Feline leukemia virus infection: age-related variation in response of cats to experimental infection.

Sixty-seven specific-pathogen-free cats of various ages (newborn, 2 wk, 1 mo, 2 mo, 4 mo, and 1 yr) were inoculated ip with either the Rickard (R) or the Kawakami-Theilen (KT) strain of feline leukemia virus (FeLV). Susceptibility to FeLV was judged by induction of a) FeLV group-specific antigens (gsa) in leukocytes, b) FeLV-related disease, c) antibody to feline oncornavirus-associated cell membrane antigen (FOCMA), and d) virus-neutralizing (VN) antibody. Susceptibility to FeLV-decreased with age. Persistent viremia and FeLV-related disease developed in 100% of cats inoculated as newborns, in 85% of cats inoculated at 2 weeks to 2 months of age, and in 15% of cats inoculated at 4 months or 1 year of age. Cats susceptible to FeLV leukemogenesis became persistently FeLV gsa-positive (viremic) at 4 weeks post inoculation and thereafter and produced little or no FOCMA or VN antibody. Cats that resisted leukemogenesis by FeLV all developed persistent FOCMA and VN titers and never became FeLV gsa-positive. The disease in inoculated cats was influenced by virus strain; FeLV-R induced predominantly thymic lymphosarcoma, whereas FeLV-KT caused fatal nonregenerative anemia without concurrent neoplasia.