Norovirus Outbreak Due to Groundwater Contamination, Alapuzha, Kerala, India, 2021

  • Water or foodborne
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Background: Between June 26–July 21, 2021, the Alapuzha municipality in Kerala, India, reported an unexpected number of acute gastroenteritis cases (613). We investigated to describe the outbreak, determine the agent, source, and risk factors to generate evidence-based recommendations.
Methods: We defined a probable case as the occurrence of at least 3 episodes of vomiting/loose stools within 24 hours since June 15. A confirmed case was a probable case with norovirus in rectal swab/stool by polymerase chain reaction (PCR). We actively searched for cases in the high-incident municipality ward. We did a 1:1 age (±5 years) and gender-matched case-control study and calculated matched odds ratio (MOR) with 95% confidence interval (CI). We tested 5 water samples from overhead tanks (2), tube wells (1) and patients’ houses (2) for norovirus. We interviewed pump operators to understand the water supply.
Results: We identified 60 probable cases by active case search, 52% (31/60) were males, and 30% (18/60) were ≤10 years of age. The common symptoms were vomiting (74%) and diarrhoea (59%). We identified 6 confirmed cases (67%) among 9 probable cases tested. The epidemic curve suggested continuous exposure, with cases increasing after heavy rain on June 25. Drinking water without adequate purification (boiling >10 minutes or using reverse osmosis) was associated with the outbreak (MOR-4.5; 95% CI=1.3–15.8). We detected group II norovirus in 4 of the 5 water samples. Pump operators mixed the untreated groundwater from tube wells with the water from the water treatment plant for uninterrupted supply. Following the heavy rain on June 25, the water supply from the treatment plant was disrupted, and only untreated tube well water was supplied.
Conclusions: The supply of groundwater without adequate treatment led to the norovirus outbreak. We recommended adequate chlorination of the water supply and boiling of water for at least 10 minutes at the households.

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