Pig disease may be spreading between humans

Vaccines to combat a deadly pig-borne disease were flown to south-western China on Sunday, where the spread of the rare illness has already killed 36 people and infected 198. The unusually high numbers of people infected by the swine disease has led scientists to speculate that it may be being spread from human-to-human – or that another disease entirely is to blame. Streptococcus suis type II, although relatively common in swine, spreads to humans extremely rarely, and the size and virulence of this current outbreak, in the province of Sichuan, has taken the World Health Organization by surprise.

The Chinese government responded on Sunday by airlifting the first batch of a vaccine for the infection – enough to treat 360,000 pigs – from the southern city of Guangzhou to the affected towns. The vaccine’s manufacturers say they will be producing enough vaccine to treat 10 million pigs in the coming days – but vaccines take three weeks to produce immunity in the pigs. Health authorities in the province have distributed two million notices to educate poor, often illiterate farmers and their children not to slaughter pigs or eat their meat. Thirty-nine temporary roadside quarantine stations have been set up to prevent dead pigs reaching markets – they will be burned instead.

China’s state-controlled media says the government has brought the disease under control, and that no human-to-human transmission of infection has been found. But there has been widespread criticism of the way the situation has been handled – with parallels being drawn to China’s handling of the SARS – severe acute respiratory syndrome – and bird flu outbreaks. The authorities knew of the first human cases on 24 June, but it only allowed the news out on 25 July. And China has banned local and foreign reporters from entering the region.

The secrecy has bred suspicion elsewhere. The WHO has said that it was baffled because S. suis has never affected so many people in an outbreak before – it usually just infected one or two people at a time. And where people have been infected previously, mortality rates have been below 10% and different symptoms have been exhibited. “It could be another disease altogether, it need not be Streptococcus suis because the presentation is so atypical,” Samson Wong, a microbiologist at the University of Hong Kong, told Reuters. “In past literature, there have been one or two cases when people died within 36 hours, but those were exceptions rather than the rule. The deaths in China are very unusual.”

Wong also says many patients in Sichuan were bleeding under the skin, a symptom that has been cited in only two or three cases in medical literature on the infection – and that deafness, which is commonly found with the disease has been little mentioned in the outbreak. Experts quoted in other news reports have also said that the swine bacterium is an unlikely cause, with the symptoms, widespread geography of those affected and the speed of infection pointing to a viral infection. Other experts question China’s denial of human-to-human spread. “The organism is carried on the pig’s tonsils and is spread pig-to-pig through nose rubbing or coughing. But it’s only found in small concentration on the pigs’ tonsils, so it’s difficult for a human to catch it that way,” says Jill Thompson from the UK’s Veterinary Investigation Centre in Edinburgh.

“When the infection spreads to the brain, causing meningitis, it’s in far greater concentration and so it can be transmitted to humans who eat raw infected pork or handle the dead animal with open cuts,” she told New Scientist. “It is so rare for humans to become infected; most farm workers develop some immunity from the endemic disease. What might have happened is that the bacteria have acquired virulence factors from another organism – a bacterium or virus that might be harmless – and the combined virulence factors have turned it into a superbug, which could be transmitted human-to-human through coughing,” she suggests.

Andrew Rycroft, a microbiologist from the Royal Veterinary College in Hertfordshire, UK, agrees. “The likelihood is that once it gets established in the human respiratory tract, it can be transmitted by the respiratory tract between humans very much more quickly,” he says, citing an analogy between the bubonic plague, which soon became the far more virulent pneumonic plague when it was transmitted by the human respiratory system.

New Scientist
August 16, 2005

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