Up to now the diagnosis of feline leukemia virus infection is mainly based on the identification of the FeLV protein p27 present in blood. A serological test to detect FeLV antibodies for diagnosis has not been available so far, because it is difficult to differentiate between antibodies to exogenous and endogenous FeLV. In this study, we evaluated four different FeLV antigens in order to develop a diagnostic tool based on the detection of FeLV antibodies. It was the goal of this study to demonstrate by indirect enzyme-linked immunosorbent assay (ELISA) wether a cat had been in contact with FeLV or not. Thus, we evaluated for use in serology a short peptide derived from the FeLV transmembrane protein, a recombinant env-gene product (p45), whole FeLV antigen, and recombinant p15E. Sera from experimentally infected and vaccinated cats as well as from field cats were examined for their reactivity to these antigens. The results of PCR detecting provirus in the blood served as reference. In contrast to the p45, whole virus, and the peptide, recombinant p15E displayed a diagnostic sensitivity of 95.7% and specificity of 100% using sera from experimentally infected cats. Using the sera from field cats and a different cutoff, p15E showed a sensitivity of 77.1% and a specificity of 85.6%. Vaccinated cats displayed only low antibody levels to p15E indicating that anti p15E antibodies are rather a sign for infection than for vaccination. The true specificity in the field may probably be even higher as many cats are PCR negative in the blood but carry the virus somewhere else in the body and thus, had contact with the virus although they are PCR negative .   

We conclude that antibodies to p15E may indicate that a cat has been in contact with FeLV and together with PCR may be helpful in the control of FeLV infection.