Bird flu hard to catch, but deadly, says new research

Bird flu remains extremely hard to catch, but strikes with deadly force in the unlucky few who become infected, new research suggests.

The good-news, bad-news message has frustrated scientists trying to understand the disease and head off a possible pandemic.

In one of the largest studies of its kind, scientists in Cambodia took blood samples from 351 people in a small village where one of the country’s six bird flu deaths traced to the H5N1 virus was confirmed.

They found no antibodies for H5N1 in any of the specimens, indicating nobody became infected, having either recovered after falling only mildly ill or displayed no symptoms whatsoever.

The results mirror similar studies conducted in Vietnam, Thailand and Indonesia – where the virus has shown up in birds and the people it has infected, but left few other clues.

While experts stress more research is needed to determine the disease’s true virulence, the research strengthens the World Health Organisation’s grim assessment that H5N1 kills about 56% of those infected.

The good news, doctors say, is that most people do not catch the virus in its current form. The bad news is that those who do are in grave danger.

“Clearly, we’re not seeing widespread classical influenza infections with this virus or we would have picked it up by now,” said Michael Perdue, a WHO virologist in Geneva who did not participate in the study.

“I think the case fatality rate probably is pretty significant given the data that we have.”

The authors said they expected to find bird flu antibodies in at least 2% of the people they sampled, and were baffled why only one 28-year-old farmer became infected and died in the March 2005 outbreak.

Dead birds were reported in the village in Cambodia’s southern Kampot province and the H5N1 virus was detected in poultry there. Many villagers surveyed said they had very close, daily contact with the birds – collecting dead or sick poultry, feeding them, cleaning up faeces, plucking and eating them.

“That supports data from all over the region suggesting that it’s actually very inefficient to transfer from birds to humans,” said Benjamin Coghlan, a WHO epidemiologist from Australia who participated in the study.

“So, what does (the transfer) require? Well, we’re not sure,” he said. “Certainly, this case in the village wasn’t doing anything unusual that everyone else in the village wasn’t doing.”

The study, conducted in March and June last year, has been submitted to The Lancet medical journal for consideration, and its results were presented at a recent medical conference in Atlanta, Georgia.

The findings dispute an earlier Swedish study suggesting many people likely become mildly infected by bird flu and recover quickly.

That survey – based solely on circumstantial evidence with no blood tests - involved more than 45,000 people in northern Vietnam. The researchers concluded that 650-750 flu-like illnesses could have been linked to contact with sick or dead birds.

Blood samples collected during January’s bird flu outbreaks in Turkey are still being analysed, said Perdue.

Some experts warn the numbers could be misleading, especially when there have been fewer than 200 cases confirmed by the WHO worldwide since the H5N1 virus began ravaging Asian poultry stocks in late 2003. Of that total, 109 people have died.

“In Cambodia, they’d only need to miss five cases that survived for the case fatality rate to halve,” said Peter Horby, an epidemiologist conducting research in Vietnam for Oxford University.

But he does not believe a large number of cases have gone undetected.

Lead author of the Cambodia study, Dr Sirenda Vong said the findings could be interpreted positively and negatively.

“It’s very reassuring because there’s very low transmission from avian to human, but how about the high case fatality rate?” said Vong, the head of the Pasteur Institute’s epidemiology and public health unit in Phnom Penh.

“A lot of people said: ‘Oh, we have a high case fatality rate because we’re not able to detect mild cases.’ But if we don’t have a lot of mild cases that means the case fatality rate may be right and very high – similar to Ebola,” he said.

Health experts fear the bird flu virus will mutate into a form easily spread among people, potentially sparking a pandemic. So far, most human cases have been linked to contact with infected birds.