In early November 2009, a small commercial pig farm in central Thailand reported respiratory problems in pigs (morbidity rate 50%, mortality rate 10%) in nursery pigs. The farm contained 3,235 pigs (700 sows, 35 boars, 1,000 piglets, 1,000 nursery pigs, and 500 finishing pigs). It has a conventional open-house production system in which both sides of the unit have natural air flow ventilation. The farm also has continuous nursery herd flow in which new pigs are continuously added when they are old enough. This process results in pigs of different ages being in the same unit. Sick pigs had clinical signs (fever, cough, nasal discharge, edematous eyelids, and conjunctivitis) of infection.

Nasal swabs from 20 nursery pigs (4–9 weeks of age) were submitted to Chulalongkorn University Veterinary Diagnostic Laboratory. All samples were positive for porcine circovirus type 2 and porcine reproductive and respiratory syndrome virus (these viruses are major causes of swine respiratory disease), and 2 samples were positive for influenza A virus by reverse transcription–PCR (RT-PCR) with primers for each specific pathogen (8–10).

Because respiratory problems in nursery pigs continued, nasal swabs specimens from 20 nursery pigs and finishing pigs, gilts (young females), and sows (10 per group) with clinical signs were submitted to the diagnostic laboratory by the end of December 2009. Two samples from nursery pigs were positive for influenza virus A (H1N1) by multiplex RT-PCR (11). Both samples were subjected to virus isolation in MDCK cells (12) and designated RA20 and RA29 (Table 1). Genome characterization identified RA20 as SIV and RA29 as pandemic (H1N1) 2009 virus (Table 2; Figure). SIV-positive nasal swabs obtained in November were then characterized. Results showed that isolates RA4 and RA9 were pandemic (H1N1) 2009 virus, which indicated that pigs on the farm were infected with this virus.

Pandemic (H1N1) 2009 investigations on the farm included clinical surveillance and sample collection from sick and contact pigs and close monitoring of swine workers and farm pets for influenza-like illness. Nasal swab specimens were obtained from pigs on January 17, 2010, January 30, 2010, and March 9, 2010. Because initial laboratory findings indicated that the outbreak involved the nursery herd, weaned pigs were moved to a separate site on the farm to control disease in the nursery pigs. Following Food and Agriculture Organization (www.fao.org) sample collection recommendations, we obtained 20 nasal swabs from pigs with SIV-like illness. In addition, nasal swab specimens (n = 10 per group) were collected from gilts, sows, and finishing pigs to test for pandemic (H1N1) 2009 virus, although no clinical signs were observed in any pigs from these age groups. All SIV-positive samples were subjected to virus isolation (12), virus subtyping by multiplex RT-PCR (11), and whole genome sequencing of subtype H1N1 viruses (13). Of 175 samples obtained during December 26, 2009–March 9, 2010, fifteen swab specimens from nursery pigs with clinical signs were positive for influenza (H1N1) 2009 virus; 8 viruses were characterized. No other SIV subtypes were found. On March 9, ≈1 month after implementing the change in handling of pigs, no pigs showed respiratory signs and 34 nasal swab specimens were negative for influenza virus.

Gene sequences were compared for corresponding genes of other influenza virus strains obtained from GenBank by using the MegAlign program (DNASTAR, Madison, WI, USA). Phylogenetic trees were constructed by using MEGA4 (www.megasoftware.net/) and the neighbor-joining method with 1,000 bootstrap replicates. Whole genome analysis showed that contemporary SIV (H1N1) and pandemic (H1N1) 2009 virus were concurrently circulating in the nursery herd (Table 2; Figure). On the basis of virus hemagglutinin 1 gene grouping (14), our findings show that newly isolated SIV (H1N1) from Thailand are grouped in the classical swine cluster with other SIV (H1N1) isolates (Figure A1). There was no evidence of gene reassortment between SIV (H1N1) and pandemic (H1N1)2009 virus during the investigation (Table 2).

To test for evidence of pandemic (H1N1) 2009 virus interspecies transmission, we obtained serum samples on January 17, 2010, from 40 pigs in 8 age groups (5/group), 15 workers, and 4 farm pets (3 dogs and 1 cat). Samples were subjected to hemagglutination-inhibition (HI) testing with SIV (H1N1) and pandemic (H1N1) 2009 virus antigens (12).

Control rabbit antibodies against SIV (H1N1) viruses did not cross-react with pandemic (H1N1) 2009 virus. Serologic results showed that only 2 (9.5%) of 21 test samples from the nursery group had positive HI titers for pandemic (H1N1) 2009 virus and 8 (38%) of 21 had positive HI titers for SIV (H1N1) virus. For pigs in other age groups, 11 (55%) of 20 had antibodies against pandemic (H1N1) 2009 virus and 14 (70%) of 20 had antibodies against SIV (H1N1) by HI test. No human cases of co-infection were observed. We found no evidence of pandemic (H1N1) 2009 virus interspecies transmission from pigs to humans or to farm pets.

Consistent with findings of previous reports (2,3), our findings demonstrate that young pigs are susceptible to infection with pandemic (H1N1) 2009 virus. Infection in pigs substantiates the hypothesis that the clinical outcome caused by infection with pandemic (H1N1) 2009 virus differs from that of infection with SIV (H1N1), which currently circulates in pigs in Thailand. Serologic results demonstrated that uninfected populations are susceptible to infection with pandemic (H1N1) 2009 virus. Results of genome analysis did not show gene reassortment between the 2 different influenza (H1N1) viruses. However, a previous report showed that reassortment of influenza virus genes occurs in pigs (15). Continued monitoring, characterization of SIVs, and serologic surveillance of pigs are necessary for future influenza pandemic preparedness.