Scientists have been hoping for a while now that they would uncover evidence that the H5N1 avian influenza virus is less virulent than has been thought, and that mild human cases of the illness have been going undetected in countries where the virus has been passed from birds to humans. That appears not to be the case, according to a report coming out of Cambodia:
New data from Cambodia presented at an international conference Monday, March 20, 2006, suggest mild or symptomless human cases of H5N1 avian flu may not be occurring. Many have hoped that the world is overestimating the virulence of the H5N1 virus, based on an assumption the disease surveillance systems in affected countries might not be sensitive enough to pick up mild infections.
But if the work presented at the International Conference on Emerging Infectious Diseases is borne out by additional larger studies, it would suggest the current case fatality rate of roughly 54 percent might indeed be close to reality.
Lead researcher Dr. Philippe Buchy of the Institut Pasteur in Phnom Penh put a positive spin on his findings. “In our experience, it seems that the virus still has difficulty jumping from avian to humans,” he said in an interview.
But the head of the influenza branch at the U.S. Centers for Disease Control said the findings have both positive and negative implications. “You can look at it in the way that it’s encouraging, because it means that the virus is not jumping to humans very readily. And that of course is very important, and that’s a positive thing,” said Dr. Nancy Cox.
“But on the other side of the coin, it means that the case fatality rate is still very high. And that is a negative thing.” Cox said it will be important to do this type of work on a larger scale, to get a more definitive picture of whether mild or asymptomatic H5N1 human cases are occurring. She said some larger studies are planned.
Buchy and his colleagues reported on work they conducted last spring in a Cambodian village that had H5N1 outbreaks in poultry and where one of that country’s four recorded cases lived. (All four Cambodians known to have been infected with the virus died.) The researchers tested blood samples from 351 of the villagers looking for the antibodies to the virus that would be proof of mild or asymptomatic infections. They found no signs of additional infections, this despite the fact that many of the villagers had significant exposure to infected poultry. Even a doctor who was involved in a difficult process to insert a ventilator tube into the windpipe of an H5N1 patient did not became infected, said Buchy, who noted the doctor wore no protective gear, because he didn’t realize what his patient was suffering from.“We didn’t find any case of H5N1,” Buchy said in an interview. “So nobody seems to have been asymptomatic or with mild symptoms during this outbreak
in Cambodia.” “It was also true with several other doctors and nurses, and none of them were positive.” Buchy said blood samples taken from two veterinarians who had autopsied birds that had died from H5N1 showed no sign of antibodies to the virus either. Like the doctor, the veterinarians hadn’t taken any precautions; they hadn’t realized the birds had been killed by H5N1.After the first human cases of H5N1 were reported in Hong Kong in 1997, so-called serological surveys — checking blood samples for antibodies —
were done to see whether all the cases had come to light. At that time, a small number of people, mainly healthcare workers, were found to have developed antibodies without having become sick enough to be diagnosed with the disease. But the virus has changed a great deal since then, and the virulence of current circulating H5N1 viruses has risen. And experts have called for more of this work to be done to ensure that the world gets a better handle on what is actually happening with the virus. Cox suggested the Cambodian study is a start. “The work in Cambodia is extremely important, because it shows that we really aren’t missing that much,” she said.
