During July of 2001, a diarrheal outbreak affected more than 7000 individuals in the urban area of San José. Even though the origen of the outbreak was not found, faecal pollution of the drinking water from the metropolitan aqueduct it is suspected. The objetive of the study was to determinate the presence of Shigella spp., as a possible aetiological agent of the outbreak. Samples were collected during two weeks (july 17^th to 31^st). Sixteen Shigella isolates were obtained out of 49 patients with diarrhoea (34,7%), 15 of the isolates were Shigella sonnei and 1 corresponded to Shigella flexneri. The most affected age groups were children under 10 and people older than 60. The antibiotic susceptibility pattern of the Shigella isolates showed that they were resistant to eigth of the antibiotics: trimethoprim/sulfamethoxazole, tetracycline, ampicillin, eritromicine, chloramphenicol, cephalotin, amikacine and amoxicillin. They were sensitive to norfloxacin, ciprofloxacin and gentamicin. Althought it was not possible to isolate Shigella directly from water, the results highly suggest this pathogen to be the aetiological agent of the outbreak. This study demonstrates the import role of Shigella spp. as cause of diarrhea affecting mainly small children and elderly people and its frequent association to outbreaks related to drinking water. This study also suggests that guidelines for treatment of shigellosis should include information on the antibiotic resistance pattern and further studies regarding the routes of transmission by water and food, will be required for strategies of prevention of such outbreaks

Shigella sonnei ; diarrhea; outbreak