After sequencing the DNA of 23 samples of the cholera-causing bacterium from Haiti and comparing them to the DNA of strains found elsewhere, researchers said the outbreak could be traced to Nepal, where the disease is endemic. They also concluded that the outbreak in Haiti came from a single source, undermining the hypothesis that the disease was repeatedly introduced to the country over the last three years.
Cholera is caused by a bacterium called Vibrio cholerae. It is typically spread through contaminated food or water, causing symptoms like diarrhea, vomiting and dehydration. Treatments include oral rehydration salts, intravenous fluids and antibiotics.
The World Health Organization estimates that 3 million to 5 million people contract cholera annually, causing 100,000 to 120,000 deaths each year. The disease spreads quickly in areas with inadequately treated sewage and drinking water, as is often the case in places that have been hit by a natural disaster.
Cholera emerged in Haiti about nine months after the January 2010 quake that killed hundreds of thousands of Haitians. The outbreak was a surprise because the disease had never been documented in the small island nation.
At first, circumstantial evidence reported by French epidemiologist Renaud Piarroux indicated that poor sanitary conditions at a U.N. camp about 40 miles outside the capital of Port-au-Prince resulted in contamination of local water supplies. But that didn't explain how V. cholerae wound up in the camp in the first place.
About 1,300 Nepalese peacekeepers arrived in Haiti in October 2010 to help with earthquake recovery efforts. The first indication that they might be responsible for the cholera outbreak was a December 2010 study that used DNA sequencing to determine that the bacterial strain most likely came to Haiti from South Asia, not from Latin America.
Another study in 2011 found that V. cholerae samples from Haiti were almost genetically indistinguishable from Nepalese samples. But some people remained unconvinced because most of the samples analyzed came from Nepal.
The study published this week in mBio, the journal of the American Society for Microbiology, considered more than 100 samples from recent cholera outbreaks in 16 different countries. Even with more candidates in the mix, the Haiti and Nepal samples were strikingly similar, perched on the same branch of the evolutionary tree that researchers constructed with their data.
"They're very closely related," said William Hanage, a study author and infectious disease expert at Harvard University's School of Public Health.
But he cautioned that the results don't rule out the existence of even more closely related samples elsewhere. The data are "consistent with a hypothesis of an introduction from Nepal, but not definitive," he said.
Hanage and his colleagues had set out to study how V. cholerae evolved since it arrived in Haiti - in particular, whether it gained genes that allowed it to adapt to its new environment. They did find DNA mutations, but these appeared to be random rather than helpful.
The team also discovered that the Haitian V. cholerae strain had a limited ability to "pick up" genes from other bacteria or the environment through a process called horizontal gene transfer, Hanage said. However, it's still possible that the strain could acquire genetic material through other means - for example, if a virus injects its genetic material into a bacterium.
The bacteria from Haiti and Nepal are both examples of "atypical El Tor" strains, which have become more virulent in Asia and Africa, resulting in higher infection rates and more severe symptoms. They have also become resistant to several cholera drugs.
"The study brings up a great new light into the epidemic that's been occurring in Haiti," said Pardis Sabeti, a computational geneticist at Harvard University's Center for Systems Biology who was not part of the study. "It provides an opportunity to watch how bacteria evolve over time and hopefully provide an opportunity to think of interventions."
The results came days before the July 6 deadline set by the Institute for Justice and Democracy in Haiti, a Boston-based human rights group, for the U.N. to compensate Haitian cholera victims or face a lawsuit in a U.S. court.
Hanage said the legal threat underscores the importance of understanding outbreaks like the one in Haiti. "Our ability to reconstruct these events have legal and ethical implications," he said. "The scientific community should be taking those seriously."
Sabeti said the battle should be waged against microbes, not the people or institutions that unwittingly spread them. Although she agreed that the Haitian epidemic has been devastating, she said she worried that litigation could chill the type of epidemiological research needed to understand these outbreaks better.
"As the world becomes more global, this kind of thing is going to happen all the time," she said. "We can't just have a lawsuit every time."
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