The effect of age on the course of experimental feline immunodeficiency virus infection in cats.

Neonatal, young adult, and aged specific pathogen-free cats were experimentally infected with cat-passaged Petaluma strain of feline immunodeficiency virus. The primary stage of illness occurred 6-8 weeks following infection in cats of all ages, but it differed in severity and clinical signs. Generalized lymphadenopathy persisted for the entire 42-week study period in neonatally infected cats, was transient in young adults, but inapparent in aged cats. Only two aged cats became chronically and severely ill during the study. One aged cat died with severe necrotizing transmural enteritis, while a second developed chronic generalized staphylococcal pyoderma that was partially controlled with antibiotics. Neutropenia appeared 6-8 weeks following infection in cats of all ages, but was more severe in newborn and aged cats than in young adults. A persistent decrease in CD4+/CD8+ T lymphocyte ratios, due to both increased CD8+ and decreased CD4+ T lymphocytes, occurred in the neonatal and aged cats. Decreased CD4+/CD8+ T lymphocyte ratios in the young adult cats was due solely to decreased CD4+ T lymphocytes. Antibody response to FIV virus, as measured by ELISA to recombinant FIV p24 antigen, was lower in aged cats than the other age groups during the first 6 weeks after infection. Antibody levels were not significantly different among the three age groups thereafter. Although there are some differences between FIV infection of cats and HIV infection of human beings, age at infection influences the severity of disease in both species.