No sign of missed H5N1 cases in Bangladesh study

Mar 13, 2012 (CIDRAP News) – No signs of H5N1 avian influenza infections were found in more than 400 Bangladeshis who were probably exposed to the virus while working on poultry farms and in markets, according to a study reported in preliminary form today.

The findings were presented at the International Conference on Emerging Infectious Diseases (ICEID) in Atlanta. They are in line with a number of previous serologic studies in the past 8 years that have found few people carrying H5N1 antibodies suggesting they had unrecognized H5N1 infections.

The question of undetected H5N1 cases has gained new prominence in the current debate over studies involving genetically modified H5N1 viruses with increased transmissibility. A few researchers have contended that the real case-fatality rate for human H5N1 is probably far lower than the 59% indicated by the official number of known cases and deaths, because, they say, many asymptomatic or mild cases have probably gone undetected.

In the new study, scientists from Bangladesh and the United States sought to measure the prevalence of H5N1 antibodies and risk factors for infection in poultry workers in 2008 and 2009, according to the abstract. The authors are from the International Centre for Diarrhoeal Disease Research in Bangladesh, the Institute of Epidemiology, Disease Control and Research, also in Bangladesh, and the US Centers for Disease Control and Prevention.

They identified 89 poultry farms that had confirmed H5N1 outbreaks from December 2007 to June 2009 and three live-bird markets in Dhaka where poultry deaths exceeded 5% to 10% for 2 consecutive days in 2008. From January to July of 2009, the investigators identified 422 workers at those sites (212 farm workers, 210 from markets) and collected serum samples, demographic information, and data about poultry exposures.

The team used a microneutralziation assay to detect antibodies to a 2008 Bangladeshi strain of H5N1 virus, using a titer of 1:40 as their threshold for a positive result.

Both farm and market workers reported many tasks that involved poultry exposures, such as feeding, collecting eggs, cleaning stalls, slaughtering, and evisceration. Despite these exposures, none of the workers tested positive for H5N1 antibodies, the report says.

"However, the recent cases of human H5N1 infection detected in Bangladesh and the continued circulation of H5N1 among poultry warrants continued surveillance among a larger cohort of poultry workers to monitor the risk of avian-to-human H5N1 virus transmission," the abstract concludes.

Nasreen S, Khan SU, Azziz-Baumgartner E, et al. Board 183. Sero-prevalence of H5N1 antibodies among poultry workers in Bangladesh, 2008-2009. ICEID program and abstract book, p 116 (Presented Mar 13, 2012)

See also:

Feb 24 CIDRAP News story "Debate on H5N1 death rate and missed cases continues"

Feb 9 CIDRAP News story "Undetected H5N1 cases seem few, but questions persist"

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