Noroviruses, a leading cause of highly disruptive gastroenteritis outbreaks in hospitals and nursing homes, may transmit through the air, meaning currently recommended contact precautions may not be completely effective at stopping spread to patients and staff, researchers report in a fascinating new study.1
Though self-limiting and rarely fatal, norovirus can spread between patients and healthcare workers, leading to furloughed staff and costly closings of entire units for environmental cleaning and disinfection. In addition to being one of the leading causes of outbreaks in hospitals and long-term care settings, norovirus strikes community settings like daycare and schools and, of course, is an infamous stowaway on luxury cruises. In a rather staggering observation, the Centers for Disease Control and Prevention (CDC) reports that someone infected with norovirus can shed “billions” of viral particles, but it takes as few as 18 viral particles to infect another person. The established routes of transmission include ingestion of stool or vomit from the infected, including through contamination of environmental surfaces, food, and water.
“It is possible for norovirus to spread through aerosolized vomit that lands on surfaces or enters a person’s mouth then he or she swallows it,” the CDC notes. “There is no evidence showing that people can get infected by breathing in the virus.”
A team of Canadian researchers set out to provide that evidence, suspecting that the nature of the outbreaks could mean another route of transmission.
“Our [airborne] hypothesis came from the fact that norovirus infections are very difficult to contain. This virus is very contagious and in some occupational settings, hundreds of people will catch the virus even if they were never in direct contact with sick patients,” says Caroline Duchaine, PhD, lead author and professor at Université Laval’s Faculty of Science and Engineering and researcher at the Québec Heart and Lung Institute (IUCPQ) Research Centre.
Duchaine and colleagues conducted the study at eight hospitals and long-term care facilities during norovirus gastroenteritis outbreaks. They gathered air samples at a distance of one meter from patients, at the doors to their rooms, and at nursing stations. Noroviruses were found in the air at six of the eight facilities studied. The viruses were detected in 54% of the rooms housing patients with gastroenteritis, 38% of the hallways leading to their rooms, and 50% of nursing stations. Virus concentrations ranged from 13 to 2,350 particles per cubic meter of air. Although norovirus is an intestinal pathogen, noroviruses could be transmitted through the airborne route and subsequently, if inhaled, could settle in the pharynx and later be swallowed, the authors theorized.
“We decided to study the viral load in healthcare settings while patients were actively sick with norovirus infections,” Duchaine says. “[We found] it can be inhaled through the mouth or the nose. If the particle is big enough -- too big to enter the lungs -- it impacts on the surface of the nose, trachea, mouth, and natural clearance would bring it to be swallowed. This is the route we think may be happening, although it is not proven.”
In vitro studies were performed to evaluate the preservation of the aerosolized norovirus infectious potential, revealing that the virus can withstand aerosolization with no significant loss of infectivity. “We tested the resistance of a murine norovirus to aerosolization -- this includes drying -- and this virus is very resistant to desiccation,” she says. “The literature states that this virus can survive for long periods on surfaces.”
Though the particles could be inhaled and swallowed to find their way to the gut, the researchers dismissed the notion that infected patients would actually be exhaling an enteric virus. “The main hypothesis is from fecal matter or vomiting,” Duchaine tells Hospital Infection Control & Prevention. “We did not take breath samples.”
Duchaine and colleagues note that – in addition to vomiting -- norovirus aerosols can result from the viral resuspension from fomites, flushing toilets and the actions of healthcare workers.
“It is well described that vomit and diarrhea contain large quantities of noroviruses,” she says. “When sick persons vomit or excrete diarrhea, aerosols are produced. Droplets will likely dry and form what is called droplet nuclei. Water evaporates and the content of the droplet will concentrate to form a smaller aerosol that will remain airborne. Our hypothesis is that such aerosol particles could [then] be inhaled and swallowed.”
The authors observe that the detection of significant concentrations of norovirus genomes in the air of corridors and nursing stations suggests that the virus can remain suspended for prolonged periods. If an airborne spread is occurring the typically recommended measures of contact isolation, frequent hand hygiene with soap and water and environmental disinfection would not be sufficient to stop all routes of transmission.
“The measures applied in hospital settings are only designed to limit direct contact with infected patients,” Duchaine says. “In light of our results, these rules need to be reviewed to take into account the possibility of airborne transmission of noroviruses. Use of mobile air filtration units or the wearing of respiratory protection around patients with gastroenteritis are measures worth testing.”
This study provides original quantitative data regarding the airborne dissemination of norovirus in healthcare facilities and documents for the first time widespread dissemination of norovirus in the air of healthcare facilities during gastroenteritis outbreaks, the authors note.
“Considering that an average human breathes approximately six liters of air per minute, a healthcare worker could inhale up to 60 copies of human norovirus during a 5-minute stay in the room of a symptomatic patient. For some individuals, this quantity could be sufficient to cause the disease,” they conclude.
- Bonifait L, Charlebois R, Vimont A, et al. Detection and quantification of airborne norovirus during outbreaks in healthcare facilities Clin Infect Dis Advance Access published April 21, 2015.