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

Because animals can transmit some diseases to people, it is wise to be cautious around animals that carry these diseases. But how do you know which animals are carrying disease? Sometimes they appear perfectly healthy. A study of 57 apparently healthy show pigs at a 2009 US state fair found that almost 20% were carrying influenza virus and at least 4 were carrying the 2009 pandemic virus. Of concern is the possibility that different types of influenza virus—pandemic, swine, avian—could combine in pigs and emerge as new viruses that then spread to humans. Swine workers, veterinarians, and other persons with pig contact may be at high risk for infection with pig influenza and should receive seasonal influenza vaccines, use personal protective equipment when working with healthy pigs, and limit their contact with sick pigs. Regular monitoring of influenza virus among pigs and testing of sick persons who have been exposed to pigs are needed.

C ross-species infections with infl uenza 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 infl uenza virus (SIV) infection (2)(3)(4). We studied swine shows as a setting for infl uenza 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 offi cials 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 infl uenza A virus. Swab specimens (run in duplicate) with cycle threshold (C t ) values <35 were considered positive for infl uenza Avirus; specimens with C t values of 35 to <40 were suspected to be positive; and specimens with C 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 infl uenza A virus isolates were performed by the CDC infl uenza 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 years of age. Some pig exhibitors were children with <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 year of age, and 40% were female. All pigs were observed to be healthy by a veterinarian before they were permitted to enter the show.
In 2008, nasal swab specimens from show pigs showed no molecular or viral culture evidence of infl uenza A virus. However, in 2009, a number of pigs were positive for infl uenza A virus. Comparing the molecular results of the 3 laboratories and using conservative rRT-PCR result interpretations on which all laboratories agreed, we determined that infl uenza A virus was detected in 12 (12%) of 102 swine respiratory samples by rRT-PCR; 11 (19%) of these were from among the 57 pigs swabbed at the Minnesota state fair (Figure). Viral culture yielded 7 infl uenza isolates, 5 of which were recultured and A follow-up telephone survey of study participants identifi ed 2 with infl uenza-like illness (ILI) within 7 days after the fair: an adult, with ILI with onset 1 day after his pigs arrived at the fair (and 4 days before pig swabbing), and his daughter who developed an ILI on the last day of the fair (3 days after pig swabbing). Three pigs exhibited by the child tested positive for infl uenza A virus.

Conclusions
We found a 19% prevalence of infl uenza A virus among the 57 show pigs swabbed at the 2009 Minnesota state fair, which occurred during the second wave of the 2009 pandemic. Temporal analysis of the results indicated that most pigs with rRT-PCR-positive results were sampled within 24 hours after arriving at the fair, suggesting that they probably were infected before their arrival (Figure). None of the pigs with molecular or viral culture evidence of infl uenza A virus infection had clinical signs of infl uenza illness at the time of sampling. This fi nding validates previous pig show-linked human cases (5,7) and suggests that exposure to apparently healthy pigs at shows is a possible source of infl uenza A virus transmission.
These detections of A(H1N1)pdm09 virus in the United States (refl ecting the ease of transmission from humans to pigs) were soon followed by multiple other detections in US pigs (8,9) and in numerous pigs in other countries. Such observations are now common (10), leading to speculation that just as the human-origin subtype H3N2 virus variant became enzootic in pigs (11), the A(H1N1)pdm09 virus and related viruses are now enzootic. Our fi ndings of asymptomatic A(H1N1)pdm09 virus infections in pigs is supported by other data suggesting that as few as 10% of infected pigs might show clinical signs of A(H1N1)pdm09 virus infection (12).
Of concern is that new reassortants between A(H1N1) pdm09, enzootic SIVs, and possibly other human-or avianorigin viruses might emerge and possibly spread to humans who have contact with asymptomatic pigs (10,13,14). Clinicians who care for persons in whom infl uenza A illness develops, particularly when human infl uenza is not widely circulating, should ask about pig exposures and consider further molecular testing to rule out human infection with a nonhuman-origin infl uenza A virus. In addition, to minimize potential interspecies transmission of infl uenza viruses, it might be prudent to develop guidelines for the exhibition of pigs.
Because of the possibility of novel virus generation in pigs and of human-to-pig and pig-to-human transmission of infl uenza virus, routine infl uenza A virus surveillance DISPATCHES 1520 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 18, No. 9, September 2012 among pigs and infl uenza A virus testing of ill persons exposed to pigs is needed to ensure timely detection of novel infl uenza viruses in humans and pigs (4). Early detection is essential for development of effective vaccines and initiation of other means to prevent the spread of novel infl uenza A viruses. However, considerable barriers exist to conducting surveillance in pigs and pig-exposed persons, not the least of which is the threat that such surveillance could economically harm the pork industry (10). To improve infl uenza surveillance, additional ways are needed for pig farmers, the pork industry, the US Department of Agriculture, and public health professionals to collaborate. Swine workers, food animal veterinarians, and persons involved in raising show pigs are at high risk for zoonotic infl uenza infection (4,15). They should be strongly encouraged to receive seasonal infl uenza vaccines and to take measures to reduce zoonotic disease transmission, including using personal protective equipment. They also should limit their contact with pigs when they or the pigs have symptoms of respiratory illness (4,10).