Heartworm infection is diagnosed through a simple blood test, and results are available immediately.
Heartworm infection is usually diagnosed through a heartworm antigen test. The dog's blood is drawn and tested for the presence of a protein shed by the female worms. This test is very reliable and will detect worm burdens of 2-3 worms or higher. Another common test is the Knotts test, where blood is examined under a microscope for the presence of microfilaria.
Usually once a diagnosis is made, X-rays, blood tests, and urinalysis are evaluated to determine how the heartworm infection has impacted the dog's health. Part of the evaluation includes "staging" the disease, which helps the vet decide which treatment protocol would be best for this particular dog:
-Stage I -
Low risk. Young healthy dogs with minimal heartworm symptoms, little or no evidence of heartworm disease on X-rays, and other tests are normal.
-Stage II -
Moderately affected dogs. Some coughing &/or some difficultly breathing is noticed, along with other clinical symptoms such as slow heartbeat, congested lungs, fever, and listlessness. Changes are seen on X-rays and blood work, possibly revealing some kidney and/or liver damage.
-Stage III -
Severely affected dogs. Weight loss, severe coughing, sever congestion, and difficulty breathing are usually observed, with more damage visible on X-rays and blood tests showing significant kidney and/or liver damage.
-Stage IV -
Vena Cava Syndrome or Caval Syndrome. The dog is collapsing in shock, all of the above abnormalities are more intense, and the dog is dying. Normal heartworm treatment will not be effective. Instead, the dog is initially treated with surgical jugular removal of some worms, if possible. Many patients with Caval Syndrome die in spite of attempts to treat.
Why A Dog Must be Tested for Heartworms
Since symptoms do not show up until heartworm infection is advanced, owners often do not have a clue that their dog is heartworm positive until the dog is seriously sick--sometimes too sick for treatment--unless heartworm testing is done. Even owners who believe that they gave the dogs preventative faithfully each and every month may not be aware that their dog spit out a tablet or vomited it, leaving the dog unprotected. Monthly heartworm preventative works against baby and juvenile heartworms but does not kill the adults, so once a dog's juvenile heartworms have matured into adults, the dog will continue to deteriorate, with damage done to the heart, arteries, and rest of the body from inflammation and immune reaction to the worms.
Adopters of new dogs must be particularly careful because heartworm infections are not detectable until about six months after a dog has been bitten by a heartworm-infected mosquito, so it's possible for a rescue dog to have received a negative heartworm test even though it was, in fact, infected. For this reason, it's recommended and reasonable to run a heartworm test on an adopted dog with an unknown medical history six months after adoption. The same would apply if there is ever a lapse in heartworm preventative dosing for more than two months. Thereafter, have the dog tested annually for the rest of his life. Puppies under 6 months old, however, are too young to test positive under any circumstances.
Overview of Heartworm Disease
What Is Heartworm Infection
How Heartworm Infection is Prevented
How Heartworm Infection is Treated
For More Information about heartworms, go to the American Heartworm Society website
or to The Pet Center