March 5, 2011
CDC: Updated Norovirus Outbreak Management and Disease Prevention Guidelines
The prototype norovirus was first identified as the cause of a gastroenteritis outbreak in Norwalk, Ohio, in 1968 (1). However, the epidemiology of noroviruses remained poorly characterized until relatively recently because of the lack of widespread availability of sensitive diagnostic methods. Electron microscopy was the first tool used for identifying noroviruses but proved to be a time-consuming and insensitive method that was rarely available outside of research settings. Immunologic assays developed during the late 1970s and 1980s improved detection rates (2) but lacked broad reactivity to detect the full spectrum of noroviruses and were not widely available, limiting their utility for routine use in outbreak investigations. Consequently, the cause of the majority of gastroenteritis outbreaks could not be determined. Because of these limitations of laboratory diagnostics, clinical and epidemiologic criteria were developed to help attribute gastroenteritis outbreaks to norovirus (3). These criteria have subsequently been validated as highly sensitive (99%) and moderately specific (68%) (4). Since the 1990s, the development and subsequent widespread use of molecular diagnostic assays have led to substantial improvements in understanding the role of noroviruses in gastroenteritis outbreaks (5), particularly among those involving foodborne transmission and those occurring in long-term–care facilities (6,7). Nonetheless, progress in the characterization and control of norovirus has been hampered by the lack of a rapid and sensitive assay for use in clinical settings and the inability to cultivate human noroviruses in cell culture.
Since 2001, when CDC published the most recent norovirus recommendations (8), substantial advances have been made in norovirus epidemiology, immunology, diagnostic methods, and infection control. This report reviews these advances and provides guidelines for outbreak management and disease prevention. Specific recommendations include standardized collection of clinical specimens during norovirus outbreaks, use of two recently launched surveillance systems for reporting of norovirus outbreaks, and use of appropriate control measures focusing on hand hygiene, environmental disinfection, and exclusion of ill persons (Box). Their implementation by public health professionals is intended to guide efficient use of public health resources for effective prevention and control of norovirus disease.
Prevention and Control
Appropriate hand hygiene is likely the single most important method to prevent norovirus infection and control transmission. Reducing any norovirus present on hands is best accomplished by thorough handwashing with running water and plain or antiseptic soap. Washing with plain soap and water reduces the number of microbes on hands via mechanical removal of loosely adherent microorganisms (106). The efficacy of alcohol-based and other hand sanitizers against norovirus remains controversial with mixed evidence depending on the product formulation and evaluation methodology. In finger pad studies, soap and water used for 20 seconds have been demonstrated to reduce norovirus by 0.7–1.2 log by RT-PCR assay, whereas alcohol-based hand sanitizers did not demonstrate any appreciable reduction of viral RNA (107). However, such studies cannot determine whether the residual virus remains viable given the inability to cultivate human norovirus in vitro.
Studies using cultivable surrogate viruses such as murine norovirus (MNV) or feline calicivirus (FCV) have demonstrated that ethanol has superior efficacy against FCV compared with other alcohols (e.g., propanol-1 and propanol-2), and formulations containing 70% ethanol were able to reduce infectious MNV by 2.5 log after 30 seconds (108–110). The sensitivity of FCV to low pH and the relatively high susceptibility of MNV to alcohols suggest that sanitizers that are effective against both surrogate viruses might be more likely to be effective against human norovirus, than those effective against only one of the surrogates (111). In addition, reduction of viral RNA (as opposed to reduced infectivity of cultivable surrogates) does not appear to be a reliable means of estimating the effectiveness of hand sanitizers against human norovirus (111).
Overall, studies suggest that proper hand washing with soap and running water for at least 20 seconds is the most effective way to reduce norovirus contamination on the hands, whereas hand sanitizers might serve as an effective adjunct in between proper handwashings but should not be considered a substitute for soap and water handwashing (106–111). As an additional preventive strategy, no bare-hand contact with ready-to-eat foods (foods edible without washing, cooking, or additional preparation to achieve food safety) is recommended (112; FDA, unpublished data, 2011).
Exclusion and Isolation
Considering the highly infectious nature of norovirus, exclusion and isolation of infected persons are often the most practical means of interrupting transmission of virus and limiting contamination of the environment. This is particularly important in settings where people reside or congregate such as long-term–care facilities, acute-care hospitals, cruise ships, and college dormitories. Empiric evidence for the effectiveness of exclusion and isolation strategies is limited (113); rather, strategies are based on infection-control principles. The principle underpinning isolation is to minimize contact with persons during the most infectious periods of their illness. This includes the acute phase of illness, a period following recovery while the person is still shedding virus at high levels (usually 24–72 hours), and, in some situations in health-care facilities, exclusion of exposed and potentially incubating persons. Isolation of both exposed and unexposed well persons might be useful during outbreaks in long-term–care facilities to help break the cycle of transmission and prevent additional cases. In health-care facilities, ill patients may be cohorted together in a unit or part thereof, with dedicated nursing staff providing care for infected persons (9). Analogously, passengers with gastroenteritis on cruise ships may be asked to remain isolated voluntarily in their cabins during their illness and for a period of 24–48 hours after their symptoms have resolved. Ill patients generally should not be transferred to unaffected units in the facility or to other facilities, except in the case of medical necessity and after consultation with infection-control staff. To minimize the risk of spread from incubating or asymptomatically infected patients and staff in health-care facilities, such persons should not be transferred to or work on (respectively) unaffected areas, typically for 48 hours after exposure. Ill staff members in health-care facilities and food handlers should be excluded during their illness and for 48–72 hours following resolution of symptoms. Asymptommatic food-service workers who have tested positive for norovirus, which might occur during an outbreak investigation, also should be excluded or restricted per the FDA Food Code (10). Regulatory authority approval might be required for excluded food-service workers to return to work (10), although requiring negative stool results prior to returning to work is not recommended. Sick pay and sick leave policies that do not penalize ill workers might help to facilitate such staff exclusion. Nonessential personnel, including visitors, may be screened for symptoms and excluded or, at the minimum, should be cautioned about the risks and made aware of the heightened importance of hand hygiene through washing with soap and water. In certain situations, units in a health-care facility may be closed to new admissions to prevent the introduction of new susceptible patients (113).
The use of chemical disinfectants is one of the key approaches to interrupt norovirus spread from contaminated environmental surfaces. Particular attention should be given to the likely areas of greatest environmental contamination such as bathrooms and high-touch surfaces (e.g., door knobs and hand rails). Sodium hypochlorite (chlorine bleach) has been widely recommended to disinfect human norovirus from surfaces, and its efficacy has been well documented (114–117). Environmental surfaces potentially contaminated with norovirus should be disinfected using a sodium hypochlorite solution or other commercial product registered with EPA as effective against norovirus. A list of EPA-approved products is available at http://www.epa.gov/oppad001/list_g_norovirus.pdf . Products containing phenolic compounds (including triclosan and quarternary ammonium compounds) are less effective against nonenveloped viruses such as human norovirus (114,118–120). Furthermore, this list should be interpreted with caution because the efficacy of these products is determined by using the surrogate FCV, which exhibits different physiochemical properties than human norovirus and therefore might not reflect a similar disinfection efficacy profile (121). In addition, several recent reports have demonstrated that FCV is not the most resistant surrogate virus to predict inactivation of human norovirus (111,122,123). Therefore, the use of multiple surrogate viruses rather than a single surrogate has been suggested to assess the efficacy of disinfectants (111).
Because of this uncertainty, whenever possible, chlorine bleach solution should be applied to hard, nonporous, environmental surfaces at a concentration of 1,000–5,000 ppm (5–25 tablespoons household bleach [5.25%] per gallon of water). Given the potential for evaporative dilution, bleach solutions should be freshly prepared for use within 24 hours, or the target concentration should be doubled (e.g., 2,000–10,000 ppm) for storage and used within 30 days. Although the upper end of this range has been recommended previously when soiling is present (124), few data are available to support the effectiveness of sodium hypochlorite on fecally soiled surfaces (125). For example, 5,000 ppm of sodium hypochlorite was not able to completely eliminate norovirus dried in 20% fecal stool suspension on surfaces (126). A recent study indicated that longer exposure times (approximately 4 minutes) of 5,000 ppm sodium hypochlorite on fecally soiled surfaces were needed to inactivate both human norovirus surrogates (MNV and FCV) by 4 log (122),which has been proposed as a satisfactory reduction level for norovirus contamination (127). In conclusion, initial cleaning of contaminated surfaces to remove organic loads such as fecal material should be performed before sodium hypochlorite disinfection.
In health-care settings, cleaning products and disinfectants used should be EPA-registered and have label claims for use in health-care settings; personnel performing environmental services should adhere to the manufacturer’s instructions for dilution, application, and contact time. Heat disinfection (i.e., pasteurization to 140°F [60°C]) has been suggested and used successfully under laboratory conditions, for items that cannot be subjected to chemical disinfectants such as chlorine bleach (128–131). Other disinfection approaches such as ozone, hydrogen peroxide, or coating surfaces with antimicrobial materials (e.g., titanium dioxide [TiO] film) also have been proposed for routine environmental control of noroviruses (132–134).