Natural feline leukemia virus infection and the immune response of cats of different ages.

Forty-two kittens and 28 adult cats were placed as tracers in leukemia cluster environments in contact with resident cats, 30% of which were persistently infected with feline leukemia virus (FeLV). After 7 months exposure, FeLV viremia had been detected in 71% of the tracer kittens, although only 55% of these remained persistently infected; in the same period, 11% of tracer adults became infected, but by 2 years the figure reached 43%. Mean latent periods before detection of viremia were 3.4 +/- 1.8 (S.D.) and 13.0 +/- 5.9 months for kittens and adults, respectively. First detection of FeLV infection was accompanied by a sharp although transient drop in peripheral white blood cell numbers, and infection onset triggered the humoral immune response which was comprised of separate antibodies with virus-neutralizing and tumor lysis activities. High titers of virus-neutralizing antibody appeared in transiently viremic cats immediately following elimination of viremia; this antibody was rarely detected in cats that remained persistently viremic. Lytic complement-dependent antibody to feline oncornavirus-associated cell membrane antigen appeared in most cats 1 to 2 weeks after FeLV infection was first detected, and subsequently high titers of this antibody remained in both transiently and persistently infected cats. If the rate of FeLV infection was summarized by using viremia and/or antibody appearance, then 95% of the kittens became infected within 1 year and 61% of the adults within 2 years. Adult cats are, therefore, susceptible to FeLV infection following long-term natural exposure, and their apparent resistance cannot be attributed to a protective humoral immune response that developed immediately after exposure commenced.