The host origin of influenza A viruses can be assessed by the intracellular cleavage of the viral nucleocapsid protein. Brief report.

The major nucleocapsid protein (NP) of many human influenza A viruses was reported to be cleaved in infected cells to reduce its molecular weight (MW) from 56 to 53 K [28]. This was not found in non-human influenza strains. In this paper two groups of viruses are described which break this rule. Two strains, A/New Jersey/8/76 (H1N1) and A/Baku/799/82 (H1N3), isolated from man possess uncleavable NP like animal viruses and a limited number of viruses of H1N1 and H3N2 subtypes isolated from animals possess NP which is cleaved like human viruses. To account for these observations it is proposed that there are two categories of influenza viruses, which cross interspecies barriers and migrate from animal to man or vice versa.