February 24, 2012
Bird Flu More Common Than Previously Believed
It was assumed that H5N1 bird flu is a rare disease that kills roughly 59 percent of the people it infects, but a new US study published in the journal Science suggests it may be more common and less deadly than previously believed.
The research could help alleviate concerns of a worldwide flu pandemic that could kill millions and millions of people, sparked by the recent lab creation of a mutant bird flu strain that can pass easily between animals.
The World Health Organization (WHO) reported 586 human cases of bird flu since 2003, with 346 of those individuals dying -- 58.6 percent. But researchers at Mount Sinai School of Medicine believe those estimates are misleading.
Virologists at Mount Sinai said they had found signs of infection in 1-2 percent of blood samples they had screened from more than 12,000 people in Asia, Europe and Africa, but most people did not seek or need treatment. And none of the screened persons died from their infections.
Their findings, which used data from 20 previously published studies, suggest that many more people have been infected with H5N1 than the 586 officially confirmed by the WHO; and if so, the fatality rate could be exponentially lower than the global health agency℠s report.
“The World Health Organization criteria that are currently being used for confirmation of H5N1 infections are good for the identification of very severe cases, but they do not pick up the cases that are mild or asymptomatic” because such patients are unlikely to seek medical treatment, said postdoctoral researcher Taia Wang, who led the study.
While there is no true scientific consensus about the actual fatalities stemming from H5N1, flu experts have speculated in the past that the WHO´s surveillance methods are flawed and misses an unknown number of cases.
WHO spokesman Gregory Hartl acknowledged today that the agency´s criteria for confirming bird flu cases may underreport mild cases. But he said it was more likely that officials were missing cases of bird flu deaths.
Peter Palese, chairman of the department of microbiology at the Mount Sinai School of Medicine in New York, and one of the study leaders, said the WHO´s mortality rate may be misleading because the agency only reports cases of people who are sick enough to go to the hospital and are lab-tested for the virus.
To be counted by the WHO, a person must have an acute illness and fever within a week and test positive for exposure to the H5 protein that gives the virus part of its name. Anyone sick enough to do that is more likely to die to begin with, noted Palese.
And in countries where avian flu is present, access to healthcare and hospitals is spotty at best, he added. Basically, there could be a lot more people out there who get the virus and either don´t show symptoms or don´t feel bad enough to see a doctor.
In the study, Palese, Wang and colleagues found that in each of the 20 previous studies on bird flu, infection ranged from 0 to 11.7 percent, though the higher figure came from people living in close quarters with those who were infected. The overall infection rate across the study was 1.2 percent.
The big question is how that translates to the rest of the population. Even a two percent infection rate is a lot of people in a group of millions. But if the WHO is only counting those who go to the hospital, those who have the most severe symptoms of the disease, it is likely that the number of people with the virus is higher, and the death rate would be much lower, the researchers speculate.
But even so, that doesn´t mean that H5N1 is benign. But it does mean that until someone studies whole populations and checks how many people with the virus show fewer severe symptoms, it is difficult to say exactly how dangerous bird flu is.
“We suggest that further investigation, on a large scale and by a standardized approach, is warranted to better estimate the total number of H5N1 infections that have occurred in humans,” the authors wrote.
But not everyone was enthused with the study findings.
Michael Osterholm, director of the Center for Infectious Disease Research and Policy, which studies threats of bioterrorism, says there are flaws in the methods used.
For instance, one of the studies included in the analysis looked at the 1997 bird flu outbreak in Hong Kong, which, Osterholm said, raises the number of people who were seropositive. “The virus was a bit different,” he noted.
In a press release from the American Society of Microbiology, Osterholm said in Hong Kong, the virus was H1N1, which is also influenza but genetically different from H5N1.
“Palese´s paper just confuses the issue because of the Hong Kong experience,” Osterholm told LiveScience, adding that only more recent studies, of a virus more similar to that plaguing humans today, should be used. In fact, doing so would reveal that 0.5 percent of the participants were seropositive.
Osterholm published a study today in the journal mBio showing that the virus might be even more deadly than the current mortality rate shows. Taking an average of the studies Palese used is therefore misleading, according to Osterholm. “If you put your head in the freezer and your feet in the oven, of course the average temperature will be just right,” he said.
“You could reduce the seriousness of this virus twenty-fold and it would still exceed that of the 1918 Spanish flu,” said Osterholm.
The 1918 H1N1 flu outbreak had a fatality rate of about 2 percent. But it killed as many as 50 million people worldwide because it spread easily between people.
Flu experts have been studying H5N1 to see if it has the potential to become more contagious in people, and researchers in the Netherlands and the United States have created strains in the lab that can pass easily between lab ferrets, which react to influenza much the same as humans do.
In December, however, fears of an H5N1 pandemic led a US government insecurity advisory panel to seek restrictions on the publication of that research. That request angered scientists who said they needed to share their findings in order to develop vaccines and prepare for a possible worldwide pandemic. But the panel, of which Osterholm is a member, said the data could be dangerous if it fell into the wrong hands.
The issue is still under debate and researchers said they will delay publication of their findings while the matter is sorted out. An international group of experts meeting at WHO headquarters in Geneva last week decided that the studies should eventually be published, but further risk assessment is needed before that can happen.
On the Net:
- World Health Organization (WHO)
- Mount Sinai School of Medicine
- Center for Infectious Disease Research and Policy
- American Society of Microbiology