Oncogenic herpesvirus infections of animals.

When considering the writing of this paper I was troubled by the fact that one series of papers in this symposium was included under the heading of 'persistent infections' and another, of which this is one, under the heading of 'latent infections'. On reflection I could not see why some subjects included in persistent infections could not have been classed as latent infections and vice versa. Consequently I resorted to the dictionary for definitions. The answer, of course, is that the two states are not mutually exclusive and in fact are frequently present together, and this is reflected in the title of this symposium. As far as virus-host relationships and resulting disease are concerned I believe the definitions given in the Shorter Oxford dictionary are pertinent. Persistent is briefly defined as something which is 'permanent or continuing'. This definition is easily applied to infection with a virus. Latent is more difficult but is defined as 'hidden or concealed'; or applied to disease, as 'present or existing but not manifest, exhibited or developed'. This definition can be used in respect of (1) the infection or (2) the development of disease, and indeed in the past has been used in both these respects. In its extreme form a latent infection could be considered that in which there is no evidence of infection, eg, where the virus genome or a copy is integrated with the host DNA and is in a repressed state. This may be one of the states of infection with RNA tumour viruses. However, more commonly and classically a latent infection has been applied to that state in which infectious virus cannot be identified, eg, Herpes simplex infection in man, although fingerprints of infection such as antibodies or antigens may be demonstrable. Latency of infection may be considered at the level of the whole animal or at a cellular level. Lastly, latency can be applied to the disease produced by a virus infection. The term 'latent period' is sometimes used to describe the incubation period when it is long or variable in length. A latent infection, and latency in respect of disease, may in some cases be one and the same thing. It is my intention to discuss those herpesvirus infections in animals other than man which have 65 been associated with neoplasia in some way or another, paying particular attention to latency and persistence of infection. First there are those herpesviruses which are known to cause neoplasia under natural conditions, which include the herpesviruses of Marek's disease and the Lucke renal carcinoma of frogs; secondly, Herpesvirus saimiri, and Herpesvirus ateles of monkeys and Herpesvirus sylvilagus of cottontail rabbits which cause lymphoid neoplasia experimentally but their significance as aetiological agents of neoplasia under natural conditions is not known; thirdly those herpesviruses which have been associated with neoplasia in guinea pigs and sheep but whose oncogenicity in either experimental or natural conditions has yet to be proven. A brief outline of the relevent features of infection with herpesviruses in groups 2 and 3 will first be given then Marek's disease and the Lucke renal carcinoma of the frog will be discussed in more detail.