What are heartworms?
Heartworms, Dirofilaria immitis, belong to the same class of worms as roundworms. In fact, they look a bit like roundworms, but that is where the similarity ends. Heartworms spend their adult life in the right side of the heart and the large blood vessels connecting the heart to the lungs.
Heartworms 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.
How do dogs become infected with heartworms?
Adult heartworms in the heart 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. 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. Thirty to eighty percent 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.
What damage do heartworms cause?
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.
What are the signs of heartworm infection and how is it diagnosed?
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.
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 (eosinophils) may increase in the blood or secretions from the lungs in heartworm infections. These additional findings can all help support the diagnosis.
What tests are available to detect D. immitis infection in animals?
There are several blood tests used to detect heartworm infection. In the 1960′s, before more sophisticated tests were available, heartworm testing involved looking for the microfilariae in a drop of blood on a microscope slide. A better test, the Knott’s test, was developed to concentrate the microfilariae from a larger portion of blood through centrifuging it. This gave veterinarians a better chance of finding the microfilariae.
Later, filter tests became available. In these tests, the blood cells in the blood were lysed (broken) by a special agent that did not affect the microfilariae. The resulting liquid was then put through a very fine filter. The microfilariae were concentrated on the filter. The filter was stained and examined under the microscope for microfilariae.
Veterinarians soon recognized that some animals could have heartworm infections without having microfilariae in the blood. This occurs if only male worms are present or if the females are not laying microfilariae at the time of the test. It was obvious that better tests were needed.
What is included in a good heartworm prevention program?
The best program for prevention of heartworm infection includes using preventives, performing routine heartworm testing, and reducing exposure to mosquitoes.
Call Summer Valley Veterinary Clinic to start your heartworm prevention today.
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