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Gene sequencing of cholera bacterium from Haiti points to U.N. source

July 03, 2013|By Melissa Pandika
  • Antonine Fizamey, 47, wails as her mother, Virginia Sencilna, 67, lies gravely ill with cholera in Gonaives, Haiti. The hospital has filled up from a cholera epidemic that hit the city and nearby villages.
Antonine Fizamey, 47, wails as her mother, Virginia Sencilna, 67, lies… (Rick Loomis / Los Angeles…)

The United Nations sent Nepalese peacekeeping troops to bring relief to Haiti after it was devastated by a 7.0 earthquake in 2010. A new study concludes the peacekeepers brought something else, as well -- cholera, triggering an epidemic that has sickened hundreds of thousands of Haitians and killed more than 8,000, according to the Centers for Disease Control and Prevention.

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 could be traced to a single source, undermining the hypothesis that the strain was repeatedly introduced to the country over the past three years.

Cholera is a disease caused by a bacterium called Vibrio cholerae. It’s typically spread through contaminated food or water, and symptoms include diarrhea, vomiting and dehydration. The disease spreads quickly in areas with inadequately treated sewage and drinking water, as is often the case in places that have been hit with a natural disaster. The World Health Organization estimates that 3 million to 5 million people are sickened with cholera annually, causing 100,000 to 120,000 deaths each year. Oral rehydration salts, intravenous fluids and antibiotics can be used to treat the disease.

Cholera first emerged in Haiti about nine months after the January 2010 quake that killed hundreds of thousands of Haitians. The outbreak was a surprise, since the disease had never before 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, 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 examined bacterial DNA and concluded that the cholera epidemic was caused by a strain most likely introduced to Haiti from South Asia, not some strain that traveled from neighboring Latin America.

A 2011 study found that V. cholerae samples from Haiti were almost genetically indistinguishable from Nepalese samples. Those results were published in mBio, the journal of the American Society for Microbiology. But some people remained unconvinced because most of the samples analyzed in that study came from Nepal.

The study published Tuesday, which also appears in mBio, 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 based on 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 just because the Haiti samples bore the most resemblance to the Nepal samples out of the genomes studied, that doesn’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,” Hanage said, adding that they underscore the importance of seeking “bigger, better data sets.” 

Hanage and his colleagues had set out to study how V. cholerae had evolved since it arrived in Haiti -- in particular, whether it gained genes that allowed it to adapt to its new environment. They found 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 study authors saw DNA sequence changes, or mutations, in the Haiti strain, but they appeared to be random, rather than helpful for surviving in its new environment.

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