Detection of influenza C virus but not influenza D virus in Scottish respiratory samples

BACKGROUND A newly proposed genus of influenza virus (influenza D) is associated with respiratory disease in pigs and cattle. The novel virus is most closely related to human influenza C virus and can infect ferrets but infection has not been reported in humans. OBJECTIVES To ascertain if influenza D virus can be detected retrospectively in patient respiratory samples. STUDY DESIGN 3300 human respiratory samples from Edinburgh, Scotland, covering the period 2006-2008, were screened in pools of 10 by RT-PCR using primers capable of detecting both influenza C and D viruses. RESULTS Influenza D was not detected in any sample. Influenza C was present in 6 samples (0.2%), compared with frequencies of 3.3% and 0.9% for influenza A and B viruses from RT-PCR testing of respiratory samples over the same period. Influenza C virus was detected in samples from individuals <2 years or >45 years old, with cases occurring throughout the year. Phylogenetic analysis of nearly complete sequences of all seven segments revealed the presence of multiple, reassortant lineages. CONCLUSION We were unable to detect viruses related to influenza D virus in human respiratory samples. Influenza C virus was less prevalent than influenza A and B viruses, was associated with mild disease in the young (<2 years) and old (>45 years) and comprised multiple, reassortant lineages. Inclusion of influenza C virus as part of a diagnostic testing panel for respiratory infections would be of limited additional value.