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

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.

Transmission of infl uenza (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 confi rmed or suspected infl uenza (H5N1) virus infection found antibodies in 9. Frequency of bird-to-human transmission was low.
T he highly pathogenic avian infl uenza (H5N1) virus has posed a serious public health threat since 1997, when the fi rst transmission of the virus from birds to humans was reported in Hong Kong (1,2). In South Korea, during December 2003-March 2004, this virus caused 19 outbreaks in 7 provinces (10 outbreaks on chicken farms and 9 on duck farms), which prompted a massive mobilization to cull birds and contain the outbreak (3). Vaccination of poultry against infl uenza (H5N1) virus was legally prohibited, and a stamping out policy was considered as a control option. Culling of ≈5 million birds was conducted on all farms with infected poultry and all poultry farms within a 3 km-radius protection zone.
All persons who participated in the culling operations were equipped with World Health Organization (WHO)recommended personal protective equipment (PPE) (4). To prevent the possibility of mixed infection with human and avian infl uenza viruses, previously nonvaccinated participants were vaccinated with a seasonal infl uenza vaccine and given oseltamivir as an additional prophylactic measure.
During the outbreaks, 142 respiratory specimens were collected from persons who had infl uenza-like illness and tested by reverse transcription PCR selective for the matrix and hemagglutinin (H) 5 genes and by virus isolation in cell culture; however, no infl uenza (H5N1) virus was detected (Korea Centers for Disease Control and Prevention [CDC], unpub. data).
The defi nition of a case of infl uenza-like illness was sudden onset of fever (>38°C) with cough or sore throat. According to previous serosurveys and outbreak investigations, infl uenza (H5N1) virus is poorly transmitted from birds to humans (5)(6)(7)(8). To trace the frequency of transmission of the infl uenza (H5N1) virus to persons who had been exposed to the confi rmed or suspected virusinfected poultry, a serosurvey was conducted by the Korea CDC.

The Study
A serologic investigation was performed among 2,512 persons who worked on poultry farms or culled birds during the 2003−2004 outbreaks in South Korea. Their use of PPE, receipt of oseltamivir, and exposure to birds with confi rmed infl uenza (H5N1) is unclear. Poultry culling was conducted during December 12, 2003-March 21, 2004. Blood was collected from cullers on the day of culling completion in each region. Convalescent-phase blood samples were collected at least 4 weeks later. Written agreement was provided before blood was collected from cullers, other poultry workers, and their household members. This study was reviewed and approved by the ethics committee of Korea CDC.
WHO-recommended laboratory tests and case defi nitions were used for serologic diagnosis of infl uenza (H5N1) virus infection in the cohort (9)(10)(11). Before this study, the laboratory staff of Korea CDC received 4 weeks of training at the US CDC on serologic testing for infl uenza (H5N1) virus. All experiments with live viruses were conducted at the biosafety level-3 facility of Korea CDC, and all serologic testing at the US CDC was conducted under biosafety level-3 containment including enhancements required by the US Department of Agriculture and the Select Agent Program.
All serum samples were tested for antibodies against infl uenza (H5N1) virus by microneutralization (MN) assay; results were considered to be positive if titers against H5 were >80 according to at least 2 independent assays. As recommended by WHO, samples that were antibodypositive by MN underwent confi rmatory testing by hemagglutination inhibition assay with horse erythrocytes or by H5-specifi c Western blot analysis (9,10 were farm workers or their household members (Table  1). Cullers included local government workers, soldiers, animal husbandrymen, and civilians. The culling periods were 1-13 days, and the average was 5.4 days. Among the 2,512 persons, MN assay results were confi rmed positive for 9. The US CDC confi rmed positive results in a single sample for 4 persons; the Korea CDC confi rmed positive results in paired samples for 5 others. Among the 9 persons with positive MN results, only 2 had positive results according to horse hemagglutination inhibition assay; however, all 9 had clear reactivity to H5 proteins on Western blot analysis and were confi rmed positive according to WHO criteria ( Table 2). All those with infl uenza (H5N1)-positive results were male, median age was 32.5 years (range 22-48 years), and all had participated in culling during the outbreaks ( Table 2). None of the other poultry farm workers had seropositive results.

Conclusions
By identifying only 9 seropositive cases among 2,512 persons, we determined that the risk for poultry-to-human transmission of the infl uenza (H5N1) virus is small. Other studies have also shown low frequencies of poultry-tohuman (H5N1) virus transmission. In provinces in Thailand, blood samples were collected from 322 poultry farmers 6 months after confi rmation of infl uenza (H5N1) virus outbreaks; all antibody titers were negative (5). Two studies of villagers in Cambodia who had frequent and direct contact with poultry with confi rmed and suspected infl uenza (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 infl uenza (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 confi rmed or suspected infl uenza (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 insuffi cient serum for adsorption assays, we cannot exclude the possibility of cross-reactivity with circulating antibodies resulting from seasonal infl uenza vaccination or previous infection with human infl uenza virus. In 2004, among 83 Vietnam hospital employees who were exposed to 4 patients with confi rmed and 1 patient with probable infl uenza (H5N1) virus infection, a positive antibody titer against infl uenza (H5N1) virus and cross-reacting antibodies against infl uenza (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 infl uenza (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 infl uenza (H5N1) virus transmission is low.