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.
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."
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.
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.
Link to complete article: http://www.guardian.co.uk/society/2012/sep/28/deadly-viruses-ebola-marburg-sars?newsfeed=true