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In 1993, rectal swabs from clinically suspected cases of cholera admitted to the Infectious Diseases Hospital (IDH), Delhi were examined for Vibrio cholerae O1 and O139. Epidemiological data of 396 cholera cases were collected before the patients' discharge from IDH. Of the 1528 laboratory-confirmed cholera cases, 46% and 54% were caused by serotype O1 and O139 respectively. Both serotypes appeared and disappeared simultaneously, and peaked during the same time of the year. However, the two serotypes affected persons of different age groups; about 65% of the O1 cases occurred in children aged less than 10 years, whereas this age group accounted for 40% of the cases due to V. cholerae O139. Although there were some focal outbreaks due to serotype O139, both serotypes had almost similar geographical distributions. Important risk factors for transmission of cholera were almost equally prevalent in the majority of both types of cholera cases. Since the seasonality, geographical distribution, and risk factors for transmission were similar for both serotypes, the study indicates that the preventive and control measures are also likely to be similar. The study also shows that the emergence of V. cholerae O139 in 1993 did not affect the incidence, seasonality, and epidemiology of endemic V. cholerae O1 E1 Tor strains in Delhi.

Hundreds of laboratory-confirmed cholera cases occur every year in Delhi. However from 1965 through 1993, no cases of cholera nor carriers of Vibrio cholerae have been detected in the months January and February of all these years. Nevertheless, two cases occurred in January 1994. Both were children who acquired their infection locally. Six hundred fifty-eight rectal swabs collected from possible contacts were negative for V. cholerae. The next isolations could be made only in April, which is the usual beginning of the cholera season. The study suggests that cholera transmission can occur during the winter months in Delhi, but that it is not sustained.

The National Institute of Communicable Diseases (NICD) has been monitoring the incidence of laboratory confirmed cases of cholera in Delhi in collaboration with Infectious Diseases Hospital (IDH) since 1965. Cholera and cholera-like cases from all hospitals in Delhi are admitted in IDH and the rectal swabs of all such cases are processed for isolation of Vibrio cholerae at NICD laboratory. Since April 1993, there has been isolation of Vibrio cholerae serotype 0139, in increasing numbers (831 out of 2,830, 29.2%) The isolates have been characterized and enterotoxin studies carried out. As a referral laboratory NICD has also confirmed the causative role of Vibrio cholerae 0139 in diarrhea outbreaks from various parts of the country. The implications of establishment of this newer serotype of Vibrio cholerae, as a potential epidemic strain are discussed.

During 1988 a large scale outbreak of cholera occurred in Delhi giving rise to a total of 1708 bacteriologically proved El Tor cholera cases, the highest number recorded in a single year in Delhi, following its first detection in 1965. Civil lines and Shahdara zones were the worst affected areas recording 85.7 and 55.7 cases per 100,000 population respectively. The highest incidence rate was observed in the age group of 1-4 years (77 per 100,000). Males and females were equally affected. The data of moving average show the endemicity of cholera in Delhi with an increasing trend. The risk factors identified are lower socioeconomic status, poor personal hygiene, drinking water and food storage practices.

Delhi experienced an outbreak of cholera during July-August 1988 which affected residents from all walks of life. A total of 1824 laboratory confirmed cholera cases were detected in two months period at I.D. Hospital, Delhi alone. The number of cholera cases in July-August 1988 was 5-10 times that of the same period during the previous years in the Capital. The outbreak was caused by Vibrio cholerae Ogawa biotype ElTor. Majority of the laboratory confirmed cases (about 74 per cent) were seen in children under the age of 15 years. Though the cases were spread all over Delhi, almost three-fourths of total cases were reported from two specific zones (Shahdara and Civil Lines). Most of the isolates were sensitive to all antibiotics tested. The proportion of isolates resistant to furazolidone during this outbreak was substantially higher than in previous years suggesting that the outbreak may have been caused by the introduction of a new strain rather than proliferation of endemic strain. The salient features of the outbreak are discussed.