Plagues,
Pestilences and Diseases
New super strain of cholera is the deadliest and most
virulent yet, say researchers
Global action plan needed for the 13% of the world's population
who still lack access to safe drinking water
* Paul Benkimoun
* Guardian Weekly, Tuesday 1 February 2011 14.04 GMT
With major cholera outbreaks in several countries, from Zimbabwe
to Haiti, through the Dominican Republic, researchers now think
the hybrid strain that appeared several years ago is causing more
frequent and more severe epidemics, with a higher death rate.
Infection is caused by the bacterium Vibrio cholerae, which is
often found in wetlands, particularly if there are stagnant ponds
and estuaries.
In an article published last month by the online journal PLoS
Neglected Tropical Diseases, Edward Ryan of Harvard University
questions the suitability of existing counter-measures. The World
Health Organisation estimates that out of 3m to 5m cases a year,
between 100,000 and 120,000 prove fatal.
Haiti, Nigeria, Angola, Pakistan, Vietnam and Zimbabwe have all
suffered recent epidemics, which suggests that "our current global
action plans against cholera are failing", according to Ryan. The
disease spread all over the world in the 19th century, starting
from the Ganges delta. Since 1817, there have been seven cholera
pandemics, all originating in Asia. The most recent, which is
still under way, started in Indonesia in 1961, making it the
longest recorded pandemic.
The current variant is known as El Tor O1, after the location of
the quarantine camp in Sinai where it was first isolated, in 1905.
Its capacity for survival has helped it to proliferate. Moreover
it is often carried by people who display none of the usual
symptoms, but may unwittingly transmit the disease to others. Ryan
thinks these characteristics explain the prolonged outbreaks of
the disease.
Over the past 20 years El Tor O1 has undergone two changes. A
change in its structure turned the O1 strain into a new serotype,
O139. The new strain can kill people who are immune to O1,
enabling it to spread through 11 Asian countries in the 1990s.
Another O1 variant has recently appeared, displaying new
characteristics but producing the same cholera toxin.
The newcomer, which can cause more severe forms of cholera,
rapidly supplanted the old El Tor strain in many areas. "The
prevalence of the hybrid strain may explain why we are seeing case
fatality rates of 1% to 5% (or higher) in recent outbreaks, as
opposed to the less than 1% historically accepted as the goal for
response teams," Ryan argues.
David Olson, a cholera specialist with Médecins Sans Frontières,
confirms the increase in the number of cholera cases worldwide in
2009 and 2010, and the greater severity of infections caused by
the new variant. "The new strain has been predominant since the
1990s in Bangladesh and India. Starting in 2004, it appeared in
Mozambique, then Zimbabwe, Zambia, Nigeria, Cameroon and Chad. It
could explain the high death rate in Haiti," Olson says.
Looking to the future, the solution is to build up the necessary
infrastructure to improve hygiene, a huge challenge given that 13%
of the world's population still lack access to safe drinking
water. This has prompted calls for a reappraisal of vaccination
policy. Until last year the WHO was against the idea, but it no
longer rules out the use of vaccination.
There are two types of oral vaccine, administered in two doses a
week apart, which is a major problem in the midst of an epidemic
and the resulting chaos. Moreover the vaccine "is effective for
about two years and manufacturers lack the capacity to produce
sufficient quantities", Olson adds.
According to Professor Renaud Piarroux, a tropical medicine
specialist at Université de la Méditerranée, Marseille, "the
answer is to combine several tools: access to water, hygiene and
vaccination".
This article was first published in Le Monde