FeLV and FIV: Testing vs. Vaccination
Clearly, one of the most significant technologic advances introduced into clinical practice within the last 10 years has been
the ENZYME-LINKED IMMUNOSORBENT ASSAY, or ELISA. The ability to perform in-hospital, same-day screening for feline leukemia virus (FeLV), feline immunodeficiency virus (FIV),
and canine parvovirus (CPV) are among the most important value-added services offered by the companion animal practitioner
today.
The feline retroviral infections, FeLV and FIV, continue to represent a major threat to the health of the domestic cat population.
Extensive surveys have consistently demonstrated that among sick cats presented to veterinarians, from 11% to 16% are FeLV-positive
while from 6% to 9% are FIV-positive. Retrovirus infection is best prevented by eliminating opportunities for exposure of
susceptible cats and kittens to infected cats. However, in the clinical setting, it is unreasonable to assume that recommendations
for avoiding contact with FeLV/FIV–infected cats constitutes a practical approach to prevention. Fundamental to the strategic
management of feline retrovirus infection is the opportunity to vaccinate at-risk cats against FeLV. However, routine FeLV
vaccination of cats presented to the practice only represents one component of a prevention strategy. Since the available
FeLV vaccines are unlikely to confer complete protection in all vaccinated cats and that there is currently no FIV vaccine
available suggests that the risk of retrovirus infection among susceptible cats is still significant.
In accordance with current guidelines, it is recommended that the FeLV and FIV status of all cats seen by the practice be established. However, the commitment to perform routine screening of cats for retroviral infection
raises important issues pertaining to interpretation of test results and follow-up actions needed to further manage those
households with confirmed FeLV and/or FIV positive cats. Current testing recommendations outlined by the AAFP/AFM have recently
been published and are ALTERNATIVES: A Veterinary Clinical Update. (A Supplement to Compendium on Continuing Education, published by Veterinary Learning Systems, Volume 19; Nbr. 12).
The current ELISA technology used for in-hospital retrovirus testing in cats is remarkably sensitive and accurate assuming
proper technique and specimen handling are used. These same tests are recommended for routine screening of both healthy and
sick cats presented to the practice.
Fundamental to the proper use of ELISA-based testing for the diagnosis of FeLV and FIV infected cats is an understanding that
FeLV tests are designed to detect the p27 antigen (core antigen) while FIV tests detect the presence of antibody. In clinical practice, these facts have important implications. For example, the FeLV test can detect FeLV virus in the blood
(serum or plasma) of young kittens; a positive test is consistent with infection. It is important to note that FeLV tests designed to detect the presence of virus in tears and/or saliva are also ELISA-based
tests. However...these tests are significantly less sensitive/specific (ie, large numbers of false-positive and false-negative results)
than tests utilizing blood, serum, or plasma. Saliva and tear tests should not be used for routine screening of individual
cats. It should also be noted that neither maternal antibody nor recent FeLV vaccination interferes with the ELISA-based FeLV test. The reader is referred to Figure 1 for guidelines on interpreting the various test results in cats at various stages in the course of FeLV infection.
ELISA-based FIV tests are not reliable in kittens less than 6 months of age. Since antibody response to FIV infection requires weeks or months to become
detectable, a negative test result could occur in an exposed, infected kitten that has not seroconverted. On the other hand, uninfected kittens from
FIV infected queens may test positive as a result of having acquired maternal FIV antibody; detectable levels of maternal FIV antibody can persist for as long as
6 months.
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