Special guest writer: @influenza_bio
For the first time in human history, we are
watching diseases jump from animals to humans on a large scale. We've seen
diseases appear for the first time in humans before; that's not new. We've seen
HIV and several new strains of influenza emerge over the past century or so,
for example. What is new is that we
can now watch this process as it happens. We are able to watch animal diseases
trickle case by case into humans, and we wonder whether any of these diseases
might some day become human diseases. We wonder whether we might be watching
pandemics develop in real time.
A disease that jumps from a non-human
animal to a human (or the other way around) is called a "zoonotic"
disease or a "zoonosis." Individual cases are called
"zoonotic" cases. When a zoonotic disease is trying to make the jump
to us permanent, we call this disease an "emerging infectious
disease."
We have certainly been watching a lot of
zoonotic MERS coronavirus and bird flu (e.g., H7N9 and H5N1) cases develop in
people lately, along with Ebola virus cases. Zoonotic cases of other diseases,
including infections with various strains of bird and swine flu, occasionally
develop, as well, and are watched closely.
When the 2009 H1N1 flu pandemic started, we
had no clue much beforehand that it was on its way. We didn't even have surveillance data about
swine flu strains that were even particularly close to the strain that emerged
in us. A large animal flu surveillance gap blindsided us that year.
And we will undoubtedly be blindsided again
by other emerging infectious diseases that we won't even see coming, although
people are doing their best to see what's out there.
When an emerging infectious disease jumps
to humans, it can cause either a relatively local outbreak or a worldwide
outbreak, called a "pandemic." If a disease becomes a pandemic, that
just means that it's spreading worldwide; the word "pandemic" doesn't
imply anything about how bad the disease might or might not be. In some sense,
the worst case can be when a disease jumps to humans and becomes
"endemic" in humans, meaning that it gets established in people and
regularly infects people, year after year. Endemic diseases can circulate
worldwide (e.g., influenza) or in more restricted geographical regions (e.g., malaria).
Our knowledge and resources have grown to
the extent that we are currently able to monitor some significant zoonotic
outbreaks of disease. We are currently watching the MERS coronavirus and the
influenza A(H7N9) virus both try to become human viruses.
Will either one succeed? We can't say.
We've never watched this process happen before. We don't know how long such a
process "usually" takes, or whether there even is a "usual"
amount of time that it takes. We don't know how long it might take, or how
quickly it has happened before. We do
know that the process is "stochastic," meaning that it involves a lot
of chance. A pathogen that in one situation might cause a pandemic might just die
out in another situation. Everything depends on the specific changes in a
pathogen that get a chance to develop and on whether those changes end up
getting passed on. We don't know how often pathogens "fail" when they
"try" to make the jump to humans.
A lot of us have watched the recent surge
in MERS coronavirus cases with some amount of concern. As of April 19, 2014, there
are two large clusters of cases in the Middle East, and at least one of them is
still growing. One cluster, in Jeddah, Saudi Arabia, now has 60 cases; 7 cases
were added to this cluster today, and 6 were added yesterday. There are perhaps
over a dozen cases in another cluster in the UAE. One patient who became ill
with MERS in Jeddah at the end of March flew to his home country of Malaysia
while ill and subsequently died in Malaysia; 79 of his contacts are now being
watched closely in Malaysia. Test results are starting to come in for a number
of these contacts, and thankfully all are negative for MERS so far. An
asymptomatically infected Filipino health care worker traveled on an airplane
back to the Philippines a few days ago. Yesterday, a MERS case was announced in
Greece; a Greek man who had been living in Saudi Arabia was recently in Jeddah
and presumably became infected there before flying back to Greece. He arrived
in Greece with a fever; his contacts are now being monitored. In other words,
MERS case numbers are growing quickly right now, at least in part through
human-to-human transmission, and infected – and potentially infectious – patients
are getting on airplanes to travel around the globe.
Does what we're seeing now represent
changes in the virus that are making it more transmissible among humans? Or are
we seeing a random fluctuation in the numbers of cases? Or, are we seeing more
cases simply as a result of improved surveillance? I would argue that what
we're seeing likely reflects one or more changes in the virus, simply because
- We've been seeing so many more symptomatic cases recently,
- We've been
seeing significantly larger clusters than we've ever observed before,
- A greater
number of health care workers appear to be getting infected than ever before,
and
- A greater proportion of cases are in health care workers than ever
before.
It's not that we've been seeing a rise only in the number of
asymptomatic cases detected, which could suggest that we're only seeing the
effects of improved surveillance. Moreover, while surveillance does seem to be
picking up more mild and asymptomatic cases, it is difficult to know whether we
are seeing more of these cases because of improved surveillance or because
there simply are more such cases now. A lot of variables are being changed at
the same time, and we don't have perfect information.
Nonetheless, the sheer numbers of recent
cases suggest to me, at least, that the virus is changing and becoming more
transmissible among humans. Until recently, we rarely saw evidence for
human-to-human transmission of MERS; most cases may have been zoonotic. Now,
however, large clusters involving roughly 1 to 4 dozen people are being seen,
with single infected individuals infecting possibly up to a dozen or more other
people. This is new. I don't think that we're seeing these clusters just as a
result of improved surveillance, although I would be very happy to be wrong.
What does the future hold for MERS? We
can't know. We might be watching MERS become a pandemic, and we might not. We
might be watching the current relatively small MERS outbreak develop into a
larger outbreak that eventually gets contained, as was seen with SARS. Or, the
whole outbreak might all just simmer down or go away. Even if the virus were currently
changing to become more transmissible, the current spate of cases could still simmer
down or go away, just stochastically, just through sheer chance.
Prudence would dictate that we remain
concerned and vigilant, however, especially as symptomatic MERS cases have had
an approximately 40% case fatality rate (CFR). If MERS did cause one or more
wider outbreaks in humans, that CFR might or might not change. Even if the CFR
dropped to 10% of what it is now, it would still be on the same scale as the
CFR for the 1918-19 influenza pandemic.
As a global society, we have an obligation
to do everything in our power to prevent the MERS coronavirus from causing
larger disease outbreaks in humans. We need more surveillance in affected
countries, including much more genetic sequence data. And in countries of the
Arabian Peninsula that are currently detecting MERS cases, infection control procedures
need to be improved to the point where nosocomial cases in health care workers
and patients are prevented. Health care workers in other countries should be
educated about the possibility of MERS patients arriving from afar and about
how to treat such patients safely. If this virus becomes more transmissible, we
should not be caught unprepared. We can see this one coming.
