Mar 4, 2011 (CIDRAP News) A new report from the Centers for Disease Control and Prevention (CDC) makes clear why norovirus is the world's leading cause of gastroenteritis and the leading cause of foodborne disease outbreaks in the United States.
The virus is so contagious that one food handler with contaminated hands can infect hundreds of people, the report says. Those who have had the virus may have little protection against future infection.
To compound the problem, recovering patients continue to shed the virus in stool for weeks. People also can become infected and pass the virus to others without having any symptoms themselves.
A decade's worth of findings about norovirus is summarized in the CDC report, published yesterday in Morbidity and Mortality Weekly Report. It's the first update of the agency's norovirus recommendations since 2001. The agency says the update reflects substantial advances in norovirus epidemiology, immunology, diagnostic methods, and infection control in recent years.
Norovirus symptoms typically include nonbloody diarrhea, vomiting, nausea, abdominal cramps, and, in some cases, low-grade fever and body aches, the report notes. The illness usually sets in between 12 and 48 hours after exposure. Otherwise-healthy people usually recover after 1 to 3 days, but the symptoms can last longer in young children, elderly people, and hospital patients. About 10% of those who get sick seek medical care.
US has 21 million cases annually
Recent improvements in diagnostic methods have clarified the role of norovirus as the leading cause of gastroenteritis in all age-groups, the report says. The CDC estimates that the virus causes about 21 million illnesses in the United States each year, about a quarter of which involve foodborne transmission.
US and European findings show that norovirus is the cause of about 50% of all reported gastroenteritis outbreaks. Outbreaks occur year-round but are more common in winter, the report says.
The virus spreads via food, water, and person-to-person contact. "Norovirus is extremely contagious, with an estimated infectious dose as low as 18 viral particles, suggesting that approximately 5 billion infectious doses might be contained in each gram of feces during peak shedding," the document states. The latest data suggest that the person-to-person route is most common.
As is well-known, outbreaks are common in institutions and other group settings. Of 660 outbreaks in known settings that were confirmed by the CDC from 1994 to 2006, 35.4% were in long-term care facilities, 31.1% were linked to restaurants and parties, 20.5% were in vacation settings including cruise ships, and 13% were in schools and communities.
The virus accounted for 822 (35%) of 2,367 US foodborne disease outbreaks in 2006-07, the report says. Because a tiny dose of the virus is enough to cause infection, one person can contaminate large amounts of food, as was illustrated by one outbreak in which about 500 cases were linked to a food handler who vomited at work.
Infected persons continue shedding the virus in stool for an average of 4 weeks, the CDC reports. But because it's not possible to grow the virus in cell cultures and there is no small-animal model for human infection, no one knows exactly how long a person remains contagious.
"Furthermore, up to 30% of norovirus infections are asymptomatic, and asymptomatic persons can shed virus, albeit at lower titers than symptomatic persons," the report adds. "The role of asymptomatic infection in transmission and outbreaks of norovirus remains unclear."
Immunity poorly understood
The report says immunity to norovirus is still a murky area. In studies, volunteers who had had the illness remained susceptible to infection with the same strain as well as other strains, and those who had preexisting antibodies "were not protected from infection unless repeated exposure to the same strain occurred within a short period."
On the other hand, some volunteers who were exposed to the virus seemed to remain uninfected, the report relates. Thus, both innate host factors and acquired immunity are thought to influence susceptibility to infection.
For identifying noroviruses, most clinical virology labs use real-time reverse transcription polymerase chain reaction, and partial capsid sequencing can be used to identify specific strains, according to the report. It says enzyme immunoassay (EIA) kits that yield fast results offer "moderate performance" and may be useful for preliminary screening of samples in outbreaks.
Hand hygiene is best defense
Hand hygienechiefly through washingis probably the single most important tool for preventing infection, the CDC says. The evidence for the effectiveness of alcohol-based and other hand sanitizers is mixed. Excluding and isolating infected persons are usually the most effective ways to stop outbreaks and limit environmental contamination. Sick healthcare workers and food handlers should stay off the job until 48 to 72 hours after their symptoms fade, the agency recommends.
For environmental disinfection, the report says the best bet is a chlorine bleach solution. Products that contain phenolic compounds, such as triclosan and quaternary ammonia, are less effective against nonenveloped viruses like norovirus.
The CDC says individual cases of norovirus are not nationally notifiable, but all outbreaks should be reported to local or state health departments. The National Outbreak Reporting System (NORS) was launched in February 2009 as an integrated surveillance tool for enteric disease outbreaks. Also in 2009, the CDC launched CaliciNet as a network that labs can use to share and compare norovirus sequences in order to identify transmission vehicles and emerging strains.
CDC. Updated norovirus outbreak management and disease prevention guidelines. MMWR Recommendations and Reports 2011 Mar 4;60(RR03):1-15 [Full text]