Two young brothers in Turkey both became infected with the H5N1 strain of the bird flu virus but show no signs of illness, according to Turkish health officials.
Some news reports have suggested that the absence of any symptoms means avian flu may be less harmful than currently thought, but several U.S. doctors said this is not a surprising development and does not reduce the threat of a pandemic.
What it does mean: The absence of illness in the brothers, who are 4 and 5 years old, suggests that the H5N1 virus may not affect all people equally.
"The severe cases thus far observed in hospitals in Southeast Asia and Turkey may in fact represent one end of a clinical spectrum in which many other cases are either mild or asymptomatic, not requiring hospitalization," said Dr. Pascal Imperato, chair of the department of preventive medicine and community health at State University New York Downstate Medical Center.
In rural areas, for example, people may be getting mildly to moderately sick from bird flu but then recover from what they believe is the ordinary flu or common cold without seeking medical attention.
This would lead to skewed statistics, since many people might be infected but not being counted in the overall number of cases.
This is not unusual for infections diseases, said Dr. Marvin Bittner, associate professor at Creighton University Medical Center in Omaha. "In medicine, as a general rule, the first cases of disease to be discovered are the most severe. As time goes on, doctors learn to diagnose cases that are milder," Bittner said.
On the surface, a milder form of H5N1 may seem encouraging, but avian flu experts say these findings do not lessen the threat of a pandemic.
Currently, the mortality rate for H5N1 flu is approximately 50 percent, meaning the virus kills about half of the people it infects. But if that number were to drop -- even by a lot -- it could still signal a major health crisis, said Dr. Frank James, at the University of Washington School of Public Health.
"In a global pandemic where [25 to 30 percent of the population] becomes ill ... even a mortality rate of 0.1 percent would be a very large number of deaths," he said.
Scientists are just beginning to understand the H5N1 strain of the avian flu virus, and most of the testing has been done on people who already show symptoms. Many believe that the best way to measure the true threat of the virus would be to take blood samples from a large portion of the population to see what percentage of people test positive for antibodies to the virus.
There were hundreds of cases of less severe infections in Hong Kong in 1997, said Dr. Marc Siegel, an associate professor at New York University Medical Center.
"Since then, [widespread blood testing] has not been routinely done, but when it has, it has continued to show exposure without illness," said Siegel, author of the forthcoming book, "Bird Flu: Everything You Need to Know About the Next Pandemic."
It is still unclear whether a recent mutation in the virus has resulted in a less virulent strain, or if the use of Tamiflu in these two young boys, who are being closely watched at Kecioren Hospital in Turkey, is affecting their infections.
Only time will tell, said Dr. Guenael Rodier, the chief World Health Organization investigator in Turkey.
"It's too early to say that the two young brothers will not develop symptoms at a later date," he said. "What is significant about the two is that they were detected early, earlier than any previous human case, so now doctors can watch the infection develop from the very beginning."
There were three new human cases of bird flu in Turkey today, bringing the total number of cases to 18.
Dr. Rajesh V. Swaminathan is a resident at Massachusetts General Hospital.