Epidemiological investigation of an outbreak of foodborne typhoid and paratyphoid

Objective　The study was conducted to identify the epidemiological factors causing the outbreak of typhoid andparatyphoid in Ningbo in 2005, providing evidence for the prevention and control approaches. Methods　Epidemiological casestudies were implemented accompanied with paired case control studies. Shellfish from seafood market was collected for laboratory tests. Information on the spot was analyzed with descriptive epidemiology and health statistical methods. Results A total of 1029 cases of typhoid and paratyphoid were found in the epidemic with the incidence rate of 13.10/lakh. Abimodal shaped curve of onsets was depicted. The cases had been distributing in the peripheral areas and centered on theurban area, 25 55 year old age adults constituting the major involved population. The 1∶1 paired epidemiological surveyrevealed a 4.0 times higher risk among the raw cockle eaters and a 23.0 times higher risk among raw oyster eaters than thecontrol group(95% CI∶1.45 11.0 and 5.1 103.6). Meanwhile, ISalmonella paratyphi/I A was detected from both cockle and oystersamples from the market. Conclusion　The epidemic is triggered by people eating contaminated seafood such as raw oysters,cockles and other shellfishes.