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How a pandemic builds

April 10, 2006

While there is currently no strain of pandemic flu virus, worldwide outbreaks of deadly diseases are an inevitable part of civilization.

In the 20th century, there were three influenza pandemics. The worst was the Spanish influenza of 1918. That pandemic killed an estimated 40 million to 50 million people worldwide. Pandemic flu emerged again in 1957 and 1968.

It is impossible to know whether the deadly bird flu strain H5N1 will lead to the next human pandemic, say doctors and scientists. However, health and government officials are taking that possibility seriously.

Here's what would have to happen for bird flu to become a virus that can be transmitted from person to person, and to start a pandemic outbreak:

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Step 1: A powerful virus emerges.

This is the only step the three listed here that has happened in the case of bird flu, which is spreading quickly among both domestic and wild bird populations. Since 2003, almost 200 people have contracted bird flu after direct contact with infected poultry. All of those cases have occurred in the Middle East and Asia. At this point, doctors and scientists believe bird flu cannot be spread easily from one person to another.

Step 2: The virus mutates.

"It's in the nature of flu viruses to adapt. They do minor drifts all the time. What we're worried about is a major shift," says Dr. Robin McFee, an expert on emerging infectious diseases and emergency preparedness.

Scientists are closely studying the bird flu strain H5N1, looking for signs of it mutating into a form that could be transmitted between humans.

Viruses can mutate slowly over time, or they can mutate within a host, such as a human or a pig. If a person who already has a human strain of influenza contracts bird flu, there's a potential for the flu strains to mix and mutate.

The same goes for swine. Pigs can contract human flu, swine flu and bird flu. If a pig contracts a strain of both a human and a bird flu, that could be another opportunity for quick mutation, explains McFee, who lives in Massachusetts.

In a common host, viruses "can reassort like a LEGO block and come up with something novel that we haven't seen," she says.

Step 3: The virus spreads quickly.

The World Health Organization (WHO) has put together strategies to contain a mutated bird flu virus if one emerges. The organization is stockpiling antiviral drugs that would be used to help contain an emerging pandemic at its source, according to a document released by the WHO.

The success of containment strategies would depend on how and where the mutated flu would emerge. In the instance of a sudden explosion of cases in a populated area, there would be little hope to keep the virus from spreading, the WHO document states.

Mutated bird flu would not be like the influenza that circulates from year to year. The human immune system would have no immunity to a mutated bird virus. That means people who would contract the mutated bird flu would likely experience symptoms more severe and serious than those of regular flu.

The World Health Organization has estimated the death toll from a pandemic outbreak of mutated bird flu could be 2 million to 7.4 million people worldwide.

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