title: Effect of herd size on subclinical infection of swine in Vietnam with influenza A viruses.
abstract: BACKGROUND
  Influenza A viruses of swine (IAV-S) cause acute and subclinical respiratory disease. To increase our understanding of the etiology of the subclinical form and thus help prevent the persistence of IAV-S in pig populations, we conducted active virologic surveillance in Vietnam, the second-largest pig-producing country in Asia, from February 2010 to December 2013.
  
  
  RESULTS
  From a total of 7034 nasal swabs collected from clinically healthy pigs at 250 farms and 10 slaughterhouses, we isolated 172 IAV-S from swine at the weaning and early-fattening stages. The isolation rate of IAV-S was significantly higher among pigs aged 3 weeks to 4.5 months than in older and younger animals. IAV-S were isolated from 16 large, corporate farms and 6 family-operated farms from among the 250 farms evaluated. Multivariate logistic regression analysis revealed that "having more than 1,000 pigs" was the most influential risk factor for IAV-S positivity. Farms affected by reassortant IAV-S had significantly larger pig populations than did those where A(H1N1)pdm09 viruses were isolated, thus suggesting that large, corporate farms serve as sites of reassortment events.
  
  
  CONCLUSIONS
  We demonstrate the asymptomatic circulation of IAV-S in the Vietnamese pig population. Raising a large number of pigs on a farm has the strongest impact on the incidence of subclinical IAV-S infection. Given that only some of the corporate farms surveyed were IAV-S positive, further active monitoring is necessary to identify additional risk factors important in subclinical infection of pigs with IAV-S in Vietnam.
authors.affiliation: Influenza and Prion Diseases Research Center, National Institute of Animal Health, NARO, Ibaraki, Japan.|Thailand-Japan Zoonotic Diseases Collaboration Center, Bangkok, Thailand.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Takemae
authors.affiliation: Division of International Health, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan.
authors.forename: Yugo
authors.initials: Y
authors.lastname: Shobugawa
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Phuong Thanh
authors.initials: PT
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, Epidemiology Division, Hanoi, Vietnam.
authors.forename: Tung
authors.initials: T
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, Epidemiology Division, Hanoi, Vietnam.
authors.forename: Tien Ngoc
authors.initials: TN
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, National Centre for Veterinary Diagnostics, Hanoi, Vietnam.
authors.forename: Thanh Long
authors.initials: TL
authors.lastname: To
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Phuong Duy
authors.initials: PD
authors.lastname: Thai
authors.affiliation: Department of Animal Health, National Centre for Veterinary Diagnostics, Hanoi, Vietnam.
authors.forename: Tho Dang
authors.initials: TD
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Duy Thanh
authors.initials: DT
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Dung Kim
authors.initials: DK
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, National Centre for Veterinary Diagnostics, Hanoi, Vietnam.
authors.forename: Hoa Thi
authors.initials: HT
authors.lastname: Do
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Thi Quynh Anh
authors.initials: TQ
authors.lastname: Le
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Phan Truong
authors.initials: PT
authors.lastname: Hua
authors.affiliation: Department of Animal Health, Center for Veterinary Diagnostics, Regional Animal Health Office No. 6, Ho Chi Minh City, Vietnam.
authors.forename: Hung
authors.initials: H
authors.lastname: Van Vo
authors.affiliation: Department of Animal Health, Epidemiology Division, Hanoi, Vietnam.
authors.forename: Diep Thi
authors.initials: DT
authors.lastname: Nguyen
authors.affiliation: Department of Animal Health, National Centre for Veterinary Diagnostics, Hanoi, Vietnam.
authors.forename: Dang Hoang
authors.initials: DH
authors.lastname: Nguyen
authors.affiliation: Influenza and Prion Diseases Research Center, National Institute of Animal Health, NARO, Ibaraki, Japan.|Thailand-Japan Zoonotic Diseases Collaboration Center, Bangkok, Thailand.
authors.forename: Yuko
authors.initials: Y
authors.lastname: Uchida
authors.affiliation: Division of International Health, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan.
authors.forename: Reiko
authors.initials: R
authors.lastname: Saito
authors.affiliation: Influenza and Prion Diseases Research Center, National Institute of Animal Health, NARO, Ibaraki, Japan. taksaito@affrc.go.jp.|Thailand-Japan Zoonotic Diseases Collaboration Center, Bangkok, Thailand. taksaito@affrc.go.jp.|United Graduate School of Veterinary Sciences, Gifu University, Gifu, Japan. taksaito@affrc.go.jp.
authors.forename: Takehiko
authors.initials: T
authors.lastname: Saito
mesh_terms: D000822:Animal Husbandry; D000818:Animals; D009980:Influenza A virus / Q000145:classification*; D009976:Orthomyxoviridae Infections / Q000453:epidemiology / Q000662:veterinary* / Q000821:virology; D013552:Swine; D013553:Swine Diseases / Q000453:epidemiology / Q000821:virology*; D014744:Vietnam / Q000453:epidemiology
keywords: Active surveillance; Influenza A virus; Pig; Pig farm; Vietnam