Chance Chan/Reuters
Chinese doctors and nurses at a training course in April for the treatment of the H7N9 virus.
Shutting down live poultry markets is extremely effective in preventing human cases of avian flu and should be considered if the disease reappears this winter, researchers in China reported Wednesday.
But experts, including the authors, warned that shutting such markets permanently would be impractical because consumers in many countries demand live birds, and small farmers cannot afford refrigerated slaughterhouses and trucks.
Even temporary shutdowns create economic problems but should be considered, they said.
The study, published by The Lancet, concerned last spring's outbreak of H7N9 flu outbreak in southern China. The flu is still rare in humans — only 135 cases were confirmed before it disappeared in the summer, but 45 were fatal.
Two new human cases appeared in China this month, raising fears that it will surge this winter.
In four cities that responded in April by closing their live poultry markets — Shanghai, Hangzhou, Huzhou and Nanjing — human cases dropped almost immediately by 97 to 99 percent, the researchers said.
The flu first appeared in March. Most victims caught it from birds and only a few passed it on to family members or medical personnel, scientists believe.
The closed markets have all reopened, and the outbreak cost the economy an estimated $1.6 billion. It also upset the poultry industry, which is believed to have "substantial political connections," said Benjamin J. Cowling, an epidemiologist at the University of Hong Kong medical school and an author of the study.
Other experts warned that during outbreaks of H5N1 avian flu — another rare but potentially lethal influenza that has circulated since 1997 — live markets in countries like Vietnam and Egypt simply ignored orders to close. China, by contrast, is very effective at enforcing public health measures when officials decide to act.
Even in cities where refrigeration is available, many markets sell live poultry "because people like to look at the bird's color, its age, to see if it looks healthy," said Dr. Juan Lubroth, chief veterinary officer of the United Nations Food and Agriculture Organization.
Live poultry markets are often crowded and filthy; bird flu spreads in droppings, and chickens, ducks, geese and other birds often wander over floors covered with them.
Some poultry is sold live in virtually every city in the world, including New York. In poor countries, live sales are the norm because slaughtered birds spoil quickly without refrigeration.
In China, live markets "are more common than Walmart is in the U.S.," Dr. Cowling said, adding that food scandals have made consumers wary of chilled or frozen meat.
To prevent H5N1 outbreaks, some Asian cities have mandated "rest days" on which markets must be empty for 24 hours while they are cleaned and disinfected.
A study published last year of 54,000 samples of droppings collected in live bird markets over 12 years found that the presence of flu viruses on market floors could be reduced by 84 percent through several measures less punitive than full shutdowns: two monthly rest days, bans on leaving poultry in markets overnight and bans on selling quail.
Some markets require that all birds not sold by closing time must be slaughtered and disposed of. But unless sellers are compensated, they will evade those rules, said Dr. Guillaume Fournié, an epidemiologist at the Royal Veterinary College in England, who wrote a comment on The Lancet's study.
Work he did in Vietnam, Dr. Fournié said, suggested that identifying "hub" markets, cleaning them regularly and disinfecting trucks hauling birds between markets could interrupt disease transmission with fewer shutdowns.
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