Heartworms are parasitic worms that live in the heart, lungs, and arteries of dogs and are spread through the bite of an infected mosquito.   Untreated, heartworm disease can result in severe heart damage and even death.  Heartworm disease in dogs has been reported in all 50 states, with the disease considered to be an endemic in South Central states.

To understand heartworm testing procedures, it is very important to first understand the lifecycle of the parasite.  When a mosquito bites an infected dog, it withdraws blood that contains immature heartworms called microfilariae.  The infected mosquito then spreads the disease to other animals when feeding.  During the bite, the mosquito injects the immature parasite into the bloodstream. The immature heartworms spread through the circulatory system to the lungs and the heart where they mature into adult worms.  Later, the adult worms mate and produce more microfilariae and the cycle continues when another mosquito bites the newly infected dog.

It takes about six months from the time of the bite of the mosquito before the immature heartworms develop into adult heartworms that are capable of producing microfilariae.  Adult heartworms can live up to seven years in the dog.  The worms damage the heart and the vessels in the lungs.  Eventually, without treatment, the dog will develop fatal heart failure.
  
Heartworm Testing

The diagnosis of heartworm infection in dogs can be made by viewing microfilariae in a blood sample under the microscope or by finding adult heartworm antigen in blood. The diagnosis is rarely made by identifying adult worms in an echocardiography or through x-ray.

Since it takes between 4 and 6 months before the adult worm can produce offspring, a test can appear negative if the blood is taken too early, before the worm can mature.  A test may also be falsely negative if the heartworms are all male or all female and are incapable of producing offspring.

There are a few types of tests that are performed:
 
Antigen Testing:
Antigen testing is the most common test performed and utilizes a small amount of blood.   This test can only detect the presence of adult female heartworms.  False negative antigen tests can be the result of an all male population of adult worms or a population of immature worms.

Antibody Testing: 
Animals with adult worms may not have microfilariae circulating in their blood; this is called an occult infection.  To rule out an infection without  the presence of microfilaria, your vet  may need to perform an enzyme-linked immunosorbent assay (ELISA)-based antibody test. This test checks for the presence of antibodies, which are formed from the animal's immune response to heartworms.  These tests can be performed in the hospital or can be sent out to a diagnostic laboratory for analysis.  This test is not commonly used because a positive test means that there is antibody circulating against heartworms, so it could detect a previous exposure and not a current infection. 

Direct Smear

In a direct smear test, a small sample of blood is placed on a microscope slide and examined. Sometimes, the microfilaria can be seen squirming around in the blood drop.

Filter test:
In this test, a sample of blood is mixed with a special solution and then is passed through a special filter that will remove microfilaria. The filter is placed on a microscopic slide and a dye is added to stain the parasite so they are easier to see. If microfilaria are present in the blood, they can be visualized around the slide when viewed under a microscope. The direct smear and the filter test are not commonly performed, because of the potential of having false negative results.

Benefits of Heartworm Tests

A group of veterinary experts developed an organization called the Heartworm Society and they have created standards of care on the prevention, treatment, and diagnosis of heartworm disease. According to this organization, a dog should be tested for heartworms before placing them on a heartworm prevention. The group also recommends to test dogs that are on preventative medications every two years, since there is a slight chance that they may become infected despite prophylactic care.