Avian Influenza A (H5N1) Virus Antibodies in Poultry Cullers, South Korea, 2003–2004

Donghyok Kwon, Joo-Yeon Lee, Wooyoung Choi, Jang-Hoon Choi, Yoon-Seok Chung, Nam-Joo Lee, Hyang-Min Cheong, Jacqueline M. Katz, Hee-Bok Oh, Haewol Cho, and Chun Kang

Author affiliations: Korea Centers for Disease Control and Prevention, Osong, South Korea (D. Kwon, J.-Y. Lee, W. Choi, J.-H. Choi, Y.-S. Chung, N.-J. Lee, H.-M. Cheong, H.-B. Oh, H. Cho, C. Kang); Centers for Disease Control and Prevention, Atlanta, Georgia, USA (J.M. Katz)

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Abstract

Transmission of influenza (H5N1) virus from birds to humans is a serious public health threat. In South Korea, serologic investigation among 2,512 poultry workers exposed during December 2003–March 2004 to poultry with confirmed or suspected influenza (H5N1) virus infection found antibodies in 9. Frequency of bird-to-human transmission was low.

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Conclusions

By identifying only 9 seropositive cases among 2,512 persons, we determined that the risk for poultry-to-human transmission of the influenza (H5N1) virus is small. Other studies have also shown low frequencies of poultry-to-human (H5N1) virus transmission. In provinces in Thailand, blood samples were collected from 322 poultry farmers 6 months after confirmation of influenza (H5N1) virus outbreaks; all antibody titers were negative (5). Two studies of villagers in Cambodia who had frequent and direct contact with poultry with confirmed and suspected influenza (H5N1) virus infection found low frequency of virus transmission from poultry to humans (6,7). Similarly, a study in Nigeria also found negative results for antibodies against influenza (H5N1) virus among 295 poultry workers (8).

Because our study was conducted as a public health response, it has the following limitations. We were unable to systematically assess symptoms, extent of exposure, compliance with PPE use, and taking of oseltamivir. It is not clear if the participants were exposed to birds with confirmed or suspected influenza (H5N1) virus or whether they wore PPE properly when culling. Because the outbreak created an emergency situation and this study had not been designed before the outbreak, epidemiologic data were limited. And because we had insufficient serum for adsorption assays, we cannot exclude the possibility of cross-reactivity with circulating antibodies resulting from seasonal influenza vaccination or previous infection with human influenza virus. In 2004, among 83 Vietnam hospital employees who were exposed to 4 patients with confirmed and 1 patient with probable influenza (H5N1) virus infection, a positive antibody titer against influenza (H5N1) virus and cross-reacting antibodies against influenza (H1N1) virus was found on MN assay for 1 employee (12). Because our study was not a case–control study, we could not identify risk factors for transmission.

Regardless of these limitations, our study shows serologic evidence of influenza (H5N1) virus transmission among groups at high risk for poultry-to-human transmission (i.e., exposed to poultry during 2003−2004 outbreaks in South Korea). However, we also found additional proof that the frequency of poultry-to-human influenza (H5N1) virus transmission is low.