Viral Disease Testing in Cats - Prevaccination
What viruses do we commonly test for prior to vaccination?
The most common feline viral tests that are performed prior to vaccinations are for Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV).
What diseases do these viruses cause?
Feline Leukemia Virus (FeLV) is one of the most serious infectious viruses in cats. It is responsible for a number of diseases in cats including leukemia. FeLV is highly contagious and is transmitted through body fluids, and may be transmitted across the placenta in pregnant cats.
Feline Immunodeficiency Virus (FIV) is another virus that is specific to cats. FIV reduces the capacity of the cat's immune system to respond to other infectious agents. FIV is highly contagious and is transmitted primarily through cat bite wounds, although it may be transmitted by other routes such as across the placenta.
Why should we test for the presence of FeLV and FIV prior to vaccination for these diseases?
Determination of your cat's FeLV and FIV viral status is important for a number of reasons. First, vaccination of an already infected cat with a FeLV or FIV vaccine will not have any beneficial impact on, or alter the natural course of disease progression.
"Determination of your cat's FeLV and FIV viral status is important for a number of reasons."
Second, the only way to help prevent the spread of viral disease amongst cats is to know the viral status of individuals. This allows us to make appropriate husbandry decisions (for example, the maintenance of a single cat household or confinement of the cat indoors) in the event of a confirmed positive viral status.
Third, on an individual basis, knowledge of your cat's viral status, especially a confirmed positive viral status, will be helpful in determining the appropriate monitoring or treatment options in the event of future illness.
What tests are used to detect FeLV infections?
FeLV screening tests look for the presence of the viral antigen (viral protein) in a blood sample. Screening tests for FeLV can be done in the clinic setting using special ELISA (enzyme linked immunosorbent assay) based test kits. These kits contain FeLV antibody, a special protein that recognizes the viral antigen and combines with it. The test kit incorporates a color indicator with the antibody so that a color change occurs when the antibody combines with the FeLV antigen.
A positive screening test result is indicative of the presence of virus particles. Since some cats are able to subsequently mount an appropriate immune response and eliminate the virus from their bodies, this viremia (literally virus in the blood) may not be permanent.
Because no test is reliable all of the time, and because of the possibility of transient (temporary) viremia, it is important to confirm a positive test result, especially in a clinically healthy animal.
Such confirmatory testing is usually done at a veterinary referral laboratory. It may involve an indirect immunofluorescent antibody (IFA) test that detects the presence of cell associated viral antigen. A positive IFA test indicates the presence of virus and that the cat is not likely to eliminate the FeLV virus from his body. An alternative to the IFA test is to repeat the ELISA test in the clinical setting in one month, thus allowing the cat time to mount an appropriate immune response and potentially eliminate the FeLV virus.
Newer polymerase chain reaction (PCR) diagnostic tests that detect viral genetic material have been developed to confirm FeLV infections. These PCR based tests do not seem to have much advantage over the ELISA based tests when used to evaluate blood samples for the presence of virus.
What tests are used to detect FIV infections?
Feline immunodeficiency virus (FIV) screening tests look for the presence of anti-viral antibodies in a blood sample. For many diseases, the production of antibodies (proteins indicating an immune response) results in recovery from disease, but unfortunately with FIV, these antibodies do not confer protection and the FIV infection is not cleared. Screening tests for FIV can be done in the clinic setting using special ELISA (enzyme linked immunosorbent assay) based test kits. These kits contain a protein (antigen) that binds the FIV antibodies. The test kit incorporates a color indicator with the antigen and a color reaction occurs when the antigen combines with the antibody.
"...kittens less than 6 months of age may result in a false positive test result.
There is a good correlation between the presence of antibodies (and therefore a positive test result) and FIV infection. Nevertheless, it is important to confirm a positive screening test result, especially in a clinically well animal, as no test is entirely accurate all of the time. It is important to know that the presence of maternally derived antibodies in kittens less than 6 months of age may result in a false positive test result. In such a situation, re-testing of these kittens after 6 months of age or after 60 days is indicated. In addition, since this test specifically looks for the presence of antibody to FIV, which take time for the immune system to produce, a single FIV test may produce a false negative result if the patient has very recently picked up the viral disease. Therefore, retesting in 2 months may be indicated if there is any history of potential recent exposure to the virus such as recent fight wounds, or exposure to a stray cat.
Confirmatory testing for FIV is done at a veterinary reference laboratory. To confirm this disease, a blood sample is analyzed for the presence of antibodies to FIV using a protocol known as the Western Blot procedure.
Newer polymerase chain reaction (PCR) diagnostic tests that detect viral genetic material have also been developed to confirm FIV infections. The clinical utility of this type of test is currently under review.
What does a positive test result mean?
"...important to confirm a positive test result in a clinically healthy animal..."
As mentioned previously, it is always important to confirm a positive test result in a clinically healthy animal as no test is completely accurate all of the time. It is also important to realize that a positive FeLV or FIV test result means only that your cat has viral infection, not necessarily viral associated disease, and that infected cats may remain symptom free for years.
What if I choose not to test for FeLV or FIV at this time, but vaccinate my cat for these diseases? Will this have any impact on future viral testing?
FeLV vaccination will not have any influence on future FeLV testing as FeLV tests look for a specific antigen (viral protein) that is only found when the virus is present.
FIV vaccination will interfere with future FIV testing as both vaccination and viral infection result in the formation of antibodies against the virus. Most FIV tests look for the presence of antibodies in the blood and cannot distinguish between antibodies present because of disease from those present because of vaccination.