I. THE ORIGIN OF VIRUSES 139 



4. Latent Viruses 



A vast number of examples of virus production by apparently nor- 

 mal cells can be shown also in animals. For instance the occurrence 

 of Theiler's mouse encephalitis virus in the intestines of normal albino 

 mice has been well established. As reported by Olitzky (25) and ascer- 

 tained by other workers, whilst foetal and sucking mice up to 12 days 

 of age are free from this intestinal virus, 20 to 25 days old mice har- 

 bour it irregularly, 30 days old mice invariably and old mice (6 or 

 more months) again irregularly. Again, it is generally known that in 

 the submaxillary gland of normal guinea-pigs a virus is contained, 

 which can cause a meningeal symptom, when injected into the brain 

 of guinea-pigs. It merits attention that this submaxillary-gland virus, 

 like that of Theiler, is found only in the gland of adult animals, 

 never in the gland of young ones. Furthermore, an extract of normal 

 rabbit skin was confirmed by Daneal (26) to be able to induce serially 

 transmissible papillomata on the skin of other domestic rabbits. 



A series of neurotropic viruses, like that of poliomyelitis, have 

 been isolated from various sources of animals, apparently having no 

 association with the disease, such as feces or excretes of animals or 

 men, and also from insect juices. A group of viruses having the 

 name of Coxackie may be regarded as a type of such viruses. It is a 

 note-worthy fact that non-biting flies are regarded as disseminating 

 agents of both poliomyelitis and Coxackie virus, both viruses being 

 frequently recovered from flies. There are striking similarities in the 

 distribution of these fwo groups of viruses in nature. 



A factor in brain tissue which induces acute disseminated encepha- 

 lomyelitis, when injected into rhesus monkeys has been found by 

 Kabat et al. (27) in human, monkey, rabbit and chicken brain. It is 

 present in the spinal cord of 3 days old rabbits, but does not appear 

 in the rabbit brain until about the 12 th day of life. 



An infectious agent, which appears to be a virus has been isolated 

 from the liver of a normal wild raccoon which has led to a highly 

 fatal type of disease characterized by conjunctivitis and an elevated 

 serum bilirubin, frequently accompanied by jaundice on inoculation of 

 raccoons. Ferrets also appears to be susceptible to infections with 

 this agent (27 a). 



Such examples appear to be too numerous to be cited, but are 

 commonly regarded as evidences of detection of "latent viruses", /. e., 

 viruses present in host cells causing an inapparent infection. Pre- 

 sumably, this may hold for some examples above cited, but it appears 



