One of the recurring issues in H5N1 is the validity of its apparent case fatality rate—currently around 59 percent, with individual countries like Indonesia and Cambodia ranging much higher. This has seemed, to coin a phrase, too bad to be true: A virus that kills off its human hosts in such numbers should burn itself out. Many have argued that we see only the really serious cases, while others catch it and shake it off without much fuss.
The problem was that surveys couldn't find many people with H5N1 antibodies. We have seen a few: the other day I mentioned the healthy Indonesian labourer who seems to have been his country's index case, and I recall some South Korean poultry cullers were later found to be carrying antibodies. But those examples were rare enough to remember.
However, Greg Folkers has sent the link to a new report in Journal of Thoracic Disease that suggests China may have a lot of H5N1 survivors, especially poultry workers: Evidence for H5 avian influenza infection in Zhejiang province, 2010-2012: a cross-sectional study. The abstract:
Background:The first outbreak of H5N1 highly-pathogenic avian influenza (HPAI) virus associated with several human deaths occurred in 1997 in Hong-Kong, China. While H5N1 virus infection in poultry workers has been studied in some detail, little is known about the environmental risk factors of the H5 avian influenza virus infection in China.
Methods: A cross-sectional study was performed to evaluate the environmental load of H5 viruses in poultry-contaminated environments and to explore potential risk factors associated with infection in poultry workers between October 2010 and March 2012. Serum and environmental samples were collected in Zhejiang province, China. The hemagglutination inhibition (HI) assay was used to analyze human sera for antibodies against H5N1 virus [A/Hubei/1/2010 (H5N1) and A/Anhui/1/2005 (H5N1)]. All participants were interviewed with a standardized questionnaire to collect information on exposure to poultry. H5 Avian influenza virus in the environmental samples was detected by real time RT-PCR.
Results: One hundred and five of 3,453 environmental samples (3.0%) tested positive for H5 avian influenza virus. Fifty-five of 1,169 subjects (4.7%) tested seropositive for anti-H5N1 antibodies. A statistically significant difference in H5 virus detection rate was found among the different environments sampled (<0.001), with the highest showed in poultry slaughtering and processing plants (14.6%). Detection rate varied according to the source of samples, sewage (4.5%), drinking water (3.1%), feces (2.3%), cage surface (2.0%), and slaughtering chopping boards (7.0%), respectively. Direct or close contact with poultry (OR =5.20, 95% CI, 1.53-17.74) and breeding numerous poultry (OR =3.77, 95% CI, 1.72-8.73) were significantly associated with seroprevalence of antibodies to avian influenza virus A (H5N1).
Conclusions: The number of birds bred more than 1,000 and direct or close contact with poultry in the workplace or the environment would be a potential risk of H5N1 infection.
In the years 2010-2012, WHO reports a grand total of only 5 Chinese H5N1 cases and 5 deaths. But here we are with 4.7 percent of those tested carrying H5N1 antibodies in one province alone. Moreover, the positives found in slaughtering and processing plants indicate a lot of sick birds were getting into the food chain without automatically making consumers (or processors) sick.
This study was of course done in Zhejiang, but we might expect to find similar results all over China, especially in regions that have seen H5N1 outbreaks in poultry or humans. We might even speculate that the unfortunate Canadian nurse contracted H5N1 through a meal prepared on a contaminated chopping board—even if what she ate wasn't chicken. And she's not the first H5N1 case with no known poultry contact.