Paws for Thought

EDITOR’S NOTE: Dr. Janice Mitchell is a veterinarian with Island Animal Hospital and Little Current Veterinary Services.

“Ants” in our pants: antigen vs antibody tests

by Dr. Janice Mitchell

With everyone’s minds focused these days, there has been much discussion about laboratory tests with regards to one virus in particular. It can be very confusing. Immunology is one complex subject and thus it takes some education and thought to figure out how to use and interpret diagnostic tests. I thought a quick, simple description of antigen vs antibody is in order, and will demonstrate some examples of the common tests that veterinarians use today.

Let’s begin with some boring definitions.

An antigen is any substance that causes your immune system to produce antibodies against it. An antigen may be a substance from the environment, such as chemicals, bacteria, viruses or pollen. An antibody is an immunoglobulin, a specialized immune protein, produced because of the introduction of an antigen into the body, and which possesses the remarkable ability to combine with the very antigen that triggered its production.

The antigen tests that are commonly selected in veterinary medicine include heartworm, feline leukemia, blastomyces and parvovirus tests. The first two involve testing the patient’s blood. Blastomyces is detected using the patient’s urine and  parvovirus is detected using the patient’s faeces. 

Heartworm is a fairly large worm that, in adulthood, lives in the heart and pulmonary arteries of an infected dog. Dogs acquire this infection through mosquito bites. Using genetic engineering, it has become possible to create extremely sensitive tests capable of detecting tiny pieces of adult female heartworm skin circulating in the blood.  Although this is the most commonly performed screening test for our canines in most of Ontario, fortunately for Manitoulin, it is not a concern as of the moment. 

Feline leukemia is a common virus for our kitty friends that can cause debilitating disease. It is a social disease and can be contracted from an infected individual through grooming or fighting. It can be diagnosed through a sensitive blood test, which detects a piece of the virus’ protein.

Blastomycosis is a disease that we are unfortunately acquainted with—it is caused by the fungus Blastomyces that lurks in the decaying vegetation here on Manitoulin Island. Dogs are particularly sensitive to acquiring this disease as a result of their sniffing, curious noses low to the ground. Thankfully, we veterinarians have a very sensitive test that will detect low levels of this fungus’s antigen in a dog’s urine. Unfortunately, there is only one lab in North America that performs this test, in Indiana. Interestingly, there is an antibody test available but it is not as sensitive as the antigen test so it is rarely used.

Finally, the last of our antigen examples—parvovirus. Parvovirus is a disease of primarily puppies and adolescent dogs. It causes dehydrating vomiting and diarrhea. To differentiate between other causes of vomiting and diarrhea, a test kit will detect the parvovirus antigen in the dog’s stool.

Now, how about those antibody tests? The results of some antibody tests can be ‘diagnostic’ for particular illnesses. This means that if a pet tests positive for a particular antibody it has a certain condition now. However, it can also mean that your pet might have had this illness in the past. The two common ones that come to mind are the Feline Immunodeficiency Virus and tick (lyme in particular) tests. 

Feline Immunodeficiency Virus (FIV) is a virus that causes AIDS in cats. With FIV, there is little circulating virus so we have to look for the infection indirectly by looking for antibody. Because infected cats never recover from this disease, the presence of antibody is confirmatory evidence of infection. Why don’t we use an antibody test for the earlier listed feline leukemia? The reason is that with feline leukemia, there is usually a bucket of virus circulating so an antigen test is very accurate.

Lyme disease is another Manitoulin present disease that we are familiar with. It is transmitted to dogs (and less likely cats) through a deer tick. Dogs with symptoms ideally should have a test to confirm or rule out Lyme disease. Since it is almost impossible to culture the Lyme spirochete, efforts have centered on detection of antibodies against the spirochete’s outer surface protein. Is there also an antigen test for Lyme? There is, but veterinarians will often use the less expensive antibody screening test instead in light of the patient’s clinical signs.

With these antigen and antibody tests available, clinical judgement is needed. When considering testing, key questions veterinarians ask themselves, “Can this test give clear results?”, “Will the results of the test change anything?” and “Is this a test in a sick pet or a healthy pet, and if healthy, why am I running it?” And should you have questions, never hesitate to ask. 

Positive thoughts and health to you all, both two-legged and four-legged!