The Lancet, Volume 383, Issue 9931, Page 1793, 24 May 2014
doi:10.1016/S0140-6736(14)60866-7
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Experts
are puzzled by an explosion of new cases of MERS-CoV, as a WHO
committee raises concerns over the handling of the outbreak. David
Holmes reports.
Researchers are
struggling to explain a rapid rise in reported infections with Middle
East respiratory syndrome coronavirus (MERS-CoV). As of May 16, the
number of laboratory confirmed cases globally had climbed to 614, with
181 deaths. 418 of those cases have been reported in the past 2 months,
mostly in Saudi Arabia.
After it was
first identified in Saudi Arabia in 2012, MERS infections have been
reported throughout the Arabian peninsula and exported to at least ten
other countries. Most confirmed cases of MERS-CoV infection have
developed severe acute respiratory illness, but the virus also often
causes kidney and other organ failure. The mortality rate is around 30%,
and there is currently no vaccine or specific treatment available.
While
stopping short of proclaiming the outbreak an international public
health emergency, the WHO Emergency Committee on MERS-CoV announced on
May 14 that its concern over the situation had “significantly
increased”, with particular worries over recent evidence that the
infection is spreading in hospitals, and apparent “gaps in critical
information”.
One of the most crucial
gaps is our lack of understanding of where the virus comes from, says
Marion Koopmans, of the Netherlands National Institute of Public Health.
So far, studies have more or less ruled out sheep, cattle, goats, and
poultry as a source, but dromedary camels throughout the Arabian
peninsula and in parts of North and East Africa have been shown to carry
the virus. However, it is still too early to say definitively that they
are the main source of primary infections in humans, says Koopmans. “I
am convinced that people and dromedary camels share the same viruses,
but how exactly this happens we do not know”, she explains. “Camels shed
virus from their nose, and sometimes in stool, which is dropped and may
cause environmental contamination. Younger animals seem to be
virus-positive more often than adult animals, so combined, the best bet
would be to look for exposures (direct or indirect) to young camels as
the highest risk factor.”
As young camels
lose their maternal immunity they become more susceptible to MERS-CoV
infection, and the fact that the latest surge in cases corresponds with
similar smaller spikes throughout April and May in 2012 and 2013 in
Jordan and Saudi Arabia also suggests that the breeding season could be a
factor. However, a seasonal increase in exposure to young camels alone
doesn't seem a sufficient explanation, according to Ziad Memish, Saudia
Arabia's Deputy Minister for Public Health and professor in the College
of Medicine at Alfaisal University in the capital Riyadh. “More than
two-thirds of the primary human cases have no links to camels”, he
notes. One possibility is that another species is acting as an
intermediary. Bats have been touted as a possible source after a bat of
the Taphozous genus was found to harbour a fragment of the MERS
sequence, but contacts between bats and people are so rare that it
“would seem an unlikely explanation”, says Koopmans. Another zoonotic
infection, Nipah virus, has been shown to stem from the consumption of
date palm sap contaminated by infected fruit bats, but Christian
Dorsten, a virologist at the University of Bonn, Germany, who is working
with local researchers in Saudi Arabia, says talk of a similar link
between bats and MERS is far-fetched. “There are several aspects in
recent hypotheses around MERS and bats that make no biological sense at
all. For example, it is not true that Taphozous feeds on dates from which the virus could be acquired. Taphozous is insectivorous”, he says.
Another
possible explanation for the recent surge in cases is that the virus
has acquired mutations enabling it to be transmitted more easily between
people, but again, Dorsten notes, the facts tell a different story.
Many of the recent cases occurred in the port city of Jeddah, and
sequences taken of the Jeddah viruses show no hints of any relevant
genetic changes says Dorsten. The Jeddah viruses do, however, seem to be
a different strain from those causing infections elsewhere in the
country, and are probably linked to a hospital-associated outbreak,
according to Dorsten. “Without having seen epidemiological data, I
predict that most cases detected in Jeddah will be linked quite directly
to the outbreak in King Fahd Hospital from where it spread to other
hospitals”, he says.
The prevention of
future outbreaks will hinge on countering the “surprising lack of
information about how this virus is transmitted from animals to humans”,
says Maria Van Kerkhove, liaison between WHO and the UK MRC Centre for
Outbreak Analysis and Modelling, Imperial College London. “Basic
epidemiologic studies have still not been done to evaluate risk factors
for infection”, she says. “We know that infection control and prevention
works to stop human-to-human transmission, but without stopping
transmission from camels, we will continue to see more cases in the
Middle East, some of whom will travel outside of the region.”
For more on MERS-CoV in health workers see CommentLancet 2014; published online May 20. http://dx.doi.org/10.1016/S0140-6736(14)60852-7