LYMPHOID LEUKOSIS VIRUSES, THEIR RECOGNITION AS PERSISTENT VIRUSES AND COMPARISONS WITH CERTAIN OTHER RETROVIRUSES OF VETERINARY IMPORTANCE- DISCUSSION

Diseases caused by lymphoid leukosis virus (LLV), a retrovirus, take a long time after infection to develop and have a wide variety of patho logical manifestations. This long latent period is characteristic of ' persistent virus infections'. Disease produced by LLV infection and it s underlying mechanisms is compared with 'persistent' infections cause d by other retroviruses in birds and mammals of veterinary importance. The diseases considered for comparison are those caused by reticuloen dotheliosis, feline leukaemia, bovine leukosis and equine infectious a naemia viruses. There are significant changes in the immunological sta tus in all diseases caused by these viruses. LLV infections follow thi s 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 th an immunological disturbances may contribute to the length of time aft er infection required for the many forms of LLV infection to appear. S uch 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 maj or roles in the maintenance and progression of persistent retrovirus i nfections.