we conducted a cross - sectional study among poultry farmers and cullers from 1 district in each of the 5 provinces ( chachoengsao , kanchanaburi , khon kaen , sukhothai , and suphanburi ) where outbreaks of avian influenza ( h5n1 ) among poultry and human infections had been confirmed since january 2004 ( figure ) . with the assistance of provincial human and animal health authorities
informed consent was obtained , and a brief interview was conducted . because the precise timing of potential exposures could not be determined , a single serum sample was collected from each patient and stored at 20c until tested under biosafety level 3 ( bsl-3 ) conditions .
( 4 ) , at the department of microbiology , faculty of medicine , siriraj hospital , mahidol university by microneutralization assay ( micro - nt ) for antibody to h5n1 viruses . before this study ,
senior laboratory staff from siriraj hospital received 2 weeks of on - site training by a visiting scientist from the us centers for disease control and prevention who had expertise with this assay . the world health organization ( who ) defines a positive test result as a microneutralization antibody titer for influenza virus ( h5n1 ) of > 80 with a confirmatory elisa or western blot assay ( 3,4 ) ( www.who.int/csr/disease/avian_influenza/guidelines/case_definition2006_08_29/en/index.html ) .
serum samples from persons > 50 years of age were excluded from laboratory analysis because the microneutralization assay for antibodies against subtype h5n1 has been reported to be less specific for older persons ( 5 ) .
gray shading indicates provinces with confirmed avian influenza outbreaks ; black outlines indicate provinces included in this study . of 350 farmers asked to participate , 322 ( 92% ) enrolled in the study , of which 167 ( 52% )
the mean age of participants was 34 years ( range 550 years ) ( table ) . among participants , 188 ( 58% ) reported handling sick or dying poultry , 107 ( 33% ) were involved in culling operations of apparently well poultry in outbreak areas , and 27 ( 9% ) reported only contact with well poultry in the context of routine farming practices .
although no study participant had an anti - h5n1 antibody titer of > 80 , 7 ( 2.2% ) farmers had lower reactive antibody titers .
of these , 4 had titers of 10 , 2 had titers of 20 , and 1 had a titer of 40 .
the small number of study participants with anti - h5 antibody titers precluded statistical comparisons to those without reactive antibodies .
poultry farmers and cullers are at increased occupational risk for exposure to avian influenza viruses . however , since 2004 , infections have been less commonly reported in cullers , while poultry farmers have made up a large proportion of cases worldwide . a study in hong kong special administrative region , people s republic of china , examined influenza virus ( h5n1 ) transmission and risk factors among poultry workers and government workers involved in culling during the 199798 outbreak ( 3 ) .
the study concluded that although no hospitalized poultry workers were identified among the 18 patients in that outbreak , 3% of 293 cullers and 10% of 1,525 poultry workers had antibody titers against influenza ( h5n1 ) of > 80 , which suggested that a substantial number of mild or asymptomatic infections had occurred in this occupationally exposed population .
in contrast , we found that no poultry workers had microneutralization titers > 80 , whereas 7 ( 2% ) had lower titers that did not meet the who definition for seropositivity .
the lower titers may have resulted from cross - reactivity with circulating antibodies after previous human influenza virus infections ( 5,6 ) .
these low titers could be the result of mild or asymptomatic influenza ( h5n1 ) infections because not all influenza virus infections invariably result in marked antibody responses ( 7 ) .
likewise , these results could reflect the decay of antibody titers over time ( 8) . finally , the micro - nt assay is a highly specific and strain - sensitive test .
although we used the same virus that was circulating in thailand at that time , these lower titers could be attributable to infections with another virus variant .
most human influenza ( h5n1 ) infections have occurred in persons who had had direct contact with sick or dying poultry ( 911 ) . while human infections with avian influenza ( h5n1 ) continue to be reported
, growing evidence indicates that this virus is not easily transmitted from poultry to humans and that mild or asymptomatic infections in humans are not common . a seroepidemiologic investigation in rural cambodia surveyed 351 participants from 93 households in an area where influenza ( h5n1 ) infections in poultry and a single fatal human case had been documented ( 10 ) . despite frequent , direct contact with poultry suspected of having influenza ( h5n1 ) infection ,
a similar study in nigeria found that all of 295 poultry workers had negative test results for influenza ( h5n1 ) neutralizing antibodies ( 12 ) . 
studies of healthcare workers suggest that transmission of influenza virus ( h5n1 ) to hospital staff who cared for infected patients also appears to be uncommon ( 1315 ) .
our study provides additional evidence to suggest that influenza virus ( h5n1 ) is not easily transmitted to humans .
however , the wide geographic distribution of this subtype , ubiquitous exposures , and the high case - fatality ratio from the infection underscore the importance of adherence to poultry - handling practices recommended by the food and agriculture organization and who ( www.wpro.who.int/nr/rdonlyres/7693baf7-13e7-42db-b92b-004cf5d517e7/0/whointerimrecommendation26012004.pdf , www.fao.org/ag/againfo/subjects/en/health/diseases-cards/avian_qa.html#8 ) .
molecular surveillance indicates that the avian influenza virus ( h5n1 ) continues to evolve rapidly ( www.who.int/csr/disease/avian_influenza/guidelines/recommendationvaccine.pdf ) .
additional seroepidemiologic studies are warranted to monitor for changes in transmissibility and the spectrum of clinical illness .
 in thai provinces where avian influenza outbreaks in poultry had been confirmed in the preceding 6 months , serum from 322 poultry farmers was tested for antibodies to avian influenza virus subtype h5n1 by microneutralization assay . 
 no study participant met the world health organization serologic criteria for confirmed infection . 
The Study
Conclusions