What viruses do we commonly test for in cats?
Feline viral testing typically encompasses testing 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 important infectious viruses in cats. FeLV is responsible for a number of diseases in cats including leukemia.
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.
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.
Further, much more detailed accounts of these diseases are given in other handouts in this series.
When is testing indicated for these viruses?
"There are a number of reasons for recommending testing for FeLV and FIV."
There are a number of reasons for recommending testing for FeLV and FIV. Because both conditions can be associated with a wide variety of diseases, including those exhibiting bone marrow and immune dysfunction, testing for FeLV and FIV is strongly recommended whenever a cat is ill. Testing might be recommended for other reasons. Examples include testing a cat that has been exposed to another cat of unknown FeLV or FIV status, or testing a new cat prior to its adoption into a household with other cats.
"...routine blood testing for feline coronavirus is probably not clinically useful."
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 tests are used to detect feline leukemia viral infections?
FeLV screening tests look for the presence of viral antigen (viral protein) in a blood sample. Your veterinarian can perform a screening test for FeLV in the clinic setting using a special ELISA (enzyme linked immunosorbent assay) based test kit. These kits contain feline leukemia 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 the FeLV antigen.
A positive screening test result is indicative of viremia, or the presence of virus. However, as a percentage of cats are able to eliminate the virus from their systems, this viremia may not be permanent. Because no test is reliable all of the time, and because of the possibility of transient 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. Testing 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 indicates that the cat is not likely to eliminate the FeLV virus from his body.
Newer polymerase chain reaction (PCR) diagnostic tests that detect viral genetic material have also 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.
What tests are used to detect feline immunodeficiency virus infections?
"FIV screening tests look for the presence of an immune response..."
FIV screening tests look for the presence of an immune response (antibodies) against the virus 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) which binds to the FIV antibodies. The test kit incorporates a color indicator with the antigen so that a color change occurs when the antigen combines with the FIV antibody.
"...a false negative result if the patient has very recently picked up the viral disease."
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 important to know that maternally derived antibodies to FIV from infected queens may confound the results in kittens younger than 6 months of age. These kittens are often not infected with FIV but their viral status should be clarified by retesting after 6 months of age or after 60 days. In addition, since this test specifically looks for the presence of antibody to FIV, which take time for the cat's 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.
For FIV, confirmatory testing is done at a veterinary reference laboratory. This infection is confirmed by analyzing a blood sample 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.
"It is very important to realize that cats infected with either FeLV or FIV may live for many years."
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 disease.
What tests are used to detect FIP viral infections?
FIP testing is somewhat more problematic. FIP is due to 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. To complicate diagnosis even further, a negative blood test result for coronavirus antibodies does not mean that the 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 histological examination of affected tissue or post mortem findings.