The United Nations sent Nepalese peacekeeping troops to bring relief to Haiti after it was devastated by an earthquake in 2010. A new study concludes that the peacekeepers brought something else too - cholera, triggering an epidemic that made hundreds of thousands of Haitians ill and killed more than 8,000.
After sequencing the DNA of 23 samples of the cholera bacterium from Haiti and comparing it 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 past three years.
Cholera emerged about nine months after the January 2010 quake that killed hundreds of thousands of Haitians. The outbreak was a surprise because the disease was new to the island.
At first, circumstantial evidence reported by French epidemiologist Renaud Piarroux indicated poor sanitary conditions at a UN camp about 70 kilometres from the capital, Port-au-Prince, resulted in contamination of local water supplies. But that did not explain how V. cholerae arrived in the camp.
About 1,300 Nepalese peacekeepers arrived in Haiti in October 2010 to help with recovery efforts. The first sign that they might be responsible for the outbreak was a December 2010 study that used DNA sequencing to determine the bacterial strain most likely came to Haiti from South Asia, not from Latin America.
The study published last week in mBio, the journal of the American Society for Microbiology, considered more than 100 samples from recent cholera outbreaks in 16 countries. Even with more candidates in the mix, the Haitian and Nepalese 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 diseases expert at Harvard University's school of public health.
But he cautioned that the results did not rule out the existence of even more closely related samples elsewhere. The data were "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 after 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.