What viruses do we commonly test for?
The most common feline viral tests that are run are for Feline Leukemia Virus (FeLV), Feline Immunodeficiency Virus (FIV) and Feline Coronavirus.
What diseases do these viruses cause?
Feline Leukemia Virus (FeLV) is one of the most serious infectious viruses in cats. FeLV is responsible for a number of diseases in cats including leukemia. FeLV is highly contagious and is transmitted through body fluids, or across the placenta in the pregnant cat.
Feline Immunodeficiency virus (FIV) is another virus that is specific to cats. It may also be referred to as Feline AIDS. FIV reduces the capacity of the cat’s immune system to respond to other infectious agents. FIV is highly contagious and is transmitted primarily during cat fights, although it may be transmitted by other routes such as across the placenta.
Feline Infectious Peritonitis (FIP) is a disease caused by a mutated strain of Feline Coronavirus. FIP can cause a variety of clinical signs including the production of fluid in the abdominal and chest cavities. It is transmitted through infected body fluids or in the feces.
Further, much more detailed accounts of these diseases are available in other handouts.
When is testing indicated for these viruses?
Testing for FeLV and FIV is indicated under a number of circumstances. Since both conditions are associated with a wide variety of diseases, including those exhibiting bone marrow and immune dysfunction, testing for FeLV and FIV is suggested whenever a cat is ill. Other reasons for testing include testing a cat that has been exposed to another cat of unknown FeLV or FIV status, or testing a new cat prior to adoption into a household with other cats.
Feline Coronaviruses are common and are found in the intestinal tract of many healthy cats. The mutated strain of coronavirus that causes disease is less common. Because the number of cats exposed to and therefore carrying antibodies to feline coronavirus is high (estimated to be up to 30% of the general population, and up to 80% of cats in catteries), but the percentage of cats that actually develop FIP is very small, routine blood testing for feline coronavirus is probably not clinically useful. Therefore testing is generally restricted to those cats in which a diagnosis of FIP is strongly suspected due to other clinical information and supportive laboratory data. Occasionally catteries or multi-cat households wishing to maintain a feline coronavirus free status may routinely test for feline coronavirus.
What sorts of tests are used to detect these viral infections?
FeLV screening tests look for the presence of 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 specifically 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 FeLV antigen.
A positive screening test result is indicative of the presence of virus. Since some cats are able to mount an appropriate immune response and eliminate the virus from their systems, 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 and may involve an indirect immunofluorescent antibody (IFA) test that detects the presence of cell associated viral antigen. A positive IFA test result indicates the presence of the 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 clinic setting in 1 month, which allows time for the cat to eliminate the FeLV virus naturally.
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 appear to have much advantage over the ELISA based tests when used to evaluate blood samples for the presence of virus.
FIV screening tests look for the presence of viral antibodies (indicating an immune response) in a blood sample. 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 and produces a color reaction
There is 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 also important to recognize that, since the test specifically looks for the presence of antibody to FIV, and since it takes time for the body to develop antibodies, a single FIV test may produce a false negative result if the patient has recently picked up the viral disease. Therefore, your veterinarian may recommend repetition of the screening test in approximately 2 months to ensure that your cat is FIV negative.
Confirmatory testing is done at a veterinary reference laboratory, and requires a single blood sample that is analyzed for the presence of antibodies to FIV using a protocol known as the Western Blot protocol.
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.
It is very important to realize that cats infected with either FeLV or FIV may live for many years. Depending upon the initial reason for testing, a confirmed positive test result should be considered only an indication of viral infection and not necessarily an indication of viral disease. However, it is important to keep all positive cats indoors so that they do not infect other cats.
FIP testing is somewhat more problematic as there are no blood tests available that will absolutely confirm the disease. FIP is caused by a mutated strain of feline coronavirus, but exposure to any strain of feline coronavirus will result in the production of an immune response (antibodies). There is no blood test that will distinguish between antibodies produced against a non-FIP strain of coronavirus and a FIP-causing strain of coronavirus.
A negative blood test result for coronavirus antibodies does not mean that a cat could not have FIP, as detectable antibody concentrations may be reduced in animals with the terminal form of the disease.
Even newer PCR (polymerase chain reaction) tests that have been designed to detect viral genetic material are unable to distinguish accurately between the different strains of coronavirus.
While the combined information derived from clinical history, laboratory data, and characteristic features of any fluid present in the abdominal or chest cavities may be supportive of FIP, the definitive diagnosis of FIP continues to rely upon microscopic examination of affected tissue or upon characteristic post mortem findings.
In conclusion, tests for FeLV and FIV are useful screening tests, while the current tests for FIP are less helpful. All 3 diseases are highly contagious, and cats that are exposed to other cats can become infected with any one of the diseases. Outdoor cats are at a higher risk of infection, and your veterinarian may recommend screening for FeLV and FIV on a routine basis.