ARTICLE - CHOLERA RESEARCH

NEW STRAIN OF CHOLERA IS THE DEADLIEST AND MOST VIRULENT YET, SAY RESEARCHERS
(Guardian.co.uk) - By Paul Benkimoun

Global action plan needed for the 13% of the world's population who still lack access to safe drinking water

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