Although everybody wants to avoid the winter vomiting bug, this especially applies to those who are hospitalised. An outbreak on the ward strikes already vulnerable patients and may cause the postponement of planned operations and therapies – not to mention the loss of much-needed staff. During the winter of 2010-2011, 30% of all wards and 43% of medical departments in Skåne suffered from at least one norovirus outbreak.
“We saw from our studies that an average of six patients and four members of staff were affected in each outbreak, although in one case as many as 90 people were infected in a single outbreak,” says Carl-Johan Fraenkel, who is now presenting his dissertation at Lund University.
In order to be classed as an outbreak, at least two patients must be infected with the virus by a common source. One reason why norovirus is so infectious is that it requires very few particles to cause the illness – and in one gram of faeces there may be as many as 10 billion virus particles.
Among other things, Carl-Johan Fraenkel has mapped the factors that can affect whether an outbreak will occur, by comparing patients who had given rise to outbreaks to those that, despite having the illness, did not start one.
“We saw that if the patient is sharing a room with others, is over 80 years of age, has another illness, becomes ill on the ward/unit or vomits, the risk of an outbreak increases. When it comes to vomiting, we have also seen that a particular strain of norovirus, GII.4, caused more vomiting than other strains and also spreads more efficiently.”
Is there any need to worry about infection if one is placed in a room in which someone suffering from norovirus was previously staying?
“We studied data from everyone admitted to infectious disease wards in Skåne over a five-year period – a total of 30,000 patients, 50 of whom were infected on the ward – and were thereby able to calculate the risk catching the winter vomiting bug. That risk is 0.2%. If you are placed in a room in which someone with norovirus has previously lain, the risk rises to 0.7%. So, it appears that there is a slight extra risk of infection depending on the room one is placed in and it should be possible to affect this with improved cleaning,” says Carl-Johan Fraenkel.
While working as an infectious disease specialist in Blekinge, Carl-Johan came across a few isolated incidents of norovirus outbreaks; however, something happened.
“When I came to Lund in 2010, i was confronted by a great many outbreaks of norovirus and there was not very much research being conducted into its spread. As I was working in a hygiene unit with overall responsibility for 10 hospitals, I saw an opportunity to gather data as the basis for future studies.
According to Carl-Johan Fraenkel, the varying severity of outbreaks of norovirus from season to season is due to strains of the virus – GII.4 for example – that have the ability to mutate at regular intervals. If the same variant of the disease presents in two consecutive years, the season will be quieter; however, if the virus has ‘upgraded’ itself, it can become worse.
Dry air, greater spread
There are relatively few studies into how norovirus spreads through the air, but together with researchers from the Faculty of Engineering at Lund University, Carl-Johan collected air samples from infected patients. With the aid of a cyclone – a piece of equipment much like a vacuum cleaner – researchers sucked air into a water reservoir for analysis of its particulates.
“We saw that norovirus is more common in the air during or before an outbreak on the ward, and most common of all if the patient had recently vomited. We cannot really demonstrate causation but it may be that airborne infection is more common than we previously thought. And, as I said, it doesn’t take many virus particles to infect someone – only a brief inhalation may be sufficient,” states Carl-Johan Fraenkel.
Particle size is significant
The size of virus particles was also significant, something that can vary due to humidity.
“In dry air, virus particles are generally smaller and more readily airborne. At higher humidity, the drops fall to the floor more rapidly. So, during the winter, the virus should be more easily spread in the air after vomiting. The infectiousness of the virus is therefore simply explained by the diseases popular name: the winter vomiting bug.”
The fact that the norovirus appears to have the ability to spread via the air may be the reason that staff are so often infected; simply washing one’s hands is insufficient in itself.
“This raises the question of whether the healthcare service provides the correct training and equipment to staff in terms of procedures for norovirus. Hands are the most important infectious agent for viral diseases but there is reason to consider whether ventilation is sufficient and if we can better protect staff and fellow patients from infection. This does however require further study.”
Tove Smeds