While Chinese poultry – particularly poultry sold in live markets – are atop the suspect list for spreading the H7N9 virus, there remain many unanswered questions regarding where this virus resides, and how it is jumping to humans. Prime among them, despite having the finger of suspicion directed at them, remarkably few birds have actually tested positive for the virus.
While only a few dozen H7N9 positive birds were detected last year out of tens of thousands tested, we saw an immediate and dramatic drop in the number of human cases once live bird markets were closed (see The Lancet: Poultry Market Closure Effect On H7N9 Transmission), making for a pretty compelling circumstantial case for live poultry being the prime vector.
As does the oft quoted (but hard to verify) statistic that 70% of human cases reported recent direct contact with live poultry.
The FAO, in their most recent update (see FAO Calls For Increased Regional Vigilance & Preparedness Over H7N9), stops short of making a definitive statement on the source of infection, but states:
There is strong evidence that people become infected following close contact with infected live poultry, mostly in live bird markets or when slaughtering birds at home.
Source of human infection
Although much remains unknown about this virus, such as (1) the animal reservoir(s) in which it is circulating, (2) the main exposures and routes of transmission to humans, and (3) the distribution and prevalence of this virus among people and animals, human infection appears to be associated with exposure to live poultry or contaminated environments, including markets where live poultry are sold. This is based on the following:
• Most human cases report a history of exposure to birds or live poultry markets.
• The viruses isolated from humans are avian influenza viruses and genetically similar to those isolated from birds and the environment.
• Targeted testing of poultry and environment in live poultry markets that are epidemiologically linked with human cases of H7N9 infection has revealed more positive results than testing in areas not linked with human cases
While none of this equates to a `smoking chicken’, warnings regarding visiting live poultry markets have been prominently featured by Hong Kong’s CHP, Mainland China’s CDC, along with our own CDC, and the World Health Organization.
Unlike other dangerous avian flu viruses we’ve seen in the past, H7N9 doesn’t make poultry ill, making it very difficult to detect, control, or eradicate. An avian virus that sickens and even kills people, carried stealthily by chickens or ducks, is the very definition of a nightmare scenario for the poultry industry.
But for China, where food insecurity remains high and poultry provides much of their diet’s protein – this scenario could prove economically, politically, and socially disastrous.
China, which produces more poultry than anyplace else on earth, reportedly raises over 15 Billion birds each year. Poultry - whether factory-farm produced or from backyard flocks - is a major source of income, and food, for hundreds of millions of people.
Anything that seriously threatens China’s poultry industry also raises the specter of mass hunger in the world’s most populous nation. And with that comes political instability, and unpredictable outcomes. Something which we looked at last April in Food Insecurity, Economics, And The Control Of H7N9.
While China’s CDC is primarily concerned with preserving the the health of the nation, their Minister of Agriculture (MOA) is concerned with feeding more than 1 billion people. With very different responsibilities and goals, and the price of failure high, I suppose we shouldn’t be too surprised to see conflicting messages regarding the H7N9 virus.
Today, on the eve of China’s biggest holiday celebration, their MOA has publicly disputed the connection between live poultry and the infection of humans by the H7N9 virus. This from CCTV.
China's Ministry of Agriculture says there is so far no proof of any direct transmission of the H7N9 bird flu from poultry to humans. The ministry said on Wednesday that it had collected over 1.6 million poultry and environment samples in 2013, of which only 88 samples had tested positive for the H7N9 virus.
So far in 2014, the ministry has taken another 33,000 samples, eight of which were found to be positive. All of the positive samples came from live poultry markets.
"So far we haven’t found any positive samples from live poultry farms. And we haven’t found any evidence to prove that poultry can pass the H7N9 virus directly to humans." Zhang Zhongqiu, bureau director of Ministry of Agriculture, said.
Admittedly, the paucity of positive tests out of more than a million samples introduces a degree of uncertainty regarding the transmission of this virus, and the possibility of there being more than one vector species can’t be ignored.
But until another vector can be identified, exposure to live market birds remains the most likely risk factor for contracting the virus.
Of course, should this virus ever acquire the ability to spread efficiently between humans, that point will quickly become moot.