FeLV and FIV testing
The Feline Leukemia virus (FeLV) and the Feline Immunodeficiency virus (FIV) are both in the retrovirus family. However, they are very different in structure, protein makeup, and the way they cause disease. The main source of transmission between cats is bite wounds from catfights. Testing is usually recommended for: new cats and kittens, whose viral status is unknown, for sick cats, and cats that may have been exposed to infected cats.
The in-house snap tests are enzyme-linked immunosorbent assay, or ELISA tests. It tests for the FeLV antigen, specifically a protein on the virus. An antigen is any substance, often a protein, which stimulates an immune response. A positive test result indicates the actual virus was detected in the cat’s blood, and a negative result means it was not.
For cats that test positive on the snap test a follow up test can be sent to the outside laboratory, called an immunofluorescene assay (IFA). This is also an antigen test, but tests for the antigen within the cat’s white blood cells, indicating that the disease has progressed to the bone marrow. Theoretically a positive snap test could occur at a very early stage when their immune system may still fight the infection off, but if the IFA is also positive then it has already moved into full-blown leukemia.
The biggest difference between the FeLV and FIV snap tests is the FIV test detects the presence of FIV antibodies, not an antigen. Antibodies are proteins produced by the immune system that recognize and eliminate foreign objects (like viruses and bacteria). This is why FIV test results are considered unreliable in cats under six months old. It’s possible for a nursing mother cat to pass on antibodies to her babies. Young kittens with positive FIV results therefore should be retested after six months of age to determine if they were truly positive or if they received antibodies from mom.
It’s important to note that a cat that was vaccinated against FIV will also have antibodies, and test positive. A stray cat with unknown vaccination history and a positive FIV test may truly have FIV, or may simply have been vaccinated against it. Another possibility would be a cat that was exposed to FIV, but whose immune system fought off the disease. These cats would also have FIV antibodies. Unfortunately there is no follow up test to distinguish between those cats who have only been exposed or vaccinated and those who actually have the disease. Keep this in mind when interpreting results from an otherwise healthy looking cat.