Pathogenicity and sequence analysis studies suggest potential role of gene 3 in virulence of swine enteric and respiratory coronaviruses

Coronaviruses have been commonly associated with enteric and respiratory diseases. Two of the swine coronaviruses, namely transmissible gastroenteritis virus (TGEV) and porcine respiratory coronavirus (PRCV) have been extensively studied. TGEV replicates in both the enteric and respiratory tracts and causes enteric disease, whereas, PRCV replicates in the respiratory tract with limited to no replication in the enteric tract. We have isolated PRCV from swine herds with respiratory disease and have reproduced moderate pneumonia in gnotobiotic and conventionally reared pigs with two of the PRCV isolates. We have also identified two PRCV isolates with low virulence. One consistent difference that we have observed between PRCV isolates of different pathogenicities is in gene 3. The gene 3 is intact in the two virulent PRCV isolates, whereas gene 3 is altered in the two low virulence isolates. A similar observation has been reported for TGEV as a nonpathogenic TGEV mutant with a small plaque morphology had a deletion in gene 3. We have also observed that one of the low virulence PRCV isolates, IA1894, which has a deletion in gene 3, replicates poorly in cell cultures. Collectively these studies suggest that gene 3 may be an important determinant for in vivo virulence and in vitro replication of coronaviruses.