The World Health Organization estimates that cholera has killed more than 4,000 Zimbabweans since the outbreak began there in August.  The ongoing epidemic has brought international attention to this devastating disease.

Cholera is a chronic problem throughout the developing world, primarily in areas where sanitation is poor and intestinal parasites are common. New research suggests that these parasites could be reducing people's ability to resist cholera infection, and compromising the effectiveness of vaccines to prevent it.

Cholera is a bacterial infection that affects millions of people worldwide. They get the disease by eating food or drinking water contaminated with cholera bacteria - often from the feces of an infected person. Cholera can cause severe diarrhea, vomiting and dehydration, which can be fatal if left untreated.

To try to better understand the immune response to cholera, Dr. Jason Harris, a pediatric infectious disease specialist at Massachusetts General Hospital in Boston, looked at cholera patients in Bangladesh.  "The ICDDR,B [International Center for Diarrheal Disease Research in Bangladesh]takes care of over 100,000 patients a year with diarrhea," Harris says. "Probably 20,000 of them on average have cholera."

The research team collected blood samples from 361 cholera patients who came to the center for treatment between 2001 and 2006, and measured their immune response over three weeks.

Fifty-three cholera patients in the study also had intestinal parasites. "Those patients went on to develop weak immune responses to the cholera toxin."

Specifically, compared to other cholera patients, those with intestinal parasites produced about half as much of a particular antibody, called IgA. "And this antibody we think is very important for protecting against intestinal infections," Harris explains.

In other words, since cholera patients who have intestinal parasites produce fewer protective antibodies against cholera bacteria, they could be more vulnerable to getting re-infected.

Harris says his findings could also help explain why cholera vaccines that have looked very promising in medical trials in the U.S. and Europe haven't worked very well in developing countries. "This might be part of the explanation for that, that there's high rates of parasitic infection in countries where cholera is common."

His work also points to a way to improve the effectiveness of existing cholera vaccines. "This study suggests that efforts to treat intestinal parasites, perhaps to treat them before vaccinating, may improve the response to cholera vaccines, and may improve the chance of developing a healthy immune response that makes the vaccines more effective."

But, Harris says, as the current epidemic in Zimbabwe shows, we still have a long way to go in controlling this widespread disease. His research is published in the journal PLoS Neglected Tropical Diseases.