Calcutta: A procession of 500-odd diarrhoea patients in one state-run hospital and nine municipal clinics since Tuesday afternoon has triggered fears that the enteric outbreak may have spread far wider than thought.

Doctors at Baghajatin State General Hospital said many of the patients who turned up on Wednesday with symptoms of an E. coli infection were from Garia and Sonarpur, which were said to be outside the perimeter of the affected zone.

"We had treated Patuli as the southern boundary. But we have since had patients coming from Sonarpur and Garia, which are further south of Patuli. We didn't have patients from those parts of the city before now," a senior official of the health department said.

The outbreak had been first reported on Friday evening.

Laboratory tests have revealed the presence of E. coli bacteria in many patients. The tests were done at the School of Tropical Medicine.

E. coli is present in the lower intestines and is medically considered harmless, except strains of the bacteria that are introduced into the body from outside.

Contamination of tap water from a possible sewer leak is suspected to be the cause of the outbreak.

An engineer in the Calcutta Municipal Corporation's water supply department said water samples had been collected from 151 places for tests to ascertain "chlorine content". All the samples showed requisite chlorine presence, he said.

Chlorine presence between .5 to 1 part per million is normal.

Civic officials are worried about diarrhoea cases being reported in areas serviced by Garden Reach Waterworks. "Nazir Bagan in Dhakuria receives water from both the Dhapa and Garden Reach plants. We have to find out from where the affected people get potable water," an official said.

The affected zone includes wards 101, 102, 103 and 105 to 110 of the CMC. Parts of Baghajatin, Patuli, Kasba, Nonadanga and a residential area behind Peerless Hospital are among the affected neighbourhoods.

MR Bangur Hospital and the Beleghata Infectious Diseases Hospital have also reported patients with diarrhoea.

Nazir Bagan resident Anubhab Bhatta, who studies in Class VI at DPS Ruby Park, had to be brought back from school before classes gave over because of repeated vomiting and a stomach upset. "He had woken up around 3am feeling queasy, but still went to school and took ill in class," his mother said.

At the state-run Baghajatin hospital, the medical team was reinforced with five additional doctors to handle the rush of patients.

A pharmacy in Nazir Bagan said sale of ORS (oral rehydration solution) and Zeoline, a water purifier, had spiked over the past few days. "I used to sell ten ORS packets in a day. Since Saturday, I have sold more than 80 each day. Zeoline is out of stock," the owner said.

The owner of another medicine store in Dhakuria said sale of antibiotics had also gone up, although he wouldn't say whether these drugs were being sold against prescriptions or not.