Norovirus (shortened from Norwalk virus) causes epidemic illness with common names that include winter vomiting disease, winter vomiting bug, viral gastroenteritis, and is also sometimes referred to as stomach flu. Symptoms usually last 2 to 5 days and include, nausea and stomach pain, vomiting and diarrhea. Fever occurs in between one-third and one-half of cases and virus shedding can extend up to a week after recovery. Unfortunately, people who get norovirus do not develop immunity against future infections. Although the body can make effective antibodies against the infecting strain, there are many strains that cause infection and the virus is constantly mutating in small ways that results in evasion of the human immune system.
Norovirus is a non-enveloped virus having a very tightly packed spiked-soccer or football-like outer shell and thus was once referred to as small-round-structured-viruses. While alcohol based hand sanitizers are highly effective against enveloped viruses (like flu and other common respiratory viruses) noroviruses and their relatives are somewhat but not entirely refractory to alcohol's denaturation effects. While experiments have shown alcohol sanitizers to be slightly effective in the 1 Log10 range, with aggressive hand washing for 20 seconds followed by hand drying significantly more effective. For this reason, public health authorities warn against reliance on our normally trusted use of hand sanitizers and instead insist on vigorous hand washing to prevent or reduce transmission. Norovirus is responsible for an estimated 25 million cases in the US each year and some 50,000 hospitalizations and 300 deaths. Studies on rates of infectious intestinal disease in the UK indicate that there around 3 million cases per year while worldwide noroviruses are believed to cause up to 90% of epidemic nonbacterial gastroenteritis. Learn more about Norovirus including guidelines for healthcare providers and food handlers from the CDC.
Norovirus spreads rapidly in venues such as hospitals, hotels, casinos, daycare centers, long-term care facilities and domestic homes. Hospital outbreaks are not infrequently involved in outbreaks resulting in ward closures to allow environmental cleaning to prevent further spread. Norovirus has been determined to have a very low infective dose of from 1 to 10 particles capable of causing infection and is excreted via diarrhea or projectile vomiting in very high numbers (6 to 7 Log10/ml). Vomiting is so forceful that the virus can be aerosolized and cause infection via inhalation although the vast majority of transmission occurs via contact with contaminated surfaces or contact with infected persons. Microbiological data show that projectile vomiting associated with norovirus infection may distribute up to 7 Log10 virus particles as an aerosol.
The likelihood of airborne norovirus transmission was demonstrated in an a famous outbreak that occurred in a concert hall and by using seat tickets from infected individuals a wide pattern of dissemination was shown. Similar outbreaks at other public venues show that air duct contamination is known to be an issue with this virus. Beside infected individuals, contaminated surfaces have been found to be the most likely factor responsible for sustaining a succession of outbreaks.
Norovirus has excellent survival characteristics in the environment, which means sanitizing of the environment with effective biocides is essential when concerned with preventing cross-contamination and reducing the risk of outbreaks. Hand washing along with environmental cleaning and sanitization are the major interventions. A wide variety of biocides including Virox Accelerated Hydrogen Peroxide (AHP) products and household bleach have been approved and registered with the US EPA for use as surface sanitizers.
Infection is accomplished by direct fecal–oral transfer, by ingestion of aerosols or by transfer to foods by infected food handlers that are eaten without further cooking. In 2002 the Caribbean and Mediterranean cruise ships fleet was hit with successive waves of norovirus outbreaks. Interventions required focusing on appropriate hygiene measures after attacks of diarrhea and vomiting. For surfaces where there was significant soiling with infected fecal or vomitus, sanitation squads smothered soils with disposable paper bath mats that were then saturated with disinfectant for easier removal and disposal before surfaces were fully disinfected with an appropriate compound. For other ‘secondary’ contact surfaces such as door and tap handles, stair railings, poker chips and everything else commonly touched by passengers, crews were assigned to continually roam the ships wiping these contact surfaces with disinfectant. For cleaning soiled surfaces after gastroenteritis incidents, the use of disposable gloves are highly recommended. Food handlers must be cleared for return to work only after the period of active shedding is over.
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