Sunday, April 26, 2009

Mexican virus could throw wrench into North America's pandemic planning

A woman and a child don masks as they walk outside the general hospital in Mexico City, Aprill 24, 2009. A deadly strain of swine flu has broken out in Mexico, killing at least 616 people and raising fears  it is spreading across Northe America.

Preparations based on assumption outbreak would surface in Asia, and take three months to arrive

By Ian MacLeod, The Ottawa Citizen

A woman and a child don masks as they walk outside the general hospital in Mexico City, Aprill 24, 2009. A deadly strain of swine flu has broken out in Mexico, killing at least 616 people and raising fears it is spreading across Northe America.

Photograph by: Henry Romero, Reuters

Based on the law of averages and the science of virology, experts agree another influenza pandemic is inevitable. The question is when.

International public health officials have warned for more than a decade that the world is due for a respiratory disease that will spread to every corner of the globe within months. Millions will become sick and untold numbers will slowly suffocate to death.

In Canada, it has been estimated one-third of the workforce could become ill. And without a ready vaccine, the federal government calculates 11,000 to 58,000 Canadians could die, depending on the virulence of the virus, the rate of infection and the availability of an effective vaccine.

But those dire warnings and grim numbers have been sounded so many times now, the alerts and headlines have taken on a Chicken Little quality, considered by many to be only slightly more credible than tips to the FBI on the whereabouts of Jimmy Hoffa's gravesite.

If the Mexican virus does, indeed, turn out to be a novel pandemic strain -- and that's still a very big if -- it would throw a wrench into emergency preparations.

Much of North America's pandemic planning is premised on the assumption that a killer virus would emerge in Asia, where all previous pandemic flu viruses have surfaced, and wouldn't arrive here for at least three months. That would allow time to begin preparing a vaccine and detailed plans to inoculate doctors, nurses, police officers, air-traffic controllers, other essential personnel and high-risk groups.

But if Mexico proves to be ground zero, that hoped-for head-start could be lost.

A flu pandemic occurs when a type A influenza virus, originated in birds but which can spread to pigs and then humans, undergoes a sudden and radical mutation called an antigenic shift, and acquires the ability to infect humans. Because people have never been previous exposed to such a germ, no one has immunity. The result is a worldwide epidemic, known as a pandemic.

An average of 25 years separated each pandemic of the 20th century. The range was 11 to 39 years. It has been 41 years since the last pandemic. Statistically speaking, we're overdue.

Like all flu viruses, the one now striking down Mexicans hijacks healthy human cells and uses them to rapidly reproduce new copies of itself. The host cells soon swell and then burst, spewing out armies of new pathogens to invade other healthy cells, particularly the epithelial cells that form the lining of the lungs.

The surface of an influenza virus is covered with hundreds of protruding spikes, each made from one of two proteins: Hemagglutinin (H) allows the virus to bind to the host cells it uses to reproduce itself; neuraminidase (N) helps the new pathogens to pour out of those cells and travel through the bloodstream.

When the virus invades, the immune system produces antibodies that target the hemagglutinin and neuraminidase, called antigens, of that particular strain. The antibodies kill the virus and prevent a repeat infection.

To survive, the virus must constantly replicate. But as it does, incremental mutations occur and over time, minor changes appear in the hemagglutinin and neuraminidase, a process called antigenic drift.

Antibodies on guard against the earlier antigens become less and less effective at recognizing the newer invaders, so that a portion of the world's population is always susceptible to some degree of infection. The result is the annual flu season.

As many as 100 million people in the Northern Hemisphere are infected each year with one of three types of flu virus, usually Type A and Type B, both capable of causing serious disease. Type C influenza is of little public health concern.

Once every generation or so, a type A influenza virus undergoes a sudden, radical mutation. An antigenic shift. Without warning, a novel subtype emerges with a combination of hemagglutinin and neuraminidase to which humans have never been exposed and have no immunity. The result is a pandemic.

The first detailed record of an influenza pandemic was in 1580. Since then, three to four have swept the planet each century.

The Spanish flu pandemic of 1918-19 was triggered when a virus to which humans had never been exposed -- called A/H1N1 and a distant relative of the virus now raging in Mexico -- crossed from a bird or pig to a human and then blazed around the globe, killing 20 million to 40 million people, mostly from hemorrhagic viral pneumonia, many within a day or two of being infected. Others were felled by secondary infections such as bacterial pneumonia.

The virus slipped into Canada with the soldiers returning from the Great War in 1918. The virus claimed 440 victims in Ottawa that October. Almost 60 per cent of the victims were 18 to 35 years old. In all, more than 500 people died and thousands became gravely ill during the final four months of the year.

But not all pandemics are apocalyptic. In July 1957, Asian flu (H2N2) arrived in Canada, but the rate of infection didn't explode until the fall of that year, killing 7,000 Canadians. The last pandemic, the 1968 Hong Kong flu (H3N2), caused far more widespread illness than the Asian flu, but was less deadly, killing about 4,000 Canadians. In a normal year, upwards of 1,500 Canadians, mostly older, die from the flu. Hundreds to thousands of others die from flu-related complications.

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