International humanitarian group Oxfam said the current cholera outbreak in the densely populated Carrefour district, on the western edge of the capital Port-au-Prince, is far worse than last autumn when the epidemic began.
In November, a maximum of 900 cases of cholera were reported there each week, but health organisations are now registering over 300 new cases per day, according to the agency. Ten people are reported to have died of the disease since May 26.
Cholera is an acute intestinal infection caused by ingestion of contaminated food or water. It produces diarrhoea and vomiting that can quickly lead to severe dehydration and death without prompt treatment.
Last week's arrival of the rains - with the chance of higher-than-normal precipitation forecast for the rest of this month - has helped spread bacteria into local water sources, Oxfam said.
"We are talking about very congested areas, where open defecation is commonly practiced because there are very few latrines and little space available to construct them," Roland Van Hauwermeiren, Oxfam's Haiti director, said in a statement.
In response to the recent deterioration, Oxfam is boosting its cholera prevention efforts in Carrefour. It will distribute water-purification tablets to an additional 70,000 people, disinfect houses where cholera cases have been detected, and carry out health promotion activities. It already provides sanitation services to the 77,000 camp residents in the district.
On June 2, the Haitian Red Cross and the Canadian Red Cross opened a 40-bed cholera treatment centre in the Carrefour-Feuille area of Port-au-Prince with the support of Medecins du Monde and the Haitian public health ministry. Between then and June 8, the centre treated 132 cholera-affected patients, including six under the age of five, the Red Cross said.
The increase in cholera cases had been limited to specific rural areas until recently, but has now hit the capital, leaving treatment facilities stretched, according to the International Federation of Red Cross and Red Crescent Societies (IFRC).
"The Red Cross has been scaling up our cholera prevention work for many months in preparation for the rains, which we know bring increased risk," Gerhard Tauscher, IFRC's cholera operation coordinator, said in a statement. "We have further stocks of medical equipment in country, ready to be released should cases continue to rise."
Medical aid group Medicins Sans Frontieres (MSF) has also raised the alarm, reporting a sharp increase in cholera patients in Port-au-Prince since mid-May, as well as outbreaks in other parts of the country. It has had to reopen emergency treatment centres to prevent existing facilities in several districts of the capital from being overwhelmed.
Cholera has killed around 5,400 people among the nearly 325,000 cases reported in the impoverished country since the epidemic began in October.
Despite the recent jump in cases, however, Oxfam said the number of deaths is much lower than last year as people are able to get help faster.
"Haitians are now more aware of how they can catch cholera, how to protect themselves against it, and what to do when cholera symptoms arise," said Van Hauwermeiren.
Cholera prevention involves simple steps, including drinking clean water and practising good hygiene. But it is difficult to follow those recommendations without basic sanitation services in place, Van Hauwermeiren added.
The Red Cross has been sending out SMS messages to people in high-risk areas with the location of their nearest treatment centre and practical information on prevention, treatment and symptoms. And a weekly radio show, which broadcasts nationwide in Creole, is covering cholera again, answering listeners' questions live on air.
Aid agencies are also distributing safe drinking water, hygiene kits, soap, water-purification tablets and oral rehydration salts - by donkey in some remote mountainous areas - as well as training health staff and volunteers.
MSF urged Haiti's government and its humanitarian partners to stop the spread of the disease by strengthening the national surveillance system and treatment facilities. Urgent improvements in hygiene, sanitation and drinking water supplies should be a national priority to protect the most vulnerable, it added.
"Workload should be shared and coordinated in order to increase cholera treatment capacity in Haiti," MSF head of mission Romain Gitenet said in a statement. "Too many public facilities are still inadequate."
Even before the devastating January 2010 earthquake, around half of Haiti's urban residents and more than 80 percent of the rural population did not have access to basic sanitation services, according to Oxfam.
"Oxfam can provide an emergency response to the cholera outbreak, but we need the national government, with the support of the international community, to provide long-term sanitation facilities and access to drinking water both in rural and urban areas, otherwise outbreaks will continue," Van Hauwermeiren warned.