The importance of picornavirus infections in respiratory disease of man and other mammals.

Picornaviruses may be divided, by physicochemical properties, into enteroviruses, cardioviruses, caliciviruses, rhinoviruses and foot-and-mouth diseases viruses. Although the respiratory tract may be the primary site of entry and multiplication for enteroviruses, cardioviruses and FMD viruses, few agents in these groups cause respiratory disease. A notable exception is coxsackievirus A21 which is an important cause of upper respiratory tract diseases in military recruits. The picornaviruses which most frequently cause respiratory illness are rhinoviruses and caliciviruses. There are over one hundred rhinovirus serotypes which infect man and they have been isolated from up to 50 percent of cases of mild respiratory illness, from 1-2 percent of healthy adults and from 5-10 percent of healthy children. About 10 percent of rhinovirus infections in adults are symptomless. The two bovine serotypes of rhinovirus and the two equine serotypes frequently infect cattle and horses respectively but seldom cause disease. Numerous calicivirus serotypes have been found in 17 percent of cats with respiratory disease and in 19 percent of clinically normal cats. However, experimental inoculation with caliciviruses has confirmed their causative role in respiratory disease of cats. The high rate of virus isolation from normal cats is probably due to their ability to carry virus in an infectious form for up to two years after initial infection and illness.