Influenza C virus is a common cause of mild upper respiratory tract illness, mainly in young adults and children. It is an understudied influenza virus because symptoms are mild compared to influenza A and B. Sometimes, however, influenza C can lead to severe lower respiratory tract infections, such as bronchiolitis and pneumonia.
Thirty original clinical samples were collected from patients in Perth, Western Australia, from 2002 to 2012, of which 15 yielded virus isolates in MDCK cell culture.
A Taqman real time assay based on primers and a probe designed in house were established and performed on all of the influenza C isolates as well as on the original samples. The CT values of the original samples from which viruses were isolated ranged from 19.3 to 36.5, and the CT values for those that failed to grow in cell culture were above 27.83. This suggests that virus isolation was influenced not only by the CT value of the original sample but also by other factors such as age of sample and sample handling.
The hemagglutinin-esterase (HE) gene of all of the isolates was sequenced. The sequences were then analysed phylogenetically using Geneious and compared with previously published HE sequences available in Genbank, including the six commonly used reference viruses representing six different antigenic and genetic groups (C/Taylor/1233.47, C/Aichi/1/81, C/Sao Paulo/378/82, C/Kanagawa/1/76, Yamagata/26/81, and C/Mississippi/80). The Perth samples fell into two clades, clade 2 and clade 6. The results suggest that at least two influenza C lineages were co-circulating in the Perth region during 2012. Overall, all the Perth viruses in the same lineage are almost identical, even those isolated from samples obtained in different years.
Influenza C outbreaks occur often and reinfection is frequent, even though the evolution rate is low. Our study highlights that there is a need to further study influenza C, especially at the full genome level to disclose the unique evolution pattern of influenza C.