How do dogs become infected with heartworms?

Adult heartworms lay very tiny larvae called microfilariae. The microfilariae can live up to 2 years in the dog's 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 until adult heartworms develop, mate, and lay microfilariae is about 6-7 months in dogs and 8 months in cats. (Remember this – it is important when we talk about diagnosis.)

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.

What damage do heartworms cause?

 

The adult worms can obstruct the different chambers of the heart and the various large blood vessels leading from the heart to the lungs. First, the right pumping chamber (ventricle) of the heart and the large pulmonary artery leading from it to the lungs becomes enlarged as the worms take up space. If worms die, they may follow the flow of blood into smaller vessels in the lung and obstruct those vessels.

 

In severe infections, the worms can also block the large vein (vena cava) bringing blood to the right side of the heart. As the blood backs up, the liver becomes enlarged and damaged.

What are the signs of heartworm infection and how is it diagnosed?

Dogs with heartworm infections may show decreased appetite, loss of weight, and listlessness. Often, the first sign of the disease is a cough. Animals with 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.

Antigen 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) are often taken to look for typical changes in the heart and lungs caused by D. immitis. 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.

Serologic tests were developed to identify antigens (small protein and carbohydrate components) of heartworms in the bloodstream. There are different varieties of this test. Some test kits run one sample at a time and can be done right in your veterinarian's office. Others are designed to test multiple samples in large batches. This batch-type of test is generally performed by outside laboratories to which your veterinarian sends your dog's blood.

Although the antigen tests were much better than the filter test, we still could not identify all cases of heartworm infection because antigen tests will only be positive if female worms are present, since the antigen detected is from the worm's uterus. If the heartworms were not fully mature, or there were only male worms present, the antigen test result in infected animals would be falsely negative. This means the test result is negative when the animal is really infected.

Serologic tests have now been developed to detect antibodies (proteins to fight off the 'invaders') the animal makes against heartworms. This is the test most commonly used in cats. This test will be positive even if only one male worm is present. But this test has a downfall, too. Although it is very good at giving positive results when an infection is present, false positive tests are more common with this test than the antigen test. A false positive result means the test result is positive when no infection is present.