Cholera flushes its loo-image

24. June 2011
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Where sewage is no (longer) being purified and clean drinking water is in shortage, epidemics such as cholera threaten. Vibrio cholerae occurs, however, not only in wastewater, but also in warm coastal waters. Is the pathogen heading back to Europe?

A distance of some 7500 kilometers is a long stretch. Not only for pathogens, but also to a threat in our head. Bangladesh and Haiti are about the same distance from Germany. Both countries are at the moment known distribution-areas for Vibrio, the cause of the dreaded diarrheal disease Cholera. Cholera is occasionally an issue in news coming from disaster areas and developing countries. But in Europe, or even Germany? We’re past those times due to sanitation and permanent fresh, controlled drinking water, right?

European vibrios love warm summer

About fifteen years ago, microbiologists found the bacteria Vibrio cholerae in seawaters around Sicily. A close relative, Vibrio vulnificus, is a regular guest in the Baltic Sea particularly when the temperatures raise to 20 degrees and above.In Denmark in 1994 – an exceptionally warm summer – the germ caused eleven cases of illness involving wound infections, in “normal” years there are three to four cases. Germs of a non-pathogenic Vibrio cholerae strain were isolated by the health authorities in 2006 in bathing waters in Thuringia but also from the Lake Neusiedl. In all cases the water was not contaminated by other faecal bacteria.

The Robert-Koch-Institute warns against excessive panic. “Instances of indigenous infections (from Cholera) have not been reported in recent years.” From 2001 to 2009 there were eight patients who had been infected with the disease abroad. However, there are renowned microbiologists warning about a return of the disease due to climate change.

Away from human settlement

What stands out to one who looks at the occurrence of the disease over the world is that in countries near the equator it is caught throughout the year via people. In countries of the subtropics, however, the frequency varies with the seasons – an indication that lack of hygiene is only responsible in part for the multiplication of germs, whereas climate also contributes a crucial part. In Peru, the disease appeared in 1991 after a hundred years of absence for the first time again and since then has resulted in the summer months in many illnesses. Even in Haiti, there had not been any cholera for a century before the earthquake.

Rita Colwell was for a long time director of the U.S. National Science Foundation and now teaches at the University of Maryland. Her research shows that the bacteria can multiply independent of humans. She discovered in her investigations in the Bay of Bengal – but also at her doorstep in Chesapeake Bay – Vibrio cholerae bacteria independent of human settlement. That cholera has a chance not only to spread via human activity was for a long time considered very contentious. The theories of the environmental scientist kept on being strongly attacked by those in medical circles. Matthew Waldor, infectiologist of Harvard University, says: “If the cholera is traveling with the environment, (…) then it can not be prevented.” Such statements are not well received by developers and marketers of vaccines.

A threat to Europe as well?

And yet there is increasing evidence that climate variations have a say in how many people the bacteria strikes. With the increasing frequency of El Niño events – the warmer ocean currents that occur periodically – there has been in Bangladesh over the last decades an increase in the occurrence and spread of the infectious disease. The risk of infection climbs with a rise in water temperature of 5° C over six weeks by a factor of two to four times. Colwell warns that this fact increases the risk for North America and Europe: “We could be standing at the beginning of a series of epidemics, such as we have not seen in almost a hundred years.”

From plankton to humans

She also questions the thesis that Nepalese UN-workers alone carrying the bacteria internally are entirely responsible for the spread of the disease in Haiti, such as a report in the New England Journal suspects. “The data we have collected over the past 40 years show that the bacteria are part of the natural water balance,” Cholera expert Sonia Shah is quoted by the science journalist, “and molecular genetic data indicate growing evidence that the outbreaks are a local phenomenon.” The determination of the genetic code alone only gives an indication of the origin of the virus, because vibrios can exchange genetic material among themselves, independently of their rate of multiplication. Therefore different serogroups get mixed up. This accounts for the way that extremely virulent epidemics occur even after long disease-free periods.
The main host of Vibrio cholerae, in addition to man, is zooplankton: tiny crustaceans that feed on plant plankton. Via the food chain, water birds also become disease vectors. Aquatic plants, protozoa and even sedimentary rocks are just as much starting points for further spread and the capacity to then make it to humans. This happens especially when nutrient-rich effluents lead to an algal bloom and so fosters the growth of plankton crustaceans.

Water filtration and vaccination

In the “Lancet” from 8 April 2011, Jason Andrews and Sanjay Basu of Boston’s Harvard Medical School forecast on the basis of their mathematical model some 800,000 cases and 11,000 deaths by December 2011 in Haiti – the death toll currently stands at around 250,000 cases and almost 5,000 deaths. Had the few Nepalese workers alone been responsible for the outbreak of the disease, according to Science (quoting French epidemiologists Renaud Piarroux), they would have had to “infect dozens of patients and introduce hundreds of litres of contaminated stools into wastewater.”

How can we now get the epidemic under control again? Just vaccinating 50 percent of the population in high-risk areas, writes Ira Longini in the newest edition of PNAS, would have reduced the total cases and death rates by about half. The costs for this action for the two cholera vaccines are respectively 1.5 and 5 dollars per dose. But it can be even easier than that: Rita Colwell showed in field trials in Bangladesh – with around 133,000 people – that simple water filtration reduces cholera rate by 48 percent.

It may well be that we central Europeans won’t need to pay regard this notice in the coming decades. Still, the view that cholera gets its chance solely due to lack of hygiene seems to be a tale from yesterday.

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