Molecular epidemiology of rotaviruses isolated from hospitalised children in Melbourne, Australia

Rotaviruses are the leading cause of severe gastroenteritis in children worldwide. The morbidity due to rotavirus related illness in developed countries is significant, and results in an estimated 610,000 childhood deaths annually. Until the mid 1990s, four major G serotypes (G1-G4) of rotavirus had been determined to circulate in Australian communities and around the world. Since then, the emergence of G9 rotaviruses has been reported worldwide, and with its unique ability to spread rapidly, many countries have reported the presence of this serotype. In Australia, serotype G9 strains were first isolated in 1997. A recent outbreak (in 2001) caused by serotype G9 rotavirus in Central Australia highlights the epidemiological importance of this serotype. The main aim of this thesis was to analyse the evolution of serotype G9 rotaviruses in Melbourne and characterise their antigenic and genetic properties. An epidemiological study conducted in this study in 2002 revealed a predominance of serotype G9 rotaviruses (58%) in rotavirus positive isolates obtained from hospitalised children. This thesis further explored the extent of genetic variation among serotype G9 rotaviruses collected over a longer period of six years (1997-2002) from children admitted to the Royal Children’s Hospital in Melbourne. Six different antigenic groups of the VP7 protein among serotype G9 isolates were defined in this study. Genetic analysis of the VP7 gene showed that the Melbourne strains were closely related to each other, however, no conserved amino acid difference were found to correlate with the antigenic group. Genetic variation was observed in two of the non-structural proteins, NSP1 and NSP4, which were analysed in this study. Phylogenetic analysis indicated the existence of two clusters of NSP1 gene present in Melbourne serotype G9 strains. Strain R1 had an NSP1 gene that was closely related (96% sequence identity) to the serotype G1 strain, Wa, suggesting a possible gene reassortment between rotaviruses of different serotypes. Strain R1 also shared close identity with the NSP4 gene of the serotype G1 strain, Wa (95% sequence identity). Most interestingly, strain R24 exhibited significant variation to the rest of Melbourne isolates studied (73-78% sequence identity) and only 66%-78% sequence identity with other prototype NSP4 genes. These findings suggest that R24 may belong to a new NSP4 genotype. These findings contribute new information regarding the extent of genetic variation among non-structural proteins of serotype G9 rotaviruses.