Foodborne Outbreak Caused by a Norwalk-like Virus in India

An outbreak of acute gastroenteritis occurred in the nurses' hostel of a civil hospital in Delhi, after a farewell party involving 130 nurses and some of the housekeeping staff. All affected persons had eaten salad sandwiches at the party. Stool samples were collected from six of these patients on the second day of infection. All six samples, when tested for the presence of common bacteria, parasites, and rotavirus, were found to be negative. The clinical features of this outbreak matched the criteria set for outbreaks caused by Norwalk-like viruses (NLVs). Reverse transcription-polymerase chain reaction (RT-PCR) was carried out on these six samples, using primers from the RNA-dependent RNA polymerase (RdRp) gene of NLVs. Immunoelectron microscopy was carried out on two of the samples, using convalescent phase serum. All six samples were positive for genogroup (GG) II NLVs by RT-nested PCR. Aggregates of 32-nm viral particles were visualized by immunoelectron microscopy in one of the two samples. Sequencing of the RdRp gene was done on amplicons from three samples; phylogenetic analysis placed the isolates NDV/1999 in a Toronto virus cluster of GG II NLVs. This is the first report of a food-borne outbreak attributable to NLVs from India.