Feline panleukopenia. III. Development of lesions in the lymphoid tissues.

Germfree and specific pathogen-free cats were inoculated with feline panleukopenia virus. Cats were necropsied 2 to 6 days after inoculation and tissues from the thymus, lymph nodes and spleen taken for histological and immunofluorescence studies. Necrosis of lymphoid cells in the thymic cortex began 3 days after inoculation and continued for 5 to 6 days after inoculation when the thymus was nearly depleted of lymphocytes. Immunofluorescence studies showed the lesions to be caused by virus. There was gross and histological involution of the thymus in both germfree and specific pathogen-free cats. The lymph nodes and spleen of uninoculated germfree cats looked "inactive" and lacked well developed lymphoid follicles and paracortical areas. In both germfree and specific pathogen-free cats there was necrosis in both follicular and paracortical areas of the lymph nodes and follicular and periarteriolar areas of the spleen 3 to 4 days after inoculation. Immunofluorescence showed these areas had virus infection. By 5 to 6 days after inoculation, these areas were populated by many lymphoblastoic cells. Even though significant destruction of lymphoid cells occurred, subsequently, in cats that develop mild clinical illness, these lymphoid tissues seemed stimulated rather than depleted of lymphocytes.