Transmission of pandemic influenza H1N1 (2009) in Vietnamese swine in 2009-2010.
BACKGROUND
The pandemic of 2009 was caused by an H1N1 (H1N1pdm) virus of swine origin. This pandemic virus has repeatedly infected swine through reverse zoonosis, although the extent of such infection in swine remains unclear.


OBJECTIVE
This study targets small and commercial pig producers in North Vietnam, in order to estimate the extent of H1N1pdm infection in swine and to identify the risk factors of infection.


METHODS
Virologic and serologic surveillance of swine was carried out in 2009-2010 in pig farms (38 swabs and 1732 sera) and at a pig slaughterhouse (710 swabs and 459 sera) in North Vietnam. The sera were screened using a influenza type A-reactive ELISA assay, and positive sera were tested using hemagglutination inhibition tests for antibody to a panel of H1-subtype viruses representing pandemic (H1N1) 2009 (H1N1pdm), triple reassortant (TRIG), classical swine (CS), and Eurasian avian-like (EA) swine lineages. Farm-level risk factors were identified using a zero-inflated negative binomial model.


RESULTS
We found a maximal seroprevalence of H1N1pdm of 55·6% [95% CI: 38·1-72·1] in the slaughterhouse at the end of December 2009, 2 weeks after the peak of reported human fatalities with H1N1pdm. Farm-level seroprevalence was 29% [95% CI: 23·2-35·7]. In seropositive farms, within-herd seroprevalence ranged from 10 to 100%. We identified an increased risk of infection for farms that specialized in fattening and a decreased risk of infection in farms hiring external swine workers.


CONCLUSIONS
Our findings suggest extensive reverse-zoonotic transmission from humans to pigs with subsequent onward transmission within pig herds.
French Agricultural Research Center for International Development (CIRAD), Animal and Integrated Risk Management Research Unit (AGIRs), Montpellier, France. carlene.trevennec@cirad.fr
Karen
K
Trevennec
Lucas
L
Leger
Faouzi
F
Lyazrhi
Eugénie
E
Baudon
Chung Yan
CY
Cheung
François
F
Roger
Malik
M
Peiris
Jean-Michel
JM
Garcia
D000818:Animals; D000914:Antibodies, Viral / Q000097:blood; D003430:Cross-Sectional Studies; D004797:Enzyme-Linked Immunosorbent Assay; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D009976:Orthomyxoviridae Infections / Q000453:epidemiology / Q000635:transmission / Q000662:veterinary* / Q000821:virology; D012307:Risk Factors; D016036:Seroepidemiologic Studies; D044967:Serum / Q000276:immunology; D013552:Swine; D013553:Swine Diseases / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D014744:Vietnam; D015047:Zoonoses / Q000635:transmission
D000914:Antibodies, Viral