Recovery of feline leukaemia virus from non-viraemic cats.

The persistence of virus in the bone marrow of cats which had ostensibly recovered from feline leukaemia virus (FeLV) infection was investigated. Nineteen cats were exposed to FeLV by natural, contact infection and 36 weeks later three were found to be persistently viraemic while the remainder were non-viraemic and had virus neutralising serum antibodies. Virus was isolated in bone marrow cultures established from nine of the 16 non-viraemic cats which were considered, therefore, to have latent infections. Cats infected soon after exposure to FeLV carrier cats were more likely to become persistently viraemic or develop a latent infection than those infected later, which tended to recover. There was no difference in serum antibody levels between the latently infected and recovered cats. Whether cats with latent infections spread virus or develop FeLV-negative haemopoietic tumours was considered. Six kittens housed together for eight months with a cat with a latent infection showed no signs of having been exposed to FeLV. Virus was not isolated from bone marrow cultures of two cats with FeLV-free lymphosarcoma or myelomonocytic leukaemia.