Feature articleNoroviruses: agents in outbreaks of acute gastroenteritis
Section snippets
Virology
Since the 1940s, viruses were suspected of causing gastroenteritis because the etiology of most cases could not be attributed to bacterial or parasitic agents. However, it was not until 1972, with the use of immune electron microscopy, that a virus was identified and clearly linked to an outbreak of diarrhea.5 The virus was named the Norwalk virus after the location of the outbreak in 1968 at an elementary school in Norwalk, Ohio. Viruses subsequently discovered were likewise assigned names
Epidemiology
Noroviruses are considered the most common viral etiologic agents of epidemic food-borne and waterborne viral gastroenteritis.15 Each year, approximately 23 million cases of gastroenteritis caused by noroviruses occur in the United States.2 According to records maintained by the Centers for Disease Control and Prevention (CDC), noroviruses were responsible for 96% of reported outbreaks of viral gastroenteritis from January 1996 to June 1997.16 However, because adequate viral detection methods
Mode of transmission
The primary route of viral transmission is the fecal-oral route or aerosol formation after projectile vomiting.20, 22 Norovirus is found in the stool and vomitus of infected persons. Primary infection results from the ingestion of fecally infected food or water.23 Secondary infection results via person-to-person contact, aerosolized vomitus, fomites, and infected food handlers.7 In one third of restaurant outbreaks, no food worker could be implicated, thus implying that food was contaminated
Pathogenesis and clinical features
Table 1 provides a summary of the signs and symptoms associated with norovirus infections. Clinically, patients present with abdominal cramps with or without nausea, vomiting, and/or diarrhea.9 Symptom development can be either gradual or abrupt. Adults generally demonstrate diarrhea as the predominant feature of the illness, whereas children experience nausea and vomiting more than diarrhea. Diarrheal stool is non-bloody, lacks mucus, and may be loose and watery.9 Symptoms most commonly
Treatment
There are no antiviral treatments available to treat noroviruses. Treatment focuses on supportive care, especially preventing and treating dehydration secondary to the disease. Hydration is usually maintained, using oral fluid replacement with isotonic liquids.4, 9 Symptomatic treatment for headache, myalgias, and nausea can be provided using analgesics and antiemetics.9 Bismuth subsalicylate has been found to decrease abdominal cramping, but has no effect on viral shedding.9, 29
Diagnostic methods
Routine laboratory tests (eg, white blood cell count) are not helpful in diagnosing norovirus infection.9 Advances in the diagnosis of noroviruses over the last 3 decades have progressed from the tests designed to identify the virus in stools using electron microscopy that require the presence of ≥ 106 virus/ml of stool19 to the current ability to identify the presence of scant amount of antigen in stool collected 1–2 weeks after the onset of illness using nucleic acid detection by Reverse
Specimen processing for the diagnosis of norovirus
Table 2 provides a summary of important steps used to collect specimens. The inability to cultivate the virus in cell lines is a major drawback in diagnosis of Norwalk-like virus.22 Endemic gastroenteritis due to Norwalk-like virus is often a self-limiting, acute diarrheal illness that resolves completely in less than 72 hours, and does not warrant laboratory tests for diagnosing the etiologic agent.
In contrast, the collection of timely and appropriate specimens becomes very important in the
Prevention and control
Understanding the mechanism of spread/transmission of noroviruses and their survivability and pathogenic potential have assisted greatly in developing guidelines for the prevention and control of spread of this virus. Table 3 lists important considerations for clinicians working with potential norovirus infections.
Noroviruses are highly contagious with a low infectious inoculum of < 100 viral particles.19 Direct spread from person to person from handling contaminated secretions or objects
Conclusion
Because long-term immunity to noroviruses does not result from infection, vaccine development is unlikely.9 However, as assays for noroviruses are improved and are more available, a better understanding of disease transmission should emerge. With improved understanding, it may be possible to develop new public health safeguards to limit the spread of the disease.1 Such safeguards may include changes in the tracing, screening, and recall of contaminated food products, closing of contaminated
Alice C. Thornton is an assistant professor in the Division of Infectious Diseases at Chandler Medical Center, University of Kentucky.
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Cited by (0)
Alice C. Thornton is an assistant professor in the Division of Infectious Diseases at Chandler Medical Center, University of Kentucky.
Karen S. Jennings-Conklin is an assistant professor in the Division of Gastroenterology at Chandler Medical Center, University of Kentucky.
Malkanthie I. McCormick is an associate professor in the Division of Infectious Diseases and the Hospital Epidemiologist at Chandler Medical Center, University of Kentucky.