Influenza C Virus mutates

When you read the news (or this weblog) you find that most articles about Influenza viruses are about Influenza A Virus and Influenza B Virus. Both are represented in your yearly Influenza vaccine.

Less well known are Influenza C viruses and Influenza D viruses appear to have limited abilities to infect humans. That situation might end quite soon, because the Influenza C Virus is mutating.

Influenza C Virus was first isolated in 1947 in the US from a human patient having mild respiratory symptoms. The virus has a worldwide distribution and and the initial exposure or infection occurs during childhood. The majority of humans thus develop antibodies against Influenza C Virus early in life. Humans are the main reservoir of Influenza C Virus, but occasionally the virus may also infect dogs and pigs.

During the COVID-19 pandemic, the influenza epidemic increased in China and caused the influenza outbreak in 2021–2022. On 14 June, 2022, there was an abnormal increase in influenza-like illness in a kindergarten in Guangzhou, involving 68 children aged from 3 to 4. Results showed that one tested positive for the influenza C virus[1].

The case infected with Influenza C Virus was a four-year-old girl, who had symptoms, including runny nose, stuffy nose, slight cough, pharyngeal congestion, breathing sound aggravated, and conjunctival congestion, no vomiting and diarrhoea. She got fever for six days intermittently, with the highest temperature of 38.7°C on June 14, 2022.

The HE genes of Influenza C Virus fall into six genetic lineages that correspond to antigenic clusters. The the newly detected virus, named C/Guangzhou/05166/2022, was triple reassortant, with the gene of HE belonged to KA176-like lineage, the genes of PB2, PB1, M, and NS belonged to PB11581-like lineage, the genes of P3 and NP belonged to MS80-like lineage.

Phylogenetic analysis demonstrated that the genotype of C/Guangzhou/05166/2022 was different from the prevalent strains isolated from Hong Kong (2015–2020) and Japan Philippines (2009–2013), though the HE gene belongs to the same evolutionary branch of KA176 as the Hong Kong epidemic strain. This similar genotype strain could be traced back to the C/Miyagi/9/96 and C/Saitama/3/2000 of Japanese epidemic strains from 1996 to 2000, which revealed that the diversity of Influenza C Virus genes pool in Asia provided conditions that various gene segments could be mixed and reassorted.

The problem that an Influenza C infection has the same symptoms as a mild and asymptomatic COVID-19 infection, so it is difficult to distinguish clinically. So, Influenza C Virus can happily mutate further and become more dangerous.

[1] Lu et al: Emerging triple-reassortant influenza C virus with household-associated infection during an influenza A(H3N2) outbreak, China, 2022 in Emerging Infections - 2022. See here.