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Norovirus: Why "Germaphobes" Might Be On To Something

Besu Teshome, Pharm.D. and Jennifer Seltzer, Pharm.D.

March 14, 2013

Noroviruses, a group of single-stranded RNA viruses previously known as Norwalk-like viruses, are the leading cause of epidemic gastroenteritis worldwide.1 In the U.S., noroviruses are estimated to cause 21 million illnesses annually, resulting in 70,000 hospitalizations and up to 800 deaths.2 This virus was also responsible for up to 35% of foodborne disease outbreaks in the U.S during 2006-2007.1

Kapikian and colleagues3 first identified noroviruses in 1972 when microscopically examining stool samples following a gastroenteritis outbreak.3 While researchers remained aware of the link between norovirus and gastroenteritis since the initial discovery, substantial strides in the understanding and epidemiology of noroviruses were not made until molecular assay advancements in the 1990s.  Noroviruses can be classified into five genogroups, GI-GV, and even further divided into separate genotypes within each group. The majority of human diseases are attributed to GI and GII strains.1 The GII strains have an additional 21 genotypes, with GII.4 being the genotype that is responsible for over 85% of the outbreaks.4 The GII.4 New Orleans subtype had been the predominant strain responsible for the majority of outbreaks from 2010-2012, but recently a new variant has emerged.4,5 In March 2012, GII.4 Sydney, identified in Australia, started spreading rapidly across multiple countries and has already been responsible for 53% of the U.S. norovirus outbreaks from September to December 2012.5

Noroviruses can infect people of all ages.  While most infections occur during winter months, outbreaks have been reported year-round.1-3 The incubation period lasts for 24 to 48 hours and is followed by a sudden onset of symptoms including nausea, vomiting, non-bloody diarrhea, malaise, headache, and abdominal cramps. Fever (101°F to 102°F) is also documented in approximately half of the cases.  Generally these symptoms are self-limiting and last 1-3 days without any treatment, but can last up to 6 days in young children, elderly and hospitalized patients.1,3 Up to 30% of infections are asymptomatic, potentially allowing viral carriers without clinical symptoms and lower viral titers to shed virus and infect others.1 As little as 18 viral particles have been reported to make up an infectious dose.6 This means that during peak shedding, each gram of feces can contain approximately 5 billion infectious doses, whichcontributes to high secondary transmission rates.1 In addition, viral shedding with norovirus infections is prolonged as virus can be detected in stool for an average of 4 weeks post infection.  Furthermore, reinfection can occur with noroviruses because long-term immunity is not maintained after exposure.1,3

Transmission can occur by person-to-person contact as well as contaminated food or water.  Person-to-person transmission occurs mostly through the fecal-oral route by direct or indirect contact, or by contact with aerosolized vomitus. Food and water transmission are attributed to either fecal contamination at the source or infected food handlers.7 The most common foods that serve as a reservoir are shellfish and foods that need handling without cooking, like salads or sandwiches.1-3 Noroviruses have also been implicated as a leading cause of outbreaks linked to fresh produce.3 Outbreaks are largely due to new viral strains, which cause an initial infection, then propagate by person-to-person transmission. Viral outbreaks can occur in various settings, but especially target closed environments such as nursing homes, hospitals, cruise ships, and schools.8

There are no antibiotics or vaccines available to treat noroviruses so management of symptoms and isolation are considered the best options for those infected. Oral rehydration solutions and, in severe cases, parenteral solutions with electrolytes may be warranted for patients with a high amount of fluid loss due to diarrhea.1,3 In addition, considering the ease of transmission of these viruses, isolation of norovirus-infected patients is important, especially those subjected to closed environments. For those not infected, instituting appropriate preventive measures such as maintaining proper hand hygiene; carefully washing fruits and vegetables; thoroughly cooking shellfish; disinfecting any contaminated surfaces; carefully removing vomitus or fecal matter (e.g., apply disposable cloth with minimal agitation, use disposable gloves, wash hands), and thoroughly washing any contaminated laundry may be the best lines of defense against this pathogen.1,2,8



  1. Centers for Disease Control and Prevention. Updated norovirus outbreak management and disease prevention guidelines. MMWR Recomm Rep. 2011;60(RR-3):1-18.
  2. Centers for Disease Control and Prevention. Prevent the spread of norovirus. Available at: Accessed February 26, 2013.
  3. Patel MH, Hall AJ, Vinje J, et al. Noroviruses: a comprehensive review. J Clin Virol. 2009;44:1-8.
  4. Vega E, Barclay L, Gregoricus N, Williams K, Lee D, Vinje J. Novel surveillance network for norovirus gastroenteritis outbreaks, United States. Emerg Infect Dis. 2011;17:1389–95.
  5. Centers for Disease Control and Prevention. Notes from the field: emergence of new norovirus strain GII.4 Sydney – United States, 2012. MMWR Morb Mortal Wkly Rep. 2013;62(3):55.
  6. Teunis PF, Moe CL, Liu P, et al. Norwalk virus: how infectious is it? J Med Virol. 2008;80:1468-76.
  7. Moe CL. Preventing norovirus transmission: how should we handle food handlers? Clin Infect Dis. 2009;48:38-40.
  8. Treanor JJ. Epidemiology, clinical manifestations, and diagnosis of noroviruses, astroviruses and saproviruses. In: UpToDate, Basow, DS (Ed). UpToDate, Waltham, MA, 2013.



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