Coronaviridae

Coronaviruses are ssRNA viruses that infect a wide range of mammalian and avian species; they are important causes of respiratory and enteric disease, encephalomyelitis, hepatitis, serositis, and vasculitis in domestic animals. In humans, coronaviruses are one of several groups of viruses that cause the common cold. The prototype of the family, avian infectious brochitis virus, is one of the most infectious of all viruses and causes an acute respiratory disease, which in young chicks can cause very high mortality. Outbreaks can be explosive, involving nearly every bird in the flock at about the same time, because of respiratory transmission and a very short incubation period. In many ways, transmissible gastroenteritis virus of swine and mouse hepatitis virus behave similarly, affecting young animals most severely, spreading very quickly to all animals at risk, and causing major economic losses before control strategies can be put into place. The strategy of expression of the coronavirus genome is unique. The input virion RNA molecule is translated directly, one of the products being an RNA polymerase that then transcribes a full-length (—) sense copy RNA, from which in turn is transcribed a 3′-coterminal nested set of subgenomic mRNAs. The nested set comprises six overlapping species of mRNAs that extend for different lengths from a common 3′ terminus. Only the unique sequence toward the 5′ end, which is not shared with the next smallest mRNA in the nested set, is translated, each product, therefore, being a unique protein.