Isolation of a Novel Swine Influenza Virus from Oklahoma in 2011 Which Is Distantly Related to Human Influenza C Viruses

Of the orthomyxoviridae family of viruses,only influenza a viruses are thought to exist as multiple subtypes and has non-human maintenance hosts. in april 2011,nasal swabs were collected for virus isolation from pigs exhibiting influenza-like illness. subsequent electron microscopic,biochemical,and genetic studies identified an orthomyxovirus with seven rna segments exhibiting approximately 50% overall amino acid identity to human influenza c virus. based on its genetic organizational similarities to influenza c viruses this virus has been provisionally designated c/oklahoma/1334/2011 (c/ok). phylogenetic analysis of the predicted viral proteins found that the divergence between c/ok and human influenza c viruses was similar to that observed between influenza a and b viruses. no cross reactivity was observed between c/ok and human influenza c viruses using hemagglutination inhibition (hi) assays. additionally,screening of pig and human serum samples found that 9.5% and 1.3%,respectively,of individuals had measurable hi antibody titers to c/ok virus. c/ok virus was able to infect both ferrets and pigs and transmit to naive animals by direct contact. cell culture studies showed that c/ok virus displayed a broader cellular tropism than a human influenza c virus. the observed difference in cellular tropism was further supported by structural analysis showing that hemagglutinin esterase (he) proteins between two viruses have conserved enzymatic but divergent receptor-binding sites. these results suggest that c/ok virus represents a new subtype of influenza c viruses that currently circulates in pigs that has not been recognized previously. the presence of multiple subtypes of co-circulating influenza c viruses raises the possibility of reassortment and antigenic shift as mechanisms of influenza c virus evolution. © 2013 hause et al.