Influenza A(H1N1)pdm09 Virus among Healthy Show Pigs, United States

Gregory C. Gray

, Jeffrey B. Bender, Carolyn B. Bridges, Russell F. Daly, Whitney S. Krueger, Michael J. Male, Gary L. Heil, John A. Friary, Robin B. Derby, and Nancy J. Cox

Author affiliations: University of Florida, Gainesville, Florida, USA (G.C. Gray, W.S. Krueger, G.L. Heil, J.A. Friary, R.B. Derby); University of Minnesota, St. Paul, Minnesota, USA (J.B. Bender); Centers for Disease Control and Prevention, Atlanta, Georgia, USA (C.B. Bridges, N.J. Cox); South Dakota State University, Brookings, South Dakota, USA (R.F. Daly); and University of Iowa, Iowa City, Iowa, USA (M.J. Male)

Abstract

Within 5 months after the earliest detection of human influenza A(H1N1)pdm09 virus, we found molecular and culture evidence of the virus in healthy US show pigs. The mixing of humans and pigs at swine shows possibly could further the geographic and cross-species spread of influenza A viruses.

Cross-species infections with influenza A viruses readily occur between humans and pigs. Pigs often have been infected by human epidemic viruses (1), and swine workers and their family members are at increased risk for swine influenza virus (SIV) infection (2–4). We studied swine shows as a setting for influenza A virus transmission (5).

The Study

After acquiring informed consent, we recruited persons >7 years of age showing pigs at 3 state fairs in Minnesota (2008, 2009) and South Dakota (2009). Exhibitors were eligible for the study if they reported working with pigs at least 1 cumulative hour per week and had no current immunocompromising condition. Enrolled participants completed a questionnaire and permitted collection of nasal swab specimens from their show pigs. Before data were collected, multiple institutional review boards, the Institutional Animal Care and Use Committee of the University of Minnesota and the University of Iowa, and state fair officials approved the study.
We used the Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA) real-time reverse transcription PCR (rRT-PCR) (6) to screen for influenza A virus. Swab specimens (run in duplicate) with cycle threshold (C_t) values <35 avirus="avirus" c="c" considered="considered" for="for" influenza="influenza" positive="positive" specimens="specimens" sub="sub" were="were" with="with">t
values of 35 to <40 and="and" be="be" c="c" positive="positive" specimens="specimens" sub="sub" suspected="suspected" to="to" were="were" with="with">t values >40 were considered negative. In a blinded fashion, aliquots of swab specimens from pigs were shared with the Minnesota Veterinary Diagnostic Laboratory (St. Paul, MN, USA), where rRT-PCRs for matrix, hemagglutinin (HA), and neuraminidase (NA) genes were performed. Specimens were then shared with the National Veterinary Services Laboratory (Ames, IA, USA) and later with CDC for further molecular and sequencing studies. Positive and suspected-positive rRT-PCR specimens were cultured in shell vials on MDCK cells by using standard techniques. Sequence-based analyses of the influenza A virus isolates were performed by the CDC influenza division, using full or partial genome sequencing approaches for all 8 gene segments. Sequences were compared by using BLASTn alignment search techniques (http://blast.ncbi.nlm.nih.gov

).
Questionnaires were completed by 121 (98%) participants. Participants were predominantly male (71%), and their median age was 34.9 years (range 9–75 years); 24% of participants were <18 age.="age." children="children" class="text-underline" exhibitors="exhibitors" of="of" pig="pig" some="some" span="span" were="were" with="with" years="years"><
1 year of pig exposure (Table); others were pig farmers with numerous years of pig exposure. Participants reported an average of 18.7 years of pig exposure. Nasal swab specimens were collected from a total of 149 pigs (from Minnesota, 47 in 2008 and 57 in 2009; from South Dakota, 45 in 2009). Almost all (97%) swabbed pigs were <1 40="40" a="a" age="age" all="all" and="and" be="be" before="before" by="by" enter="enter" female.="female." healthy="healthy" observed="observed" of="of" p="p" permitted="permitted" pigs="pigs" show.="show." the="the" they="they" to="to" veterinarian="veterinarian" were="were" year="year">