of the dog).

Treatment:
Melarsomine (adulticide) is the medication currently used to treat heartworm infection. It is given by injection deep in the muscles of the back. The treatment protocol depends on the severity of infection. In less severe cases, the dog may be treated for four months with a heartworm preventive to kill any migrating heartworm larvae and to decrease the size of the female worms. Then an injection of adulticide is given to kill the adult heartworms. Five weeks later, the dog is treated with two more injections. Four months after the treatment, the dog should be tested for heartworms. Some animals may need to undergo a second round of injections if repeat antigen tests remain positive. It is recommended that dogs remain on a monthly heartworm preventive during the treatment.

Regardless of which drug is used, when the adult heartworms die, they can obstruct blood vessels to the lungs (these are called pulmonary embolisms). If only a small part of the lung is involved, there may be no clinical signs. However, if the vessels to a large portion of the lung, or a small area of an already diseased lung are blocked, severe signs may result. These include fever, cough, coughing up blood, and even heart failure. Because of the risk of these embolisms, any dog being treated with an adulticide must be kept very quiet during treatment and for at least 4 weeks thereafter. In very severe infestations adult worms are removed from the heart surgically.

No vaccine exists.

ATTENTION: these are only general tips, every treatment is on case by case basis so if your dog has this disease you should consult the vet.