Emergence of norovirus GI.2 outbreaks in military camps in Singapore

• An uncommon GI.2 norovirus strain was the cause of acute gastroenteritis outbreaks in two military camps.
• Essentially identical norovirus strains were found in preceding community outbreaks.
• Differences in norovirus antigenic sites may have contributed to susceptibility.
• The bigger camp had a larger and longer acute gastroenteritis outbreak.
• There is a need for research and improvements in surveillance and control measures.

SummaryBackgroundSimultaneous acute gastroenteritis (AGE) outbreaks occurred at two military camps. This study details the epidemiological findings, explores possible origins, and discusses preventive measures.MethodsInvestigations included attack rate surveys, symptom surveys, hygiene inspections, and the testing of water, food, and stool samples. DNA/RNA was extracted from stool samples and amplified via real-time reverse transcription PCR (RT-PCR). Partial and full-length capsid nucleotide sequences were obtained, phylogenetic relationships inferred, and homology modelling of antigenic sites performed.ResultsThe military outbreaks involved 775 persons and were preceded by two AGE outbreaks at restaurants in the local community. The outbreak was longer and larger in the bigger camp (21 days, attack rate 15.0%) than the smaller camp (6 days, attack rate 8.3%). Of 198 stool samples, norovirus GI.2 was detected in 32.5% (larger camp) and 28.6% (smaller camp). These were essentially identical to preceding community outbreaks. Antigenic site homology modelling also showed differences between identified and more common AGE outbreak strains (norovirus GII.4).ConclusionDifferences observed highlight difficulties in controlling person-to-person outbreaks among large groups in close proximity (e.g., military trainees). Distinct differences in antigenic sites may have contributed to increased immunological susceptibility of the soldiers to infection.