After the fact discoveries of bird flu cases

Reports of missed mild bird flu cases raise questions about scope of spread
17:54:50 EDT Sep 17, 2006
Canadian Press: HELEN BRANSWELL

(CP) - Recent reports from South Korea and Indonesia of after-the-fact discovery of a handful of mild human cases of H5N1 avian flu have again raised questions about whether the disease's extraordinarily high death rate is being inflated because mild cases are being missed.

Experts say the evidence to date points away from that notion. But they add that it is important to continue to search for mild cases. Understanding the true number of human infections and the range of symptoms experienced could help scientists better assess the pandemic risk posed by the virus.

As well, tracking mild cases over time could provide an early warning if important changes to H5N1 occur, they suggest. Climbing rates of mild cases might signal the virus was adapting to become a human flu strain, moving closer to triggering a flu pandemic.

"We need to keep monitoring it," Dr. Michael Osterholm, director of the University of Minnesota's Center for Infectious Diseases Research and Policy, said Sunday.

"Because frankly, one of the indications that there may be a changing epidemiology (disease pattern) with this is in fact if we start seeing larger and larger percentages of individuals who are asymptomatic or only mildly ill that we can clearly confirm as having H5N1 infection."

The World Health Organization announced last week blood tests done on an Indonesian man showed he became infected, likely in May when he helped nurse his sister, a confirmed H5N1 case. The man reported having a cough and abdominal discomfort but his symptoms weren't diagnosed as H5N1-related at the time.

And South Korean officials revealed that testing of blood samples from more than 2,000 workers who culled diseased poultry in late 2003 and early 2004 showed five had developed antibodies to the virus - a sign they had been infected. An earlier round of testing in South Korea turned up four cullers with antibodies.

None of these people had serious illness at the time; none has yet been added to the WHO's official case list.

To the best of the scientific community's knowledge, the H5N1 virus is not adept at infecting people. But when it does, it appears to cause severe disease. Nearly 60 per cent of the 246 confirmed cases have died; many of the survivors battled for their lives.

But there has always been a suspicion in some quarters that a significant number of mild cases are being missed - a theory these new reports may fuel.

Many - though not all - diseases cause a spectrum of illness ranging from undetectable to life-threatening infection. With polio, for instance, it is estimated there are 200 inapparent infections for every person the virus paralyzes. On the other hand, rabies is almost 100 per cent fatal.

If the confirmed H5N1 cases were the proverbial tip of the iceberg, the death rate attributed to the virus would tumble. That in turn might ease worries about the threat posed by H5N1 - though a flu virus that killed even 10 or 20 per cent of its victims would still be a source of serious concern.

Dr. Angus Nicoll, an influenza expert with the European Centre for Disease Control, said these new asymptomatic cases don't change the fact that the bulk of evidence to date suggests such cases are rare.

A number of small studies have been done to test the blood of people who've been in contact with confirmed cases. They have turned up few or no cases that missed earlier detection.

The most recent study - published in the October issue of Emerging Infectious Diseases - looked at blood samples from 351 Cambodians who had been in contact with a man who died from the disease. None developed antibodies to the virus.

"It's still a plausible hypothesis but it's becoming a less likely one," Nicoll said of the notion that there may be a significant number of asymptomatic and mild cases of the disease.

"My bottom line would be: I'm sure we're going to find the occasional mild case. I'm sure we're going to find, eventually, an asymptomatic (case)," he said from Sweden.

"Even if you find just a few asymptomatic and mild cases, that's not going to change the our risk assessment."


© The Canadian Press, 2006