Evaluation of immunosuppressive effect and efficacy of an improved-potency feline leukaemia vaccine.

An inactivated feline leukaemia vaccine was tested to determine if process improvements would permit it to be effectively used in a two-dose primary regimen versus the three-dose regimen required for a previous vaccine. Twenty-five cats were vaccinated with two subcutaneous doses given 3 weeks apart. Vaccinates and controls were artificially immunosuppressed to enhance susceptibility to challenge, and inoculated with virulent feline leukaemia virus (FeLV) 2 weeks after the second dose. Following challenge, seven (28%) vaccinates became persistently viraemic, versus six (60%) controls. Nine (36%) other vaccinates became transiently viraemic and nine were aviraemic. The mean postvaccination ELISA gp70 antibody value was 0.796 for aviraemic vaccinates, 0.575 for transiently viraemic vaccinates, and 0.350 for persistently viraemic vaccinates. Eighteen of 25 vaccinates had a postvaccination antibody response greater than or equal to 32 to feline oncornavirus associated cell membrane antigen. Seven of 25 (28%) vaccinates developed postvaccination virus neutralization (VN) antibody titres, and 16 of 25 (64%) developed postchallenge VN antibody titres. One of 10 controls developed postchallenge VN antibody titres. An in vitro lymphocyte blastogenesis assay to evaluate immunosuppressive effects of vaccination revealed no significant difference in the mean stimulation index for vaccinates versus controls. Results indicated that vaccination with two doses was an effective immunoprophylactic regimen in the face of FeLV challenge severe enough to produce persistent viraemia in 60% of non-vaccinated controls.