Testing for Dog Heartworm Symptoms

Heartworm is a serious and potentially fatal disease in dogs caused by parasitic worms blocking the heart and associated blood vessels. It is easily preventable by administering one of a number of medications.

If your dog has a history of missing heartworm preventative medication, has never been on medication, or has any clinical signs suggesting the disease, your veterinarian will want to test for the presence of heartworm.

There are two main types of tests which look for evidence of the worms in blood, with both relying on having adult worms in the heart. It takes about six months from the time a mosquito infects a dog with heartworm until the parasites reach the heart and mature into adults, and these tests cannot diagnose an infection until this point. There are no diagnostic tests available to detect a heartworm infection before the adult stage. Therefore, if you miss a month or two of preventative medication, just remember that it will be necessary to wait for a few months to know if your dog has contracted heartworm.

Symptoms of heartworm
Symptoms suggesting a dog may have heartworm disease include:
  • a cough
  • exercise intolerance
  • lethargy
  • difficulty breathing
  • weight loss
  • loss of appetite
  • fluid accumulation in the abdomen
  • sudden collapse
  • brown urine.
These signs are not confined to heartworm but may provide some useful clues to guide your veterinarian. Some dogs with heartworm disease will not show any signs at all.

The test used is an "antigen" test. It only requires a small amount of blood to be collected from your dog. This test detects antigens (substances on the worm which provoke an immune response from the dog) from adult female heartworms. It can show a positive result from about five months after infection, if the worms mature quickly.

There are simple test kits available which enable vets to perform these tests in the clinic fairly quickly, and they will give either a positive or negative result. These are fairly reliable tests but if there are no female worms present or the test is performed too early (before the worms are mature) it will yield a false negative result. This is the most common test performed in a clinic to diagnose heartworm disease.

The other common heartworm tests performed in clinics are those which look for baby worms, called microfilariae, in the blood. When present, these can be seen wriggling between blood cells in specially-prepared blood smears examined under a microscope. These tests are less reliable than the antigen tests and are more difficult to perform, so are used less frequently. Microfilariae are not usually produced until approximately six and a half months after infection, so if the test is performed too soon, there will be a false negative result.

"If you miss a month or two of preventative medication, just remember that it will be necessary to wait for a few months to know if your dog has contracted heartworm."

Infections in which the worms are all one sex and so cannot reproduce will also produce a false negative. Both the dog's immune system and heartworm preventative medication can clear all the microfilaria from the bloodstream. An infection of heartworm where adults are found in the heart ,but no microfilariae are found in the blood, is called an occult infection. There are also other types of larvae from harmless parasites which may be seen in the blood and confused with heartworm microfilariae. Iit takes close examination to differentiate them.

In addition to these tests, your veterinarian may wish to perform further tests which indicate the severity of the disease. These may include:

Anti-heartworm treatments
Pet Shed's most popular solutions for ridding your pet of heartworm:
  • x-rays to look at the heart and lungs
  • blood tests to check on organ function, which can deteriorate when the heart is failing due to the presence of heartworms
  • and an ultrasound examination of the heart walls and chambers to look for any wriggling worms.

Animals more than six months of age beginning a heartworm prevention regime for the first time should have a heartworm test prior to starting the medication. Animals missing doses of heartworm should also be tested approximately six months after the dose gap. In addition, if an animal is changing the type of heartworm medication, a heartworm test is advisable to check that the previous medication has been effective. Many veterinarians will also recommend periodic testing to verify the medication being given is effective.

American Heartworm Society. http://www.heartwormsociety.org/heart.htm
Payne, P.A., Dryden, M.W., Carter, G.R. External Parasitic Diseases of Dogs and Cats. In: A Concise Guide to Infectious and Parasitic Diseases of Dogs and Cats International Veterinary Information Service, Ithaca NY. www.ivis.org
McCall J.W. et al. Recent advances in heartworm disease. Veterinary Parasitology 125:105-130, 2004.
Heartworm. Michigan Department of Agriculture. www.michigan.gov/mda
Tilley, L.P., Smith, F.W.K. The Five Minute Veterinary Consult Canine and Feline. Second Edition. Lippincott Williams & Wilkins, Baltimore, 2000.