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Published on: February 25, 2021
Shellfish poisoning outbreaks in Cuddalore District, Tamil Nadu, India
Anoop Velayudhan1, Janardhan Nayak1, Manoj V Murhekar2
1EIS Officer, Epidemic Intelligence Service India Programme, National Centre for Disease Control, New Delhi, India.
Background:
Two suspected shellfish poisoning events were reported in Cuddalore District in Tamil Nadu, India, between January and April 2015.
Objectives:
The study was conducted to confirm the outbreaks and to identify the source and risk factors.
Methods:
For both outbreaks, a case was defined as a person with nausea, vomiting, or dizziness. Sociodemographic details and symptoms were noted down. Data were also collected in a standard 3-day food frequency questionnaire, along with a collection of clam samples. A case-control study was initiated in the April outbreak. Stool samples were collected from cases, and clam vendors were interviewed.
Results:
In an outbreak that happened in January, all the twenty people reported to be consumed clams were diagnosed as cases (100% attack rate, 100% exposure rate). In the April outbreak, we identified 199 cases (95% attack rate). In both outbreaks, the clams were identified as genus Meretrix meretrix. The most common reported symptoms were dizziness and vomiting. The clams heated and consumed within 30-60 min. No heavy metals or chemicals were detected in the clams, but assays for testing shellfish toxins were unavailable. All 64 selected cases reported clam consumption (100% exposure rate) as did 11 controls (17% exposure rate). Illness was associated with a history of eating of clams (odds ratio = 314, 95% confidence interval = 39-512). Of the six stool samples tested, all were culture negative for Salmonella, Shigella, and Vibrio cholerae. The water at both sites was contaminated with garbage and sewage.
Conclusion:
Coordinated and timely efforts by a multidisciplinary team of epidemiologists, marine biologists, and food safety officers led to the outbreaks' containment.
