Norovirus Outbreaks on Three College Campuses — California, Michigan, and Wisconsin, 2008

Editorial Note

This report highlights the effect of norovirus outbreaks on these three college campuses and the demand for campus medical services. College campuses are at particularly high risk for norovirus outbreaks because of the extensive opportunities for transmission created by numerous shared exposures and living areas.^[3–5] Notably, the Wisconsin school had experienced a previous norovirus outbreak in 1999 attributed to direct person-to-person and fomite transmission in the shared living and bathroom areas of a dormitory.^[6] The ready access to health-care services that is typically present on college campuses also likely encourages increased reporting of illness relative to the general public, which can facilitate outbreak reporting.

The identification of a source of infection and targets for intervention is complicated by the multiple potential routes by which norovirus can be transmitted.^[1] The California university outbreak exhibited a sharp increase in cases suggestive of a point source, although no single facility or campus event was implicated. During the Michigan college outbreak, foodborne transmission was suggested by reports of ill food workers immediately before the spike in reported norovirus cases; however, no analytic investigation was conducted to support this hypothesis. In contrast, the propagation of cases, association with a specific residence hall, and the shape of the epidemic curve during the Wisconsin university outbreak are suggestive of primarily person-to-person transmission.

Control measures implemented in response to the Michigan outbreak included cancellation of all campus activities and closure of all buildings (excluding dormitories and the medical clinic) to enable extensive disinfection and promote social distancing. During norovirus outbreaks, particularly in institutional settings, temporary closure of public areas for the purpose of disinfection and cancellation of large gatherings often are indicated to help break or slow the cycle of transmission.^[7] In health-care settings, rapid closure of units experiencing norovirus outbreaks to new admissions has been associated with shorter outbreak duration.^[8] The number of cases declined after closure of the Michigan campus; however, the direct effect of campus closure on limiting further transmission is unclear.

The findings in this report are subject to at least four limitations. First, analytic studies were not performed during the outbreak investigations at the Michigan and Wisconsin schools, so specific exposures and risk factors could not be assessed. Second, because multiple control measures were implemented simultaneously in response to these three outbreaks, the efficacy of any single intervention could not be determined. Third, the majority of the data were self-reported through mostly passive electronic surveillance surveys that had relatively low response rates, likely resulting in underestimation of cases and attack rates. Finally, different case definitions were used in each of the three outbreaks because no standard case definition for norovirus infection exists. As such, outbreak-specific case definitions typically are developed during suspected norovirus outbreaks and tailored to the desired sensitivity and specificity of the investigation.

Norovirus exhibits many characteristics that can facilitate spread of infection and complicate interventions, including multiple potential modes of transmission, prolonged asymptomatic shedding, environmental stability of the virus, and lack of persistent cross-protective immunity (i.e., failure of prior infection to confer immunity to other norovirus strains).^[1] Consistent with recommendations for general norovirus outbreak management,^[7] strategies to prevent and control norovirus on college campuses should focus on hand hygiene, environmental disinfection, and exclusion of ill food workers (Box). Additionally, the use of e-mail, text messaging, and the Internet all facilitated communication during these outbreaks, although the usefulness of these media for case ascertainment is unclear considering the relatively low response rates. Given the widespread access to these technological resources on college campuses, such methods might be helpful during future outbreaks for rapid health communications and to supplement traditional case ascertainment methods.

Morbidity and Mortality Weekly Report. 2009;58(39):1095-1100. © 2009 

Cite this: Norovirus Outbreaks on Three College Campuses — California, Michigan, and Wisconsin, 2008 - Medscape - Oct 09, 2009.