Key Point

The possibility of a pandemic of the avian flu H5N1 is real, but steps can be taken to ensure preparedness and minimize outbreaks.

MONTREAL—At the annual meeting of the American College of Chest Physicians, Ronald F. Grossman, MD, Chief of Medicine at Credit Valley Hospital in Mississauga, Ontario, presented an update on the possibility of an avian flu (H5N1) pandemic.¹ Should we be worried? “There is some reason for concern,” he said, “and we need to act sooner rather than later to prevent some of the consequences that may ensue if this organism becomes adapted to humans.”

As an example, Dr. Grossman used the well-known SARS outbreak. The organism that caused the SARS outbreak is an RNA virus (as is H5N1) that started out in animals but became transmissible to humans. However, transmission of the SARS virus is not nearly as profound as transmission of H5N1 could be. As small as it was, said Dr. Grossman, the SARS outbreak in Canada decreased the national gross domestic product by 1%. On a global scale, the financial impact of a pandemic could be devastating.

WHAT DO WE KNOW ABOUT AVIAN INFLUENZA?

Birds are the natural hosts of all influenza A subtypes. Influenza A accounts for more than 80% of influenza infections and has been associated with all pandemics. Subtypes B and C are not associated with pandemics and circulate only in humans.

“Influenza A lacks a proofreading mechanism during translation that allows small errors to occur, which are not corrected,” explained Dr. Grossman. “This is an adaptive mechanism that helps the virus evade immune system defenses and is also why influenza vaccines have to change annually.”

Wild and domestic birds are the natural hosts for all 15 hemagglutinin types present in influenza A, while humans are hosts for only the first three. The natural hosts come in close contact with another animal and the virus recombines and is then capable of attacking humans. Many other animals can be infected with the influenza virus, Dr. Grossman pointed out.

Under some circumstances, particularly with an intermediary host, an avian strain will mix with a human strain, thereby creating a brand new virus against which the human host has no natural defenses, Dr. Grossman explained. This is the setting in which a pandemic can occur.

To complicate matters, the clinical presentation of influenza is nonspecific, so it is hard to separate from other respiratory disorders. The period of communicability is three to five days in adults and up to seven days in children. “The problem is that you might be contagious yet have no symptoms,” Dr. Grossman observed. “Here, the issues of isolation and containment become very problematic.”

RECIPE FOR A PANDEMIC

Three influenza pandemics occurred in the 20th century: in 1918, the 1950s, and 1968-1969. Since then, there have been other viral strains that could have caused a pandemic but didn’t.

Will H5N1 become the next pandemic? The following are the prerequisites for the start of a pandemic:

• A novel virus subtype.

• A general population that has little or no immunity.

• Virus must be able to replicate in humans and cause serious disease.

• Virus must be efficiently transmitted from person to person.

“You need to have efficient transmission capable of causing community-acquired outbreaks,” Dr. Grossman said. “That has not occurred as of yet. So far, the virus is highly pathogenic, resulting in illness and death in both young and old.”

Why is this happening in Asia? The density of poultry per square kilometer is extremely high, answered Dr. Grossman. H5N1 will likely spread into Russia, Indonesia, Eastern Europe, and probably Western Europe, shortly. “If one bird gets sick, all of the birds get sick,” he stressed. “If you have a virus that can potentially attack humans, repeated exposure gives the virus more of an opportunity to adapt and become transmissible among humans.”

PREPARING FOR THE WORST

The H5N1 case fatality rate is approaching 50%. The first H5N1 outbreak was in Hong Kong in 1997-1998. To date, there have been 121 proven cases, and the mortality rate is 50%. Most cases have been the result of direct contact with sick birds. For comparison, the mortality rate during the 1918 influenza pandemic was about 2% to 3%, Dr. Grossman said. “In 1918, the time it took the influenza virus to get around the world was four months; now it’s four days. The number of estimated dead in 1918 was 20 million people. If a pandemic were to hit today, the death toll could reach 60 million.”

How do you plan for a pandemic? “If there’s a 35% attack rate, a pandemic in Ontario for a population of 12 million people would translate to 12,000 deaths, 52,000 hospitalizations, over four million ill but not hospitalized, and two million who become ill and need some kind of care,” Dr. Grossman illustrated. “These are overwhelming numbers for the health care system.” He added that in Canada, hospitals are coming up with individual plans to help alleviate any anticipated need for hospital beds.

Influenza vaccines can dramatically reduce illness, hospitalization for respiratory tract illness, and mortality. “On August 7, 2005, a human vaccine for H5N1 was developed, but we don’t know that it actually works,” Dr. Grossman said. “The existence of the vaccine is not a guarantee that a pandemic will be averted,” he warned.

The most commonly used antivirals all have side effects, and there is the possibility of resistance. In addition, Dr. Grossman pointed out, it is likely that at least 100 days of antiviral therapy will be needed per person in the event of a pandemic.

What do we need to combat an outbreak of avian influenza?

• Global cooperation.

• Expanded surveillance.

• Expanded capacity for tracking H5N1 around the world.

• Agreement on a research plan about the virus.

• Knowledge about the presence of the virus and the attack rate in humans.

And in the event of a pandemic:

• Broad surveillance and detection.

• Identification of clusters.

• Tracing of contacts.

• Targeted use of antivirals.

• Vaccine research and development.

“We must be ready to intervene,” Dr. Grossman maintained. “Human transmission of bird flu is predictable and probably manageable, and failure to take action would be a mistake of historic proportions.”

—Gale Jurasek

Reference
1. Grossman RF. Pandemic influenza: should we be worried? Presented at: annual meeting of the American College of Chest Physicians; October 31, 2005; Montreal, Quebec.

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