Several serological studies have been conducted to assess the transmission of H5N1 virus in poultry workers. The reported seropositive rates vary from 0 to 10% [9–11]. In the present study, the overall positive rate we found was 2.61% and in different sampling districts the rates varied from 0 to 5.38%. Many factors could be responsible for the variability, such as the H5N1 infection rate and vaccination rate of the poultry, the precautions and job duties of the workers. In our study, poultry number was identified as a novel risk factor associated with human infection with avian H5N1 virus, which could be accounted for the distinction of seropositive rate among districts and studies.
Poultry workers were expected to have the highest level of exposure to avian H5N1virus and more intensive poultry exposure was associated with having anti-H5N1 antibody . Compared with poultry workers from large scale poultry farms, backyard workers may have an even higher risk due to the lack of preventive measures and healthy environment. Wang et al. conducted a serologic surveillance study in Guangzhou, China, founding workers in scale poultry farms all seronegative for H5N1 virus . Certain job duties such as butchering poultry, feeding poultry and preparing poultry for restaurants may also be associated with the infection risk . In our study, enrolled poultry workers were all from backyard poultry farms. Their job duties were almost identical, mainly including feeding poultry, collecting eggs and cleaning poultry stalls.
High poultry density could raise the infection risk of poultry with avian influenza H5N1. Tiensin et al. conducted an ecologic investigation on risk factors of clusters of avian H5N1 virus infection. Fighting cock flock density, meat and laying duck flock density were found significantly higher in case subdistricts than in control subdistricts . However, the association between increasing poultry number and risk of humans infected with avian influenza H5N1 was identified for the first time in this study. This finding could help public health officers recognize high-risk population and institute countermeasures.
Because up to 40% of the poultry workers enrolled in our study were ≥60 years old and the microneutralization and western blot assays were found to be less specific for persons ≥60 years old , the horse RBC HI assay, which has high reliability and good agreement with MN assay results without age limit was used in this study .
Our findings suggested that avian-to-human transmission of influenza H5N1 virus remains low in China. Workers associated with raising larger poultry flocks have a higher risk of seropositivity. There were several potential reasons for that, such as high poultry density could raise the infection risk of poultry with avian influenza H5N1 , and more intensive poultry exposure was associated with having anti-H5N1 antibody for poultry workers . As we didn't know the infection rate of the poultry and the moderate sample size limited the statistical significance of our research, further studies are warranted to validate our results.