Bird Flu Targeting the Young

Bird flu targeting the young

As death toll nears 100, scientists scramble to explain why H5N1 virus is killing healthy people under 40

Mar. 11, 2006. 07:57 AM

RITA DALY

STAFF REPORTER

With the World Health Organization set to announce the 100th death from bird flu any day now, data compiled by the Toronto Star lead to one particularly compelling question: Why does the H5N1 virus attack the young?

The Star's analysis shows that all but six of the 97 people who have died globally so far from bird flu were under 40.

People, in other words, with the strongest immune systems and not, as one might expect, the elderly and those already sick. The median age was 19, and a quarter of them were under age 12.

Children, teenagers and young adults are the unfortunate victims of the deadly H5N1 bird flu sweeping through poultry farms in Asia, Africa and now Europe.

Hooked up to breathing tubes and dialysis machines in local hospital beds, bodies soaked in sweat, and blood oozing from their nostrils and mouth, they have a mere 50 per cent chance of pulling through. The rest die in a matter of days.

Any day now the World Health Organization will announce the 100th death from the bird flu that re-emerged in late 2003.

Yesterday, health officials confirmed a 4-year-old Indonesian boy died last month, bringing the number of confirmed cases to 176 and the world death toll to 97. Another three deaths in Azerbaijan are under investigation.

Although human cases are uncommon, it is now apparent the H5N1 will eventually reach North American shores, possibly via migratory birds in Alaska within six to 12 months. So what health experts know about how and whom it strikes is crucial.

So far, they know nearly everyone who died of the respiratory disease was in close contact with infected domestic birds, and most were young and previously healthy. Yet scientists still aren't sure why they fell ill, while others equally exposed to H5N1-infected chickens and ducks were spared.

"There are still a lot of unanswered questions and that's one of them," Sonja Olsen, an epidemiologist for the Atlanta-based U.S. Centers for Disease Control, said in an interview from Thailand where she is studying human cases of the H5N1.

There are other unanswered questions, like why is it some family members become infected and others not? Why aren't health-care workers in hospitals or unprotected agricultural workers slaughtering chickens also getting sick?

Originally surfacing in Hong Kong in 1997, killing six, then again in early 2003, killing one, the bird flu re-emerged later with a vengeance, decimating poultry stocks and infecting more people in areas of Southeast Asia.

The 97 deaths in the third wave are now spread across seven countries — Vietnam, Cambodia, Indonesia, China, Thailand, Turkey and Iraq. The infection rate is already double this year over last, with more than three human cases a week as infected birds spread the virus further afield.

"A lot of the human cases of bird flu have occurred in people under 25 and we're still not exactly sure why that is," said the WHO's Maria Cheng.

"They may have different behaviour patterns, they may be exposed to the virus in closer ways, they may be more susceptible to it. But there's such a small number of cases that it's difficult to draw any conclusions about how it's transmitted in target populations."

WHO officials stress the number of deaths from H5N1 bird flu is extremely low compared to the 250,000 to 500,000 who die annually from seasonal human flu, or the nearly 800 people who died during the SARS epidemic in 2003, 44 of them in Toronto. But health experts also warn no bird flu has ever sickened and killed so many people as H5N1.

The virus has only a limited ability to infect people, but experts fear it could mutate and spread easily among humans, sparking a pandemic that could kill millions within months.

Canadian officials have devised an emergency plan in the event of a pandemic, but say as long as H5N1 remains bird flu there is little cause for alarm in a country where people and poultry live separate lives.

In the past century we've had three human flu pandemics: in 1918, 1957 and 1968. The most lethal — the 1918 Spanish flu — also targeted the young and healthy, killing 20 million to 50 million worldwide.

In a study published in the online medical journal Respiratory Research in November, Hong Kong scientists noted the H5N1 was creating what's called a "cytokine storm" in its healthy victims, causing their immune system to overreact to the virus, flood the lungs with an overabundance of antibodies and cause extensive lung damage, eventually shutting them down. It's the same response scientists believe caused so many deaths during the Spanish flu.

The H5N1 virus has already earned the notorious reputation of being the worst flu in birds. An unprecedented 200 million have died or been slaughtered. It is so highly pathogenic, infected chickens drop dead in 48 hours. This month, the virus showed up in several domestic cats and a weasel-like animal called a stone marten in Germany and Austria, creating fear in the European Union that it might easily be infecting other species.

Earl Brown, an avian flu expert and professor of immunology and virology at the University of Ottawa, said the behaviour of small children playing among infected fowl could logically account for the high infection rate among the young.

A recent news report saw 15 Iraqi children running through an area where thousands of culled chickens were dumped, tying them to sticks and waving them in the air.

A 14-year-old Turkish boy and his two sisters, 15 and 11, died in January after playing with the head of an infected chicken the family slaughtered and ate. And an 8-year-old Turkish girl died after kissing and hugging her dying pet chicken.

But each person's immunity, even genetic factors, may also play a part in determining who falls ill and dies, Brown said.

"On average, you'll get influenza once every five or 10 years, so kids are less likely to have antibodies from prior exposure," he said. "Adults will have had experiences with different influenza viruses."

It still doesn't explain the disproportionate number of people in their 20s and 30s who have succumbed to the disease. One theory is that some people have immunity to the N1 antigen of the bird flu virus developed from the H1N1 Spanish flu. That virus was still circulating in a milder form until 1957 and also re-emerged as a milder strain in 1977.

Each time there is a suspected case, WHO officials quickly send a team of field experts to investigate.

Swab samples are sent to the organization's reference labs for further tests and to determine whether the virus has changed genetically in a way that might allow it to transmit more easily between people.

So far, they have found most confirmed cases involve people with backyard poultry farms who had close contact with infected or dying birds — in some cases slaughtering, defeathering and preparing them for dinner.

"When the chickens get sick and die, they get plucked, eviscerated and put into a pot, so maybe it's the mother and kids that are exposed at this point," Brown said.

Virologists know infection occurs through contact with blood, feces and other body fluids, and WHO officials recently reiterated the flu virus is also airborne, posing even a greater threat than AIDS.

If the virus were to start spreading easily among people, the first warning signal of a possible pandemic will be more and more clusters of people getting sick.

The CDC's Olsen and a team of researchers looked for this while examining 15 family clusters of infected cases in Vietnam, Thailand, Cambodia and Indonesia between Jan. 2004 and July 2005 — a mother and child; siblings; cousins; a niece and aunt; a teenager, her older brother and grandfather.

They found no increase over time.

But Olsen said most had not been investigated thoroughly enough to say for certain there was no person-to-person transmission.

"There was only one where you could clearly say there was person-to-person transmission and the others left you sort of wondering," she said.

WHO officials said this week there are three confirmed cases of suspected person-to-person transmission:

In January 2004, Ngo Le Hung, a 31-year-old Vietnamese schoolteacher, became infected and died from a chicken he bought for his wedding, and his two sisters also died.

In September 2004 a dying 11-year-old Sakuntala Premphasri infected her mother Pranee, 26, in Thailand and both died. And in July 2005 a 38-year-old father is believed to have infected his two daughters, aged one and eight — all three died.

Cheng said there may be other cases in which people became infected through human-to-human transmission, but there isn't enough evidence to prove it. There may also be many less severely ill people going unnoticed.

"But we haven't seen any substantial change in the virus and that is really the trigger we're watching for."