• Open Access

Transmission of influenza A viruses between pigs and people, Iowa, 2002–2004


  • Current addresses: Pauline Terebuh, currently on sabbatical. Tim Kniffen, Intervet/Intervet/Schering-Plough Animal Health, Waukee, IA, USA. David Madsen, Premium Standard Farms, Princeton, MO, USA.

  • Virus stock had some evidence for a mixed isolation of both H1N1 (predominant) and H3N2 (minor component) swine viruses.

Christopher W. Olsen, University of Wisconsin, School of Veterinary Medicine, Madison, WI, USA. E-mail: olsenc@svm.vetmed.wisc.edu; Carolyn B. Bridges, Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, GA, USA. E-mail: cbridges@cdc.gov


Please cite this paper as: Terebuh et al. (2010) Transmission of influenza A viruses between pigs and people, Iowa, 2002–2004. Influenza and Other Respiratory Viruses 4(6), 387–396.

Background  Triple-reassortant (tr) viruses of human, avian, and swine origin, including H1N1, H1N2, and H3N2 subtypes, emerged in North American swine herds in 1998 and have become predominant. While sporadic human infections with classical influenza A (H1N1) and with tr-swine influenza viruses have been reported, relatively few have been documented in occupationally exposed swine workers (SW).

Methods  We conducted a 2-year (2002–2004) prospective cohort study of transmission of influenza viruses between pigs and SW from a single pork production company in Iowa. Respiratory samples were collected and tested for influenza viruses from SW and from pigs under their care through surveillance for influenza-like illnesses (ILI). Serial blood samples from study participants were tested by hemagglutination inhibition (HI) for antibody seroconversion against human and swine influenza viruses (SIV), and antibody seroprevalence was compared to age-matched urban Iowa blood donors.

Results  During the first year, 15 of 88 SW had ILI and were sampled; all were culture-negative for influenza. During the second year, 11 of 76 SW had ILI and were sampled; one was culture-positive for a human seasonal H3N2 virus. Among 20 swine herd ILI outbreaks sampled, influenza A virus was detected by rRT-PCR from 17 with 11 trH1N1 and five trH3N2 virus isolates cultured. During both years, HI geometric mean titers were significantly higher among SW compared to blood donor controls for three SIV: classical swine Sw/WI/238/97 (H1N1), tr Sw/IN/9K035/99 (H1N2), and trSw/IA/H02NJ56371/02 (H1N1)] (P < 0·0001).

Conclusions  SW had serologic evidence for infection with both swine and human influenza viruses and were exposed to diverse influenza virus strains circulating in pigs. Influenza virus surveillance among pigs and SW should be encouraged to better understand cross-species transmission and diversity of influenza viruses at the human–swine interface.