Disease
Norovirus infection

Author           
Kristina Garner, BIOL 230, Spring 2009

Causative Agents
Noroviruses are a non enveloped virus, possessing a positive-sense single stranded RNA genome. Structurally the viron capsid is of icoshedral (def) symmetry in shape and typically range in size from 27-38 nanometers (def). Norovirus belongs to the family Caliciviridae. Noroviruses are very common viral causes of gastroenteritis (def).

Epidemiology
Norovirus previously known as Norwalk virus and currently known as Norwalk like viruses are responsible for between 60- 93% of non bacterial gastroenteritis (def) in industrialized countries. The name norovirus stems from its original name and most common form, the Norwalk virus, which was named after an outbreak occurring in Norwalk, OH, in 1968.  Noroviruses are very contagious and need as few as 10 viral particles achieve infection. Typically, noroviruses spread very quickly in large institutions. According to the Center for Disease Control approximately 23 million cases of gastroenteritis are caused by noroviruses in U.S annually. 
           
There are several genotypes of noroviruses (27). Some cause infection to humans and others to bovine animals (cows and pigs), and mice. There are 5 genogroups classified under noroviruses: genogroup I, genogroup II, genogroup III, genogroup IV, and genogroup V. Typically those that affect humans are from GI, GII and GIV. Some examples of GI and GII noroviruses strains most common in humans are: Norwalk virus (GI), Desert Shield virus (GI), Southampton virus GI, Bristol virus (GII), Lordsdale virus (GII), Toronto virus (GII), Mexico virus (GII), Hawaii virus (GII), and Snow Mountain virus (GII).

Viral specificity for noroviruses seems to be based on histo-blood group antigens (def) as receptor cites within gastro-epithelial cells. Particular genotypes of noroviruses pick host cells based on histo-blood group antigens. Viral specificity seems to be strain specific. Noroviruses affect all age groups but are mostly found in school age children and adults.

Transmission
Most profoundly noroviruses are transmitted via fecal-oral transfer, often person to person, or by food born contaminants from infected food handlers. Approximately 50% of all noroviruses are transmitted via food born contaminants found in uncooked foods. Some outbreaks have been traced back to contaminated water source during high flood periods due to run off of sewage into groundwater and well water. Aerosolized particles from emeses have been theorized as another source of transmission, although this has not yet been proven.

Signs and Symptoms
The most common symptoms associated with noroviruses are nausea, vomiting, low grade fevers, stomach ach, diarrhea (non-bloody), fatigue, head ach, and myalgia (def). Noroviruses are most commonly associated with gastroenteritis, specifically lesions on the jejunum (def). Possible complications arising from norovirus infections are sever dehydration mostly affecting infants, the old, and the immunosuppresed.

The average incubation period for noroviruses are one to two days although can show symptoms as early as 12 hours after infection. Upon infection symptoms last between one and three days typically. Infected individuals are typically prone to viral shedding from the onset of symptoms and for up to two weeks after recovery from infection. Second transmission occurs from viral shedding but is self limiting.

Prevention and Treatment
Currently there are no vaccines available for treatment of noroviruses. Noroviruses typically are very hardy and can resist freezing and temperatures as high as 60 C, pH of 2.7, and chlorine levels of .5-1.0 mg/l. Noroviruses are also resistant to ether, ethanol and detergent based cleansers. The only real means of control is prevention of contaminants by infected individuals. Treatment for sever cases is limited hydration and electrolyte replacement. Noroviruses are self limiting and rarely fatal. Hypochlorite at 5000 ppm (def) can work as a means to clean surfaces of viruses.

Bibliography

CDC. National Center for Immunization and Respiratory Diseases, Division of Viral Diseases. Norovirus: Technical Fact Sheet. Retrieved April 19, 2009 from http://www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-factsheet.htm.

Raphael Dolin.  (2007). Noroviruses -- Challenges to Control. The New England Journal of Medicine, 357(11), 1072-3.  Retrieved April 20, 2009, from Research Library Core database. (Document ID: 1335172031).

Microbe Wiki. A Microbial Biorealm page on the genus Norovirus. Last modified Dec. 19, 2008. Retrieved April 19, 2009 from http://microbewiki.kenyon.edu/index.php/Norovirus.

Wikipedia. Norovirus. Retrieved April 19, 2009 from http://en.wikipedia.org/wiki/Norwalk_virus#Epidemiology.2C_prevention_and_infection_control.