By Kieran Guilbert

DAKAR (Thomson Reuters Foundation) - Aid agencies in the Central African Republic said on Thursday they were racing to prevent the first outbreak of cholera in five years from spreading across the conflict-stricken country.

The outbreak, which was declared last week, started in the southern Kemo prefecture before spreading to the capital Bangui, and has infected more than 150 people and killed at least 18 people to date, according to the latest government figures.

Medical charity Médecins Sans Frontières (MSF) has opened a cholera treatment center in Bangui, and the International Organization for Migration (IOM) is supporting an awareness-raising campaign along the country's entire southern border.

MSF is also monitoring communities along the Oubangui River who use it as their main water source, and said it was working to prevent the river from spreading the epidemic further.

"It is important to react quickly to prevent the spread of the disease because the risk of infection is higher in areas where clean water is limited," said Jose Antonio Sanchez of MSF.

Cholera, which spreads through contaminated food and drinking water, causes diarrhea, nausea and vomiting.

The disease can lead to death by dehydration and kidney failure within hours if left untreated, but most patients recover if treated promptly with oral rehydration salts.

The IOM said it was concerned about the risk of the cholera outbreak spreading throughout the capital's 26 sites for people uprooted by fighting, which host some 50,000 displaced.

Central African Republic descended into chaos in March 2013 when mainly Muslim Seleka fighters seized power, triggering reprisal attacks by Christian anti-balaka militias.

President Faustin-Archange Touadéra took office in March after elections aimed at drawing a line under the crisis, but insecurity has since persisted and fresh fighting broke out in Bangui at the end of June.

"Early action to prevent a further spread of the disease is crucial, especially considering the state of many displacement sites here in Bangui," said the Anne Schaefer of the IOM, adding that water and sanitation programs were hugely underfunded.

The U.N. children's agency (UNICEF) said it was working with the health ministry to give medicine, clean water, purification tablets and hygiene kits to the cholera-hit communities.

"Children are at the highest risk of cholera, but so far we have been lucky that not many children have been infected," Souleymane Sow of UNICEF told the Thomson Reuters Foundation.

Central African Republic was last hit by cholera in 2011, when an epidemic swept through West and Central Africa - one of the biggest in the region's history - infecting more than 85,000 people and killing at least 2,400.

(Reporting By Kieran Guilbert, Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org)