Live bird markets (LBMs) In China have been consistently linked to the spread of avian influenza viruses (H5N1, H7N9, H10N8, and H5N6) to humans in recent years, with China's 2014 case-control study on H7N9 (see Risk Factors Involved With H7N9 Infection) concluding:
Exposures to poultry in markets were associated with A(H7N9) virus infection, even without poultry contact. China should consider permanently closing live poultry markets or aggressively pursuing control measures to prevent spread of this emerging pathogen.
While attempts have been made to shut down live bird markets - at least temporarily when human cases are reported - and that has helped to reduce infections (see The Lancet: Poultry Market Closure Effect On H7N9 Transmission), these live markets remain ubiquitous and operational across much of Asia.
The detection of viral RNA (or sometimes live virus) in LPMs isn't new (see Macao Detects H7 In Poultry Market - Live Sales Halted 3 Days), and some studies (see H5N1: Hiding In Plain Sight) have shown these viruses may survive for days or even weeks under the right conditions.
But these detections are usually on fomites (inanimate objects like knives, table tops, cages, etc.) or in environmental contamination from chicken manure, feathers, entrails, or dust.
Less well defined is the airborne spread of avian viruses in these markets, although it may explain how a few people just walking past a live market have reportedly been infected.
At the end of May, in Detection Of Airborne H9 Nucleic Acid In Chinese Live Poultry Market, we looked at an article appearing in the Chinese Medical Journal which found ample environmental evidence of viral contamination, and reports the first positive detection of Airborne H9 (presumably H9N2) in a Chinese LPM.
Another study, performed over last winter at 3 LBMs in Zhongshan (a Prefecture level city in Guangdong Province),has obtained similar results with HPAI H5N6 and is described in a letter to the Editor of the Journal of Infection.
Yanheng Wu , Wuyang Shi , Jinsi Lin , Man Wang , Xueqin Chen , Kangkang Liu , Ying Xie , Le Luo, Benjamin D. Anderson , John A. Lednicky
We are the first to report the isolation of viable influenza A (H5N6), a likely reassortment of H5-subtype and H6N6, from air samples collected in LPMs in China. The ZS01 strain isolated from live poultry shared high homology with other recently isolated H5N6 poultry strains, suggesting that ZS01 might circulate between poultry and aerosols.
Five of the ZS01 gene segments were closely related to the human A/Guangzhou/39715/2014(H5N6) strain. Hence, it seems biologically plausible that ZS01 could be transmitted across species (poultry to humans) via the air.
If cross-species aerosol transmission is confirmed, the already complex ecology of influenza A is further complicated. Then biosafety measures for persons occupationally exposed to poultry may require reexamination and more efforts should be made in the future to study the situation of AIV in the aerosol from LPMs.
As avian influenza is primarily a gastrointestinal disease in birds and virus is shed in copious amounts in feces. It has been theorized that when it dries - its dust (and any viral hitchhikers) - can become temporarily airborne.
And we've seen some limited evidence that avian viruses might be spread from farm to farm - at least over short distances - by prevailing winds (see Bird Flu’s Airborne `Division).
Adding to this, in 2015 (see CIDRAP: H5N2 Roundup & Detection In Environmental Air Samples) we looked at air sampling conducted by the University of Minnesota around infected poultry farms that found evidence of airborne virus particles.
Given the millions of people exposed each week at LBMs in China, and the small number of people who actually contract avian flu, it is safe to say this hasn't been a particularly efficient method of transmission.
But as these viruses become more adapted to mammalian hosts (including humans) - a direction many of these novel viruses appear to be taking (see here, here, and here) - LBMs could end up being a far riskier venue in the future.