To assess possible transmission modes of, and risk factors for, gastroenteritis associated with Norwalk-like viruses (NLVs) in a geriatric long-term-care facility.

During a prolonged outbreak of acute gastroenteritis, epidemiological data on illness among residents and employees were collected in conjunction with stool, vomitus, and environmental specimens for viral testing. NLVs were identified by electron microscopy in stool and vomitus specimens, and further characterized by reverse-transcriptase polymerase chain reaction and nucleotide sequencing. Potential risk factors were examined through medical-record review, personal interview, and a self-administered questionnaire sent to all employees.

During the outbreak period, 52 (57%) of 91 residents and 34 (35%) of 90 employees developed acute gastroenteritis. Four case-residents were hospitalized; three residents died at the facility shortly after onset of illness. A point source was not identified; no association between food or water consumption and gastroenteritis was identified. A single NLV strain genetically related to Toronto virus was the only pathogen identified. Residents were at significantly higher risk of gastroenteritis if they were physically debilitated (relative risk [RR], 3.5; 95% confidence interval [CI95], 1.0-12.9), as were employees exposed to residents with acute gastroenteritis (RR, 2.6; CI95, 1.1-6.5) or ill household members (RR, 2.3; CI95, 1.4-3.6). Adherence to infection control measures among the nursing staff may have reduced the risk of gastroenteritis, but the reduction did not reach statistical significance.

In the absence of evidence for food-borne or waterborne transmission, NLVs likely spread among residents and employees of a long-term-care facility through person-to-person or airborne droplet transmission. Rapid notification of local health officials, collection of clinical specimens, and institution of infection control measures are necessary if viral gastroenteritis transmission is to be limited in institutional settings