The link between China's live poultry markets (LPMs) and the spread (and reassortment of) avian flu strains has been well established, with a case-control study released in 2014 (see CDC: Risk Factors Involved With H7N9 Infection) citing even casual exposure to poultry in live bird markets as the primary risk factor for infection.
Meanwhile, people who owned, raised, or slaughtered birds at home, on farms, or in the wild were not found to be at any increased risk.LPMs bring together large quantities of birds of varying species (chickens, ducks, geese, quail, and others) - brought in from different, often distant farms - which are housed in cramped quarters, and then slaughtered (and often de-feathered) in open air booths as thousands of people walk by.
The temporary closing of LPMs has been shown to slow, or even halt, the spread of H7N9 in China (see The Lancet: Poultry Market Closure Effect On H7N9 Transmission), and every year we see (unsuccessful) calls for the complete ban on live markets.While most H7N9 human infections have been linked to exposure to infected birds, a not insignificant number of cases have no known poultry exposure. Some of those, however, either lived near or regularly passed by live bird markets, leading to speculation that viable virus particles may travel beyond the confines of the LPM.
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 have generally been on fomites (inanimate objects like knives, table tops, cages, etc.) or in environmental contamination from chicken manure, feathers, entrails, or dust.Over the past few years we've seen experiments designed to detect airborne virus particles inside barns, live bird markets, and even hospitals. While it requires sophisticated sampling gear - and the detection of viral RNA is a much lower bar than isolating live virus - we've seen a growing body of evidence supporting the idea.
Although more controversial, we've also seen some evidence suggesting avian influenza viruses can be spread longer distances - even from farm to farm - by prevailing winds (see Bird Flu’s Airborne `Division).
The science of all of this even has a name; aerobiology – the study of how bacteria, fungal spores, pollen and even viruses can be passively transported in the air.Two and a half years ago - during the height of North America's HPAI H5Nx epizootic - we looked at testing conducted by the University of Minnesota around infected poultry farms that found evidence of airborne virus particles (see CIDRAP: H5N2 Roundup & Detection In Environmental Air Samples).
All of which brings us to a new study (alas, behind a paywall) in the journal Building & Environment that finds evidence of airborne AI RNA at least 100 meters outside of LPMs in Guangzhou, China.
Assessing the risk of downwind spread of avian influenza virus via airborne particles from an urban wholesale poultry market
Jianjian Weia, 1, Jie Zhoub, 1, Kitling Chengb, Jie Wuc, Zhifeng Zhongc, Yingchao Songc, Changwen Kec, Hui-Ling Yenb, , , Yuguo Lia, ,
- Airborne transmission potential of avian influenza virus in a poultry market was investigated.•
- Viral RNA are readily detectable by air sampling, predominantly from particles larger than 1 μm.•
- CFD modeling reveals the combined effect of wind direction and surrounding buildings.•
- The virus concentration has a slow decay rate in air in the downwind direction.
Interspecies transmissions of avian influenza viruses (AIV) occur at the human-poultry interface, among which the live poultry markets (LPMs) are easily assessed by urban residents. Thousands of live poultry from different farms arrive daily at wholesale markets before being sold to retail markets. We assessed the risk of AIV downwind spread via airborne particles from a representative wholesale market in Guangzhou.
Because of the large volume of poultry trade daily, wholesale markets located in urban areas may pose considerable AIV infection risk to neighboring residents via wind spread, even in the absence of direct contact with poultry.(Continue . . . )
Readers with long memories will recall in December of 2012 (see Barnstorming Avian Flu Viruses?) we looked at a study in the Journal of Infectious Diseases called Genetic data provide evidence for wind-mediated transmission of highly pathogenic avian influenza that found patterns that suggested farm-to-farm spread of the 2003 H7N7 in the Netherlands due to the prevailing wind.
Another study of the same outbreak, Modelling the Wind-Borne Spread of Highly Pathogenic Avian Influenza Virus between Farms (PloS One 2012), found that wind borne transmission could have accounted for up to 24% of the transmission over distances up to 25 km.
And over the summer of 2015, APHIS released a 38-page partial epidemiology report on the spread of HPAI H5 across the United States (see APHIS: Partial Epidemiology Report On HPAI H5 In The US) - that while coming to no conclusions - acknowledged the possibility that prevailing winds may have carried contaminated dust particles from farm to farm.All of which adds plausibility to the notion that airborne contaminated dust and debris from live poultry markets could potentially spread avian influenza viruses to humans across urban environments; perhaps over distances of hundreds of yards.