‘Symptomless’ bird flu cases raise concerns

The H5N1 bird flu virus might be acquiring a greater ability to spread from human to human, recent cases in Vietnam suggest. But as two elderly relatives of patients killed by the bird flu test positive for the virus and yet have no symptoms, there are also indications that it may not be as lethal as currently thought.

The 2004 outbreak of H5N1 in Vietnam stopped in spring after the country killed millions of infected and exposed poultry. But outbreaks resumed in December, probably because the virus persisted in ducks showing no symptoms, say flu experts. Since December, 22 people have tested positive for H5N1 in Vietnam, of whom 14 have died, including one woman from Cambodia. Five of the cases occurred in clusters that suggest the virus passed from person to person. In the most recent, a 14-year-old girl fell ill on 14 February, her 21-year-old brother on 21 February, and a 26-year-old male nurse who cared for the brother, on 26 February.

Spread of the virus to health care workers would be worrying, says leading flu expert Robert Webster at the St. Jude Children’s Research Hospital in Memphis, US. Attending a conference on microbial threats in Lyon, France, last week, he told New Scientist: “That’s where we’d expect to see the first cluster if this virus starts spreading among humans.” This is what happened in 2003 with the previously unknown respiratory virus SARS – Sudden Acute Respiratory Syndrome. The Vietnamese cases might have acquired the virus from poultry, especially from raw duck products eaten during the recent Tet New Year’s festival. But because they each developed symptoms several days apart, investigators from the World Health Organization suspect human to human transmission.

The investigation has uncovered other surprises. The WHO found antibodies to H5N1 in the 81-year-old grandfather of the brother and sister, meaning he has been infected but survived. And the healthy wife of a 69-year-old man who died from H5N1 on 24 February has also been found to have the antibodies. It is unclear if the people with antibodies were infected with H5N1 at the same time as their relatives, but did not get sick. This would suggest that many infections have been going undetected, as the healthy contacts of people who get H5N1 have not been widely tested. That in turn would mean that the virus might not be as lethal as it appears from the confirmed cases, of whom nearly three-quarters have died. But it also means that many more people than suspected have harboured H5N1, and each of them is an opportunity for the virus to adapt further to humans.

H5N1 flu is thought to have been circulating in Vietnamese poultry for several years, however. The people with antibodies might have fallen ill and recovered some time ago, before anyone tested for H5N1. Or low-level exposure to the virus in poultry might have immunised them without significant illness.

New Scientist
March 29, 2005

Original web page at New Scientist