People gather around a working water fountain in the courtyard of a Christian mission on the outskirts of Port-au-Prince January 18, 2010. Credit: Reuters/Ana-Bianca Marin

So far, 11 laboratory-confirmed cases of malaria have been reported among emergency responders and those traveling in the United States from Haiti, the U.S. Centers for Disease Control and Prevention said on Thursday.

Haiti already had a problem with malaria, which is spread by mosquitoes that will have more places to breed in the cities and towns wrecked by the giant quake.

Thus, displaced persons living outdoors or in temporary shelters and thousands of emergency responders in Haiti are at substantial risk, researchers at the Pan American Health Organization and colleagues wrote in the CDC's weekly report on death and disease.

Each year, Haiti reports about 30,000 confirmed cases of malaria to the Pan American Health Organization, but the CDC estimates as many as 200,000 may occur each year.

Three cases the CDC cited occurred among Haitian residents traveling to the United States and one case involved a U.S. resident who was visiting Haiti. All are expected to recover fully.

Six out of eight patients, including seven emergency responders, had been advised to take drugs to prevent malaria but had not done so, the PAHO experts said.

According to the CDC, malaria transmission peaks after the two rainy seasons -- November to January and again during May to June.

There is no vaccine against the parasite that causes the illness and it quickly evolves resistance against drugs.

The CDC said the reported cases do not require a change in policy, but said anyone traveling to Haiti should take drugs to help prevent infection.

The CDC said it is continuing to monitor malaria in Haiti.