Influenza C virus is an enveloped, segmented, negative‑sense RNA virus within the Orthomyxoviridae family. It primarily infects humans and pigs and is associated with mild upper respiratory illness, particularly in children; it is not known to cause large epidemics.
Virology & Pathogenesis
Influenza C virus differs from influenza A and B viruses in several respects. Its genome consists of seven RNA segments rather than eight and encodes a single surface glycoprotein, hemagglutinin‑esterase‑fusion (HEF), which combines the receptor‑binding, receptor‑destroying and membrane fusion functions of hemagglutinin and neuraminidase. The virus replicates in the nuclei of epithelial cells lining the upper respiratory tract. Infection begins with HEF binding to 9‑O‑acetylated sialic acid receptors, followed by endocytosis and fusion of the viral envelope with endosomal membranes. Viral mRNAs are transcribed and replicated through the virus‑encoded polymerase complex, and progeny virions bud from the cell surface. Influenza C virus tends to infect young children, with most people acquiring antibodies by adolescence. Reinfections occur, but immunity acquired from early exposure tends to reduce disease severity. The virus undergoes antigenic drift but at a slower rate than influenza A or B viruses. According to the CDC, influenza C virus infections generally cause mild illness and are not thought to cause human epidemics.
Clinical features and significance
Clinical manifestations of influenza C infection include mild upper respiratory symptoms such as nasal congestion, cough and low‑grade fever. Lower respiratory involvement or systemic symptoms are uncommon, and hospitalization is rarely required. Outbreaks have been documented in childcare settings and schools, but widespread epidemics have not been observed. Transmission occurs through respiratory droplets and close contact, similar to other influenza viruses. Because disease is typically mild and cross‑protective immunity develops early in life, influenza C virus is not included in seasonal influenza vaccines and specific antiviral treatments are not routinely used. In pigs, influenza C virus can cause mild respiratory disease and acts as a potential reservoir for reassortment; however, cross‑species transmission to humans is infrequent.
Although influenza C virus contributes little to the overall burden of influenza, its unique genomic features and limited pathogenicity distinguish it from the more clinically significant influenza A and B viruses. Continued surveillance of human and swine populations helps track its evolution and detect any changes in virulence or host range.
Related Terms: Orthomyxoviridae, Hemagglutinin‑esterase‑fusion, Influenza A virus, Influenza B virus, Pigs