Monday, October 1, 2012

Killers on the loose: the deadly viruses that threaten human survival

Could the next big animal-human disease wipe us out?

The Next Big One is a subject that disease scientists around the world often address. The most recent big one is Aids, of which the eventual total bigness cannot even be predicted – about 30 million deaths, 34 million living people infected, and with no end in sight. Fortunately, not every virus goes airborne from one host to another. If HIV-1 could, you and I might already be dead. If the rabies virus could, it would be the most horrific pathogen on the planet. The influenzas are well adapted for airborne transmission, which is why a new strain can circle the world within days. The Sars virus travels this route, too, or anyway by the respiratory droplets of sneezes and coughs – hanging in the air of a hotel corridor, moving through the cabin of an aeroplane – and that capacity, combined with its case fatality rate of almost 10%, is what made it so scary in 2003 to the people who understood it best.
A poultry farm in China A worker disinfects a poultry farm in China after an outbreak of the deadly H5N1 strain of bird flu in 2006. Photograph: China Photos/Getty Images
Human-to-human transmission is the crux. That capacity is what separates a bizarre, awful, localised, intermittent and mysterious disease (such as Ebola) from a global pandemic. Have you noticed the persistent, low-level buzz about avian influenza, the strain known as H5N1, among disease experts over the past 15 years? That's because avian flu worries them deeply, though it hasn't caused many human fatalities. Swine flu comes and goes periodically in the human population (as it came and went during 2009), sometimes causing a bad pandemic and sometimes (as in 2009) not so bad as expected; but avian flu resides in a different category of menacing possibility. It worries the flu scientists because they know that H5N1 influenza is extremely virulent in people, with a high lethality. As yet, there have been a relatively low number of cases, and it is poorly transmissible, so far, from human to human. It'll kill you if you catch it, very likely, but you're unlikely to catch it except by butchering an infected chicken. But if H5N1 mutates or reassembles itself in just the right way, if it adapts for human-to-human transmission, it could become the biggest and fastest killer disease since 1918.

It got to Egypt in 2006 and has been especially problematic for that country. As of August 2011, there were 151 confirmed cases, of which 52 were fatal. That represents more than a quarter of all the world's known human cases of bird flu since H5N1 emerged in 1997. But here's a critical fact: those unfortunate Egyptian patients all seem to have acquired the virus directly from birds. This indicates that the virus hasn't yet found an efficient way to pass from one person to another.

Two aspects of the situation are dangerous, according to biologist Robert Webster. The first is that Egypt, given its recent political upheavals, may be unable to staunch an outbreak of transmissible avian flu, if one occurs. His second concern is shared by influenza researchers and public health officials around the globe: with all that mutating, with all that contact between people and their infected birds, the virus could hit upon a genetic configuration making it highly transmissible among people.
"As long as H5N1 is out there in the world," Webster told me, "there is the possibility of disaster… There is the theoretical possibility that it can acquire the ability to transmit human-to-human." He paused. "And then God help us."

We're unique in the history of mammals. No other primate has ever weighed upon the planet to anything like the degree we do. In ecological terms, we are almost paradoxical: large-bodied and long-lived but grotesquely abundant. We are an outbreak.

And here's the thing about outbreaks: they end. In some cases they end after many years, in others they end rather soon. In some cases they end gradually, in others they end with a crash. In certain cases, they end and recur and end again. Populations of tent caterpillars, for example, seem to rise steeply and fall sharply on a cycle of anywhere from five to 11 years. The crash endings are dramatic, and for a long while they seemed mysterious. What could account for such sudden and recurrent collapses? One possible factor is infectious disease, and viruses in particular.

The dangers presented by zoonoses are real and severe, but the degree of uncertainties is also high. There's not a hope in hell, as Webster told me, of predicting the nature and timing of the next influenza pandemic. Too many factors vary randomly.

I have asked not just Webster, but many other eminent disease scientists the same two-part question: 1) will a new disease emerge, in the near future, sufficiently virulent and transmissible to cause a pandemic on the scale of Aids or the 1918 flu, killing tens of millions of people?; and 2) if so, what does it look like and whence does it come? Their answers to the first part have ranged from maybe to probably. Their answers to the second have focused on various viruses prone to mutation, especially those for which the reservoir host is some kind of primate.
But the difficulty of predicting precisely doesn't oblige us to remain blind, unprepared and fatalistic. We can at least be vigilant; we can be well prepared and quick to respond. The scientists are on alert. They are our sentries. But we, too, should understand in some measure the basic outlines and dynamics of the situation. We should appreciate that these recent outbreaks of new diseases, as well as the recurrence and spread of old ones, are part of a larger pattern, and that humanity is responsible for generating that pattern. We should recognise that they reflect things that we're doing, not just things that are happening to us.

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