Reassortant human/swine H1N1 and H1N2 influenza virus infections in US swine

摘要

Swine influenza is a common respiratory disease caused by type A influenza viruses of several subtypes including H1N1, H3N2, and H1N2.! Nucleotide sequencing has enabled genetic characterization of influenza viruses within a subtype, such that we candistinguish viruses as to their genotype or likely genetic origin. Using nucleotide sequencing, we discovered new reassortant H1N2 and H1N1 influenza viruses in pigs in the U.S. In early November 2003, an influenza A virus was isolated from three, 4-week-old pigs with respiratory disease. The virus was initially detected by a TaqMan RT-PCR test for the nucleoprotein (NP) gene in late October 2003 in pooled nasal swabs received at the Minnesota Veterinary Diagnostic Laboratory (MVDL). Serotyping of the virus by hemagglutination inhibition test was non-diagnostic, having no reaction with antisera against A/Sw/IA/1973 H1N1 or A/Sw/ TX/1998 H3N2. The isolate was identified as HI when the first 900 nucleotides of the HA gene were se-quenced and compared toreference HI sequences in GenBank. Comparison of the first 597 bases of the HA1 gene segment between this isolate and reference strains of classical H1N1 and reassortant H1N2 SIV showed less than 76% similarity. Until that time, most HI sequences from MVDL SIV isolates were greater than 80% similar to the H1N1 swine reference strains in our analyses. Further phylogenetic analyses of the HA gene demonstrated high similarity (> 90%) to HI human influenza viruses circulating worldwide in 2002/2003. The NAgene was determined to be N2 of swine origin by a multiplex NA typing RT-PCR test conducted at the MVDL and by nucleotide sequencing and phylogenetic analyses at the St. Jude Children's Research Hospital in Memphis, TN. Influenza viruses of similar human-like H1 genotype continue to be isolated from pigs within the same production system, suggesting that the virus is adapted to swine and capable of causing infection and respiratory disease.