CDC to U.S. hospitals: Be wary of Asian avian influenza threat

Flu has pandemic potential, high mortality

If infectious disease specialists used a color-coded alert system, the color would be on yellow, for elevated. And it’s edging up to orange. While the threat of severe acute respiratory syndrome (SARS) has subsided, public health experts are becoming increasingly concerned about the potential for pandemic influenza coming from a highly pathogenic avian influenza strain in Asia.

Hospitals need to be as vigilant as ever for cases of pneumonia among travelers from Asia, say epidemiologists from the Centers for Disease Control and Prevention (CDC). They should consider avian influenza as a differential diagnosis in patients with severe respiratory illness who have traveled to Asia.

The highly pathogenic influenza strain H5N1 remains widespread in domestic poultry in six Asian countries. China, Vietnam, Thailand, and Indonesia have reported increased outbreaks among poultry. And according to a report in the journal Nature, the avian flu strain H5N1 is endemic in domestic ducks in southern China and in wild birds throughout Asia, making it difficult, if not impossible, to contain the disease.1

"This has been an unprecedented outbreak of H5N1 among poultry in Asia," says Tim Uyeki, MD, medical epidemiologist in CDC’s influenza branch.

Even more troubling, this strain is associated with extremely high mortality for people who become infected. Of 34 confirmed human cases, 23 people have died. "That’s a case fatality proportion of 68%. All these human cases were hospitalized," says Uyeki, although he adds that more mild cases may have occurred without being identified as avian flu. Most of the hospitalized cases occurred among children and young adults.

In some of the countries, it almost is impossible to control the avian influenza by culling or killing infected poultry because of the prevalence of backyard farms, he explains.

International public health authorities are facing the possibility that this avian influenza strain will remain a long-term threat.

"There has been no conclusive evidence of efficient human-to-human transmission. That’s what we worry about. That’s what we fear," Uyeki says. "If there is efficient human-to-human transmission of H5N1 viruses, then it will result in a global influenza pandemic.

"If there is a global influenza pandemic, it will dwarf SARS," he adds. "It will make SARS look like it was really a minor problem. If this continued to cause this [68%] mortality, it would be one of the most devastating infections ever to be introduced into the human populations," Uyeki says. "We have no way to know whether a 68% case mortality proportion is going to apply to a larger population or not. We hope not."

If avian influenza becomes transmissible among humans, its spread would be rapid and community-based, says CDC medical epidemiologist L. Clifford McDonald, MD. Although SARS was most contagious when people were very ill (and often hospitalized), influenza is contagious very early in the illness, he notes. "It’s generally understood that if influenza took off as a pandemic, the battle to contain it wouldn’t be primarily waged in the hospital, like SARS was," he says.

Yet the protection of health care workers will be crucial both to avoid nosocomial spread and to maintain a healthy work force that can care for infected patients. Even in last year’s influenza season, a relatively moderate one, about a third of hospitals reported staff shortages due to influenza.

The public health concern is heightened by past experience with avian influenza. Pandemics in 1957 and 1968 occurred when other avian strains re-assorted and began infecting humans, who had no immunity. Avian flu caused about 70,000 U.S. deaths in 1957-58 and 34,000 U.S. deaths in 1968-69.

In 1997, public health authorities averted spread of H5N1 avian influenza by killing a million chickens in Hong Kong; 18 human cases were linked to the strain. In a retrospective study, researchers found evidence of person-to-person transmission of avian influenza to health care workers caring for the hospitalized patients.2

These findings give greater weight to the efforts to immunize health care workers against the known, circulating influenza strains. (A vaccine for H5N1 avian influenza is under development.) One nightmare scenario would be for a health care worker to become infected with both avian influenza and a human strain, allowing the virus to re-assort and become more easily transmissible, McDonald says.

Even without dual infections, "each human infection [with avian influenza] is an opportunity for the flu to adapt further," he notes. "If there is an avian flu-infected patient, we want to make sure health care workers are vaccinated against the human virus."

Hospitals should focus on preparedness for pandemic influenza — and awareness of this avian influenza threat, McDonald says. "We need to use what we learned from SARS and rechannel [the efforts] to what is now the present threat, the major threat. It’s going to stay with us."

References

1. Li KS, Guan Y, Wang J, et al. Genesis of a highly pathogenic and potentially pandemic H5N1 influenza virus in eastern Asia. Nature 2004; 430:209-213.

2. Bridges CB, Katz JM, Seto WH, et al. Risk of influenza A (H5N1) infection among health care workers exposed to patients with influenza A (H5N1), Hong Kong. J Infect Dis 2000; 181:344-348.