Viral Disease Testing in Cats - Prevaccination

By Krista Williams, BSc, DVM; Kristiina Ruotsalo, DVM, DVSc, ACVP; Margo S. Tant, BSc, DVM, DVSc

What viruses do we commonly test for prior to vaccination?

The most common feline viral test that is performed prior to vaccinations is for feline leukemia virus (FeLV).

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. See handouts "Feline Leukemia Virus Disease Complex" and "Feline Leukemia Virus Vaccination" for further information on this disease.

Why should we test for the presence of FeLV and FIV prior to vaccination for these diseases?

Determination of your cat's FeLV viral status is important for a number of reasons. First, vaccination of an already infected cat with a FeLV vaccine will not have any beneficial impact on, or alter the natural course of disease progression.

Second, the only way to help prevent the spread of viral disease amongst cats is to know the viral status of individuals. Cats affected by this virus should be kept indoors – both to prevent them from spreading the virus and to keep them more protected against infectious diseases they can pick up outside.

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 test kits. The test kit uses a color indicator so that a color change occurs when the viral protein is detected in the blood.

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 (presence of the virus in the blood) may not be permanent.

Because no test is reliable all of the time, and because of the possibility of 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. A positive 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 referral laboratory test is to repeat the in-clinic test in one month, thus allowing the cat time to mount an appropriate immune response and potentially eliminate the FeLV virus.

Newer DNA tests (PCR) 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 screening tests when used to evaluate blood samples for the presence of virus.

What does a positive test result mean?

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 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.

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