Norovirus infection may be under-recognized

October 2006

The incidence of norovirus outbreaks in hospitals may be underreported, according to Eileen L. Yee, MD.

A recent study by the Centers for Disease Control and Prevention and Georgia Division of Public Health suggests that hospital outbreaks of nosocomial acute gastroenteritis (AGE) are under-reported in Georgia and the data suggests that the true burden from outbreaks may be underestimated.

Norovirus affected more patients and staff and lasted longer than other causes of AGE, “which may actually pose a large economic burden on our health care system,” said Yee, an investigator in the nosocomial AGE outbreak study, during the 2006 Annual Meeting of the Association for Professionals in Infection Control and Epidemiology in Tampa, Fla.

U.S. outbreaks

Based on unpublished CDC data from outbreaks that the CDC Calicivirus Lab assisted with, Yee said 4% of norovirus outbreaks in the United States occurred in acute care hospitals and 23% in long-term care facilities, which is vastly different from surveillance data reported other countries such as England and Wales.

In collaboration with the Georgia State Division of Health, the CDC investigated the frequency of AGE outbreaks in Georgian acute care hospitals in an attempt to characterize the outbreaks and assess reporting of these outbreaks.

A total of 163 infection-control practitioners were asked to fill out surveys from Jan. 1, 2002, until June 30, 2004. The survey results were then compared with the Georgia Division of Public Health database, a passive surveillance system.

An outbreak was defined as two or more related cases at a time or location clustering and illness among staff or patients hospitalized for more than 72 hours with either an acute onset of diarrhea (liquid stools for longer than 12 hours), with or without vomiting or fever. Or, two or more of the following symptoms: nausea, vomiting, abdominal pain and headache, excluding all other noninfectious causes.

Thirty-four percent of the 163 hospitals responded. Of those hospitals, 15% reported having at least one AGE outbreak, for a total of 11 outbreaks, according to Yee, a second-year Epidemic Intelligence Service officer at the Respiratory and Enteric Viruses Branch at the CDC. Since Hurricane Katrina in August 2005, she has investigated the outbreak of norovirus among hurricane evacuees in Houston.

The two of the 11 outbreaks were originally reported with unknown etiologies, but application of the Kaplan criteria revealed they strongly resembled norovirus.

Three norovirus outbreaks were identified out of the 11 total. Predominate clinical features included vomiting, nonbloody diarrhea and fever, with the average duration of individual illness lasting from one to two days.

In contrast, patients included in the five outbreaks of Clostridium difficile reported only fever and diarrhea. No bloody diarrhea was reported in any of the outbreaks.

All three outbreaks of norovirus affected patients and staff, whereas the C. difficile outbreaks affected only patients. The average duration of the norovirus outbreak lasted longer than 15 days. These outbreaks all resulted in either a ward closure, staff sick leave, relocation of case patients and/or patient discharge delays.

Conversely, the C. difficile outbreaks caused little relocation of case patients and few discharge delays.

The CDC concluded from the study that this survey was able to capture more outbreak data when compared with the Georgia Division of Public Health database. However, Yee said this was a small sampling, low response rate and included retrospective data, so there were limitations.

However, according to Yee, it is possible to conclude from the study that hospital outbreaks of AGE caused by norovirus are underestimated in Georgia, as is the true burden from outbreaks.

Control measures

Control measures used by the CDC during norovirus outbreaks included education of all staff, including food handlers, on norovirus infections and how they are transmitted and work restrictions for ill staff. Interventions for handwashing, prompt cleaning and disinfecting with effective products, isolation and cohorting of ill patients and also ward/unit closure.

“I know this can be extremely costly and also dramatic, but sometimes you need to do that, especially when the outbreaks have been widespread and prolonged,” Yee said.

Proper waste management should not be overlooked, according to Yee. Vomitous droplets can be aerosolized in an attempt to quickly clean up everything, which can lead to further spread.

Regarding cleaning, bleach is the recommended agent against norovirus on environmental surfaces, according to Yee, adding that the correct concentration of bleach is important; 1,000 ppm, even with a shorter contact time appears to be more effective. In contrast, a dilution of 100 ppm was not effective. And there are other products that have been approved by the EPA that may be less corrosive to surfaces; however, more studies on their effectiveness is needed.

What’s next?

The biggest issue currently is finding an effective cleaning and disinfecting agent against the human norovirus agent because the studies to date have been with the feline version of norovirus, according to Yee.

Norovirus assays for food and a rapid detection assay for use in hospitals are reportedly both in development. A vaccine may eventually be available, according to Yee.

Epidemiology

Norovirus, formerly known as Norwalk virus, originated in Norwalk, Ohio. Norovirus occurs among all age groups.

In the original outbreak from which the virus was discovered, 50% of 232 students and teachers suffered from AGE. There was a 30% secondary attack rate. At that time no vehicle or etiologic agent was found, according to Yee. In 1972, Kapikian and colleagues were the first to see noroviruses under electron microscopy.

Norovirus is transmitted through multiple routes such as contaminated food and water, direct person-to-person contact and also through large vomitus droplets and contaminated surfaces. They are stable in the environment and diagnostics are not always available.

Clinical presentation includes acute onset of nausea, vomiting, low-grade fever and/or abdominal cramps. There are multiple routes of transmission but the primary mode is fecal/oral. Incubation period tends to be 24 to 48 hours. It is a self-limiting illness lasting 12 to 60 hours.

Differing from rotavirus, generally norovirus affects all ages and some studies have shown asymptomatic infection.

Norovirus outbreaks tend to be widespread with high attack rates and immunity isn’t long lasting, so reinfection is common. Norovirus outbreaks are frequently seen on cruise ships, schools, camps and shelters and nosocomial outbreaks are seen in long-term care facilities and hospitals.

People infected with norovirus secrete 10⁴ to 10⁶ viruses per gram of stool and as little as 10 to 100 particles can cause infection. As with rotavirus, immunocompromised patients with norovirus tend to shed virus longer periods.

A diagnosis is based on history and clinical presentation. However, confirmation relies on send-out laboratory results. State laboratories and the CDC use reverse-transcriptase polymerase chain reaction to diagnose norovirus.

Treatment is supportive with oral rehydration and IV fluids. At this time, no vaccine is readily available.

Challenges associated with noroviruses include the lack of cell culture or animal model and most studies have relied on volunteers. There is no rapid diagnostic assay available in hospitals.

Many studies have used the feline version of norovirus as a surrogate and these have been used for studies of effectiveness of disinfectants.

Because of the challenges faced with diagnosing norovirus outbreaks, Kaplan developed criteria in 1982 as a diagnostic tool. The criteria included more than 50% affected people reporting vomiting, mean incubation time of 24 to 48 hours, mean duration of illness from 12 to 60 hours and no other enteric pathogens detected in stool specimen.

For more information:

• Yee EL. GI outbreaks in healthcare. Session 3504. Presented at: 2006 Annual Meeting of the Association for Professionals in Infection Control and Epidemiology; June 10-14, 2006; Tampa, Fla.