In China the link between live poultry markets (LPMs) and the spread (and reassortment of) avian flu strains is already well established, and it is easy to understand why.
Large quantities of birds of varying species (chickens, ducks, geese, quail, and others) are brought in from different farms, housed together in cramped quarters, and then slaughtered (and often de-feathered) in open air booths as thousands of people walk by.
Not only does this expose humans to avian viruses, it facilitates the sharing of different avian flu subtypes (H5, H7, H9) among the birds, promoting continual reassortment and the creation of new clades, or subtypes.
In 2014, a year after the H7N9 virus emerged in China, in CDC: Risk Factors Involved With H7N9 Infection, we looked at a case-control study that pretty much nailed LPMs as the prime risk factor for infection.
While even casual exposure to poultry in live bird markets was cited as the primary risk factor, people who owned, raised, or slaughtered birds at home, on farms, or in the wild were not found to be at any increased risk.
Last December, a Who's Who of avian flu experts, writing in The Lancet: Interventions To Reduce Zoonotic & Pandemic Risks From Avian Flu In Asia listed immediate actions that should be taken by LPMs to curb the evolution and spread of avian flu in 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 have generally been 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.
Three years ago, in How to Aerosolize A Chicken, we looked at one plausible way this can happen, through the use of mechanical de-feathering machines used in some chicken stalls.
And there is some evidence that avian viruses can be spread from farm to farm - at least over short distances - by prevailing winds (see Bird Flu’s Airborne `Division).
Adding more weight to this idea, a year ago, in 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.
But until now, little has been published on the search for airborne avian flu viruses in and around live poultry markets in China.
Which brings us to a study, published last week in the Chinese Medical Journal, which describes two years of environmental and air testing from one of the hundreds of LPMs in Nanning, China (Guangxi province).
They collected Aerosol samples from one (of 12) random stalls (including chickens, geese, and ducks) every week. At the same time they collected floor sewage and feathers, and swabbed poultry cages and chopping blocks.
Samples were cataloged and tested for influenza A viral RNA and positive samples were then analyzed to determine their (H5/H7/H9) subtype.
RT-PCR testing is very sensitive, and the detection of viral RNA is a much lower bar than isolating live virus, and doesn't tell us about the viability of the viruses they detected.
The (open access) study found ample environmental evidence of viral contamination, and reports the first positive detection of Airborne H9 (presumably H9N2) in a Chinese LPM.
Although not a huge threat to human health, H9N2 is increasingly on our radar because the number of human infections appear to be increasing, we've seen signs of increasing mammalian adaptations, and because it has contributed its internal genes to a number of reassorted avian flu viruses.
Guangxi Province has only reported 3 cases of H7N9, and so it isn't terribly surprising that H7 viruses were not detected, and while H5 viruses were found in roughly 20% of environmental samples, no airborne H5 was detected.
The full report can be read at the link below, I've included some excerpts from the discussion:
First Positive Detection of H9 Subtype of Avian Influenza Virus Nucleic Acid in Aerosol Samples from Live Poultry Markets in Guangxi, South of China.
Overall, the present results suggest that AIV nucleic acid exist in aerosol and other environmental samples from LPMs of Guangxi. Aerosol transmission might be an important mode of human infected by AIV after visiting LPMs; thus, it is needed to monitor virus distribution in aerosol sample from LPMs.
Previous studies have shown that LPMs were closely linked to H9N2, H5N1, and H7N9 infection in human in 2003, 2006, and 2013, respectively.
LPMs promote the transmission of AIV from avian to human. It is still needed to confirm the transmitted mechanism through exposure or visiting LPMs. Chickens were successfully infected with AIV H5N1and H9N2 by aerosol and caused higher titers of virus. [6,7]
Avian influenza H9 has been isolated animals from LPMs[8‑11] and from air in chicken houses in China. Our results first reported H9 subtype of AIV nucleic acid existing in aerosol sample from LPM of Guangxi, China, providing support for aerosol transmission of AIV in LPM.
H5 subtype of AIV nucleic acid was not detected positive in aerosol while it was positive in other four styles of environmental samples. H9 subtype was positive in aerosol and other four styles of environmental samples in 2014.
It is still needed further study to investigate whether there are differences in viral viability between H5 and H9 subtypes in aerosol and other environmental samples. As LPMs play an important role in the dissemination of AIVs, active surveillance to monitor AIV in LPMs should be carried out as an early warning system for AIV outbreaks.