The American Association of Feline Practitioners (AAFP) aims to inform small-animal clinicians about the newest and best methods for keeping cats and kittens healthy. During the 2010 AAFP Spring Conference, Susan Little, DVM, DABVP, outlined guidelines for feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) testing, focusing on kittens, the affects of vaccination on test results, and the latest research available to support the association's recommendations.
FIV infection is routinely diagnosed based on the results of positive antibody ELISA screening. While it can take some cats a long time to produce antibodies to FIV, most cats will do so within 60 days of exposure. A Western blot is recommended as a confirmatory test in cats with positive ELISA results, especially for those that are thought to be at low risk for infection. Although rare, false negative results on both tests are possible.
For kittens, the guidelines suggest early testing. Kittens younger than 6 months may have positive results if they are born to infected queens, getting FIV antibodies in colostrum. These kittens are not truly infected and will have negative results if re-evaluated at 6 months of age. Between the ages of 4 and 6 months, kittens that have positive results are most likely infected. Since a negative result at any age is considered reliable, kittens should be tested even at the early ages. This practice will prevent infected kittens from being a source of infection for other cats, and those with positive results can be re-evaluated at an older age. Early screening also improves testing compliance, increasing overall infection surveillance.
FIV vaccination can interfere with testing, and young kittens of vaccinated queens should be retested since antibodies can persist in these kittens for more than eight weeks. High-risk unvaccinated cats should be tested regularly, and all cats should be tested before vaccination. When a cat's vaccine status is unknown, positive ELISA test results can be difficult to interpret. In these cases, polymerase chain reaction (PCR) testing may be an option but is of unknown reliability.
Unlike FIV, FeLV is diagnosed based on the detection of antigen in the blood. Available ELISA test kits are recommended for regular use and have the advantage of using whole blood, serum, or plasma for screening. Saliva testing is not considered reliable, so it is not recommended.
Results are usually positive just one month after exposure. Immunofluorescent antibody (IFA) testing can also detect antigen and should be used to confirm infection. Blood or bone marrow is required for this test. False negative results can occur if IFA testing is performed before the development of secondary viremia (six to eight weeks after exposure) or when cats are neutropenic or thrombocytopenic. Young kittens are at the highest risk for infection and can be tested at any time. Maternal antibodies do not interfere with testing, but kittens born to FeLV-infected queens may not have positive results themselves for several weeks to months.
Veterinarians are cautioned not to test just one member of a group or litter or to pool samples. These practices are not reliable and will not isolate infected individuals, nor allow for appropriate follow-up. Any cat thought to have had virus exposure but with negative test results should be retested after a minimum of one month as the results may not be positive during the incubation period.
According to Dr. Little, new research suggests that most cats exposed to FeLV remain infected for life. Some of these cats may become nonviremic, a state called regressive infection. In these cats, neither antigen nor virus is detectable in the blood. However, proviral DNA can be found using PCR testing. These cats are not likely to shed virus but may be able to transmit disease if used as blood donors.
While testing is not generally affected by vaccination, blood collected immediately after vaccination may give false positive results, so it is suggested that samples be collected before vaccination.
When FeLV screening and confirmatory testing results do not agree, repeated testing at 60 days and then yearly is encouraged. Usually the cat's status will emerge with repeated testing. When performed by a qualified laboratory, PCR testing may be the most sensitive test for FeLV and could help identify regressive infections and resolve cases where other testing methods disagree. Most laboratories do not routinely perform FeLV or FIV PCR testing. So, while it is now available through several commercial laboratories, its efficacy has not been determined.
Source: Little S. Testing kittens for FeLV and FIV, in Proceedings. Am Assoc Feline Pract 2010.
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