F. M. BURNET 



F. Arborviruses 



No official nomenclature for this group of arthropod-borne viruses has yet 

 been accepted. It is, however, already clear, from the work of Casals, Theiler, 

 and others, that these viruses typified by those of yellow fever and western 

 equine encephalomyelitis form a relatively homogeneous natural group 

 (Report, 1956). Physically, the infective particles of all the well-studied 

 members of the group are spherical and about 30 m/z in diameter. Those of 

 western equine encephalomyelitis contain a high content of lipid, and all 

 their nucleic acid is RNA. They all can be shown to act as hemagglutinin for 

 erythrocytes from day-old chicks when the tests are made under strictly 

 controlled physical conditions. By hemagglutination-inhibition tests most of 

 the well-known viruses fall into one or another of two serological groups: 

 A, containing the equine encephalitis viruses and a few others, while B con- 

 tains a large number of important human pathogens, including yellow fever, 

 dengue, and Japanese B viruses. Two other serological groups (C and D) 

 have been demonstrated and there are still a number of viruses, too inade- 

 quately studied to be sure of their relation to the better-known forms. The 

 encephalomyocarditis (EMC) viruses may represent a related group. 



G. Enteroviruses 



Recent findings have obscured the demarcation lines originally drawn 

 between the polioviruses, the Coxsackie viruses, and the ECHO * viruses and, 

 despite the great differences in their medical significance, it seems likely that 

 all these denizens of the human intestinal tract fall into one natural group. 

 Polioviruses can be adapted to infect suckling mice and to lose all capacity 

 to produce lesions in the central nervous system of rhesus monkeys. Sero- 

 logically similar viruses may include strains, some of which show typical 

 Coxsackie lesions in suckling mice, others which can be propagated only in 

 tissue culture and would, in the absence of that serological relationship, be 

 classed as ECHO viruses. This interrelationship has recently been regarded 

 as adequate to justify referring to the whole group as enteroviruses and we 

 shall adopt this name (Committee Enteroviruses, 1957). 



All of these viruses have small, spherical, infective particles which are 

 uniform enough to pack into crystal lattices. They are highly resistant to 

 environmental damage and probably contain only protein and RNi\. as 

 significant components. 



To date, attention has been mainly concentrated on human material, but 

 it is already clear that similar forms can be isolated from bovines, monkeys, 

 and other mammals. The fact that foot-and-mouth disease viruses are of the 



* Enteric cytopathogenic human orphan. 



