Submitted to: American Association of Veterinary Laboratory Diagnosticians
Publication Type: Abstract Only
Publication Acceptance Date: 7/15/2007
Publication Date: 10/18/2007
Citation: Gramer, M., Goyal, S., Patnayak, D., Lager, K., Ma, W., Vincent, A., Richt, J., Webby, R. 2007. Isolation of reassortant H2N3 avian/swine influenza virus from pigs in the United States [abstract]. American Association of Veterinary Laboratory Diagnosticians 50th Annual Conference. p. 122.
Technical Abstract: In April 2006 and September 2006, outbreaks of respiratory disease occurred in growing pigs housed in 2 separate multi-site commercial swine farms, Farm A and Farm B. The swine farms were located 4 miles apart and did not share pigs, feed, personnel, or transportation sources. At both farms, pigs had gross lesions of pneumonia at necropsy. The attending veterinarians submitted formalin-fixed and unfixed sections of lung tissue with coolant to the Minnesota Veterinary Diagnostic Laboratory (MVDL). At the MVDL, the formalin-fixed tissue was routinely processed for histopathology, and the unfixed tissues for bacteriology, molecular diagnostics (RT-PCR and PCR tests), and virus isolation. Pig lungs from both farms had lesions of influenza virus and bacterial co-infection characterized by subacute bronchopneumonia and interstitial pneumonia with metaplastic and necrotizing bronchiolitis. Swine influenza virus was detected in the lungs by RT-PCR and virus isolation. The swine influenza viruses were untypable by hemagglutination inhibition serotyping tests with reference antisera against A/Sw/IA/1973 H1N1, A/Sw/NC/2001 H1N1, and A/Sw/TX/1998 H3N2. Multiplex subtyping RT-PCR tests for swine H1N1, H3N2, and H1N2 were negative. Hemagglutinin (HA) gene sequencing attempts with H1 and H3 specific primer sets were also negative. The viruses, A/Swine/FarmA/22454/2006 and A/Swine/FarmB/49644/2006, were forwarded to the NADC for sequencing and to St. Jude for serotyping. Through full genomic sequencing of the eight RNA segments of each virus, the viruses were subtyped as H2N3 influenza viruses with HA and neuraminidase (NA) genes of avian origin. Serotyping with ferret anti-duck H2N3 antisera was positive for both viruses. This is the first description of avian/swine H2N3 influenza virus isolated from pigs in the United States. The introduction of the virus into the swine farms is thought to be through the use of surface (pond) water used in the swine barns. The ponds used as water supplies for both Farms A and B are frequented by migrating waterfowl in the spring and fall of each year. The H2N3 viruses continue to circulate in both Farms A and B as indicated by positive (>1:40) H2N3 antibody titers found in sera collected from replacement breeding gilts and weaned pigs 6 months or more after onset of disease. The impact of this virus on swine health and disease control strategies will be monitored through expanded swine influenza virus surveillance and characterization efforts.