Will emerging norovirus become nosocomial bug?

Will emerging norovirus become nosocomial bug?

Problem for cruise ships, nursing homes

Emerging as the bane of cruise ships and nursing homes, norovirus — with its ability to cause severe gastroenteritis, persist in the environment, and spread via contaminated food or human contact — would seem to be the perfect candidate for a nosocomial pathogen of the most troublesome variety.

The question is, why isn’t it?

"That is an extremely interesting question," says Marc-Alain Widdowson, DVM, a medical epidemiologist in the respiratory and enteric viruses branch at the Centers for Disease Control and Prevention (CDC).

"It is in certain countries. In the United Kingdom, for instance, and in Canada, they have a lot of problems with hospital outbreaks with norovirus in the winter.

"In the UK, they have to shut down hospitals regularly because of it. In the United States, we don’t seem to hear about outbreaks as much," he explains.

Widdowson recently published an article in the CDC’s Emerging Infectious Diseases journal, which argues the norovirus is an emerging pathogen.¹ Still, he cannot quite explain why noroviruses are not being reported in more nosocomial outbreaks in U.S. hospitals. Is it a surveillance artifact — a case of not finding something you’re not looking for?

"We weren’t quite sure whether this was because there is no reporting of nonfoodborne [gastroenteritis] outbreaks to CDC or whether it was actually a genuinely low incidence," he tells Hospital Infection Control.

"In the UK, for instance, if you have an outbreak in a hospital — because it is a national health [system] — it has to be reported. There is a reporting system for all outbreaks of gastroenteritis. In the U.S., there is only an official mechanism for foodborne [outbreaks of gastroenteritis]. So our concern was that there actually are hospital outbreaks, but we just don’t hear about them, at least through formal channels," Widdowson notes.

But a CDC survey of hospitals in Georgia suggests the pathogen is not spreading in hospitals to the degree it has been found in other countries. "Sure, there were some [outbreaks] that haven’t been reported to the state, but the general feeling was that there does seem to be fewer outbreaks of norovirus illness in U.S. hospitals than in the UK and Canada," he said.

Still, the CDC is concerned enough about noroviruses emerging as a nosocomial pathogen that it posted guidance for health care facilities on the web site of its Division of Healthcare Quality Promotion. (See guidance, below.)

Noroviruses are highly contagious, with as few as 100 virus particles thought to be sufficient to cause infection.

Noroviruses are transmitted primarily through the fecal-oral route, either by direct person-to-person spread or fecally contaminated food or water.

In health care facilities, transmission additionally can occur through hand transfer of the virus to the oral mucosa via contact with materials, fomites, and environmental surfaces that have been contaminated with either feces or vomitus, the CDC advises.

Patients who have suspected norovirus infection should be managed with standard precautions with careful attention to hand hygiene practices.

However, contact precautions should be used when caring for diapered or incontinent people, during outbreaks in a facility, and when there is the possibility of splashes that might lead to contamination of clothing, the CDC recommends.

No longer Norwalk

In 1972, noroviruses — previously called "Norwalk-like viruses" — were discovered as the first viruses definitively associated with acute gastroenteritis, Widdowson explains.

But for years, researchers were unable to easily detect the virus or to find the etiologic agents of nonbacterial gastroenteritis outbreaks and hospitalizations, he notes.

"Until the mid-1990s, most of the diagnoses were based on complicated tests [such as] electron microscopy," Widdowson continues. "So lab confirmation was rare before the mid-1990s."

Not unlike the old diagnosis of exclusion for non-A, non-B hepatitis (before it was identified as hepatitis C), norovirus was identified by epidemiological clues and the absence of a bacterial agent.

"A lot of the diagnosis was based on epidemiological and clinical criteria, like if you have more than 50% of people vomiting, a certain incubation period, a certain duration of illness, and no bacteria are found, then you can attribute the infection to norovirus," Widdowson explains.

"For many years, that is what people used. Beginning in the mid-1990s [diagnostics improved so] that we were able to say for sure that this outbreak was due to norovirus because we actually detected it," he adds.

Indeed, of more than 2,500 foodborne outbreaks reported to the CDC from 1993 to 1997, less than 1% were attributed to noroviruses, and 68% were listed as "unknown etiology," Widdowson found.

"Over the last 10 years, there has been an expansion in the use of [new diagnostics], and more and more state public health authorities are using this technology," he says. "More and more specimens are being tested at the local level."

Common cause of gastroenteritis

As a result, noroviruses now are recognized as the most common cause of infectious gastroenteritis among people of all ages, Widdowson notes in his paper.

They are responsible for around 50% of all foodborne gastroenteritis outbreaks in the United States, a major contributor to illness in nursing homes and — to a less extent — hospitals. Norovirus infection has put apparently healthy people in intensive care and has been associated with chronic diarrhea among transplant patients, he explains.

But is it truly an emerging pathogen or merely one that has been an elusive etiologic agent? Widdowson points to the fact that there are myriad strains of norovirus, which have been classified into five genogroups.

The diversity "represents a dramatic increase from the single calicivirus strain discovered more than 30 years ago," he writes in the article.

In addition, the viruses are being found in an expanding array of animal hosts including mice, cows, and pigs. Antibodies to bovine strains have been found in humans, raising speculation that some zoonotic transmission may be occurring, Widdowson points out.

Another important factor is the emergence of predominant norovirus strains. The so-called Farmington Hills strain plagued cruise ships in 2002 and 2003 with the grim persistence of water rising up through the decks of the Titanic.

"What we find in cruse ships — and often with nursing homes and hospitals — is that you can close down, you can dry-dock a cruise ship for a week, put a whole bunch of new passengers on it, and the infection will still be on board," he says.

"Somehow it is in the environment. The new cohort of passengers will get sick. Similarly, with nursing homes, the virus is very persistent in the environment and can stay around for one to two weeks and act as a source of infection," adds Widdowson.

Hard to kill

The ability of the viruses to persist in the environment is explained in part by the fact that it is not phased by some commonly used disinfectants. The CDC reports, for example, that because noroviruses are nonenveloped, most quaternary ammonium compounds (which act by disrupting viral envelopes) do not have significant activity against them.

With no U.S. disinfectant approved as having specific activity against the virus, the CDC currently advises the use of chlorine bleach (1 part household bleach diluted with 50 parts water) for cleaning environmental surfaces during a norovirus outbreak. Another factor is that the viruses may become so pervasive in the environment during an outbreak that small reservoirs persist despite cleaning.

"If you have viral contamination everywhere on lots of surfaces, it is extremely difficult to make sure that you have cleaned all of the surfaces that people are going to touch," Widdowson says.

Though an obvious cause of morbidity, another unknown is to what degree norovirus infections actually may lead to attributable mortality in nursing homes.

"We haven’t really been able to nail down mortality in nursing homes, but it is something we are interested in doing by looking at hospital discharge dates and looking at gastroenteritis in the elderly," he says.

"Most gastroenteritis in the elderly, I suspect, would be due to norovirus. So it would be interesting to look at deaths and hospitalizations due to norovirus in the elderly," Widdowson explains.

He points out that if noroviruses are an increasingly common cause of infectious gastroenteritis — with some cases resulting in diarrhea-related deaths and hospitalizations — then substantially greater investments are required in their diagnosis.

Increased use of diagnostics along with improved surveillance, such as in sentinel sites, will permit identification of new strains and shifts in the epidemiology of norovirus disease, Widdowson argues in his paper.

The development of easy-to-use, sensitive assays for use by clinical and public health laboratories also should have a high priority, he urges.

"Putting it bluntly, it is an appeal for resources to try and address it," Widdowson tells HIC. "Particularly, I think one thing I tried to address in the article is control measures.

"Person-to-person outbreaks are extremely difficult to control. The virus, unlike influenza, will [not only] spread from person to person but also stay in the environment. It is extremely difficult to get rid of when you have an outbreak that is ongoing," he adds.

Reference

1. Widdowson M-A, Monroe SS, Glass RI. Are noroviruses emerging? Emerg Infect Dis [serial on the Internet]. May 2005. Web site: www.cdc.gov/ncidod/EID/vol11no05/04-1090.htm.