Lymphoid leukosis viruses, their recognition as 'persistent' viruses and comparisons with certain other retroviruses of veterinary importance.

Diseases caused by lymphoid leukosis virus (LLV), a retrovirus, take a long time after infection to develop and have a wide variety of pathological manifestations. This long latent period is characteristic of 'persistent virus infections'. Disease produced by LLV infection and its underlying mechanisms is compared with 'persistent' infections caused by other retroviruses in birds and mammals of veterinary importance. The diseases considered for comparison are those caused by reticuloendotheliosis, feline leukaemia, bovine leukosis and equine infectious anaemia viruses. There are significant changes in the immunological status in all diseases caused by these viruses. LLV infections follow this trend with, in manifestations of neoplastic disease, a perturbation of the normal switch that occurs from IgM to IgG synthesis. There are also indications of other immunological disturbances. Factors other than immunological disturbances may contribute to the length of time after infection required for the many forms of LLV infection to appear. Such additional factors may include the operation of 'biological clocks', such as the arrival of sexual maturity, and also the very nature of retroviruses. These factors, like the immunological changes, play major roles in the maintenance and progression of persistent retrovirus infections.