An Influenza Arms Race - Are Humans Birds of a Feather?

In 1997, news of an influenza outbreak in Hong Kong would have been unremarkable. After all, the flu has been identified as a human disease at least since Galileo's time. What made the 1997 Hong Kong flu outbreak remarkable was that it was the first known direct transmission of flu from birds to humans. Eighteen people were hospitalized, and six of those people died.

This winter, three different episodes of transmission from birds directly to humans may have occurred - two in southeast Asia and one in the Netherlands. What's happening?

Can we get influenza from chickens and eggs? Is direct transmission from birds to humans becoming common? How does it happen in the first place?

As we all quickly learn as children, influenza A - the flu - is a common respiratory illness in people, caused by viruses. Most of us who get the flu wind up sacked out on the couch for a week, feverish, achy, and congested, but we recover fully. If we're not allergic to eggs, we can even get vaccinated for the flu every fall and early winter. Most cases of human flu are from a subtype of the virus called influenza A. Influenza A viruses, like other viruses, are nothing but tiny little packages of genetic material with protective protein shells. Lacking the ability to make more copies of themselves on their own, viruses seek other cells to do their work of replication. When we catch the flu, the influenza A virus has already latched on to our cells and injected its tiny package of genetic material into the larger cell, telling the larger cell to make more copies of the virus. Our immune systems fight this invasion by recognizing the protein coating on the invading viruses, breaking open the virus, and destroying its little package of replication instructions.

Influenza A viruses can infect not only people and birds, but pigs, horses, seals, and whales too. Not all of the different kinds of influenza A viruses infect all animals, but all of them circulate among wild birds. That's why influenza A is often called avian influenza or bird flu. Most wild birds carry low levels of many types of influenza A with few or no ill effects. When influenza A gets into domesticated bird populations, it can make those birds very sick. Unlike their wild cousins, domestic chickens, ducks, and turkeys can suffer high mortality. Even when there are outbreaks of domestic avian influenza, people rarely catch influenza from them. The virus subtypes that make domestic birds sick just don't seem to infect or circulate among humans or other animals-at least until 1997.

That year, about a hundred people caught influenza A from chickens in Hong Kong. The subtype of flu those people caught was previously unknown to affect humans, and it was the first known episode of direct transmission from birds to people. In an effort to control the spread, 1.5 million chickens in southeast Asia were killed. In late 2003, the same subtype of flu virus infected 33 people in southeast Asia, 22 of whom died. A different subtype of avian influenza, also previously unknown to affect humans, sickened 80 people in the Netherlands. Why is this happening?

It's the evolutionary equivalent of an arms race. As soon as an animal's immune system can recognize a virus's protein shell coating, the virus's ability to replicate itself plummets. But the virus's genetic material, DNA or RNA, can have random mutations that change its protein shell. Most mutations are not helpful, but every once in a while, one that benefits the virus can appear. When that happens, the immune systems of infected animals don't recognize the "new" virus. Those strains can spread fast in humans, causing pandemics like the 1918 Spanish flu, 1957 Asian flu, and 1968 Hong Kong flu. There's no good way to anticipate outbreaks and pandemics, but the World Health Organization and the Centers for Disease Control monitor outbreaks closely.

So what does this mean for those of us who consume chickens, turkeys, and eggs? First, the old advice of careful preparation applies. Wash your hands and non-porous preparation surfaces before and after handling uncooked chicken and turkey. Cook thoroughly, to 180 degrees Fahrenheit or until juices run clear. Cook eggs thoroughly and don't eat raw, unpasteurized eggs. Refrigerate leftovers promptly. Mom was right on about that.

As consumers, we can also be choosy about how our chickens, turkeys, and eggs are produced. Truly free-range birds and laying hens typically suffer fewer outbreaks of sickening influenza because the density of their living groups is lower. So it's harder for the virus - and other pathogens like Salmonella - to spread. The United States doesn't import many fowl from southeast Asia, and when we do, those eggs or birds are always quarantined and tested for avian influenza. In this country, the CDC and USDA are responsible for monitoring avian influenza. When there's an outbreak in chickens or turkeys, those birds are quarantined and monitored to see whether their strain of influenza can infect people.

But remember - birds always carry many forms of the influenza A, and it's impossible to halt evolution of new, virulent strains in the long run. So get a flu shot (if you can) in the fall or early winter each year. WHO and CDC have developed sophisticated models of which strains are expected to spread that year, and vaccines are cultured and manufactured to combat those strains. You can also keep up on the outbreak and spread of avian influenza by checking the WHO's and CDC's web sites, especially if you need to travel to areas where human infection is an issue.