Last updated March 8, 2018 at 11:56 am
New mathematical modelling suggests that the crew are to blame.
You’re on a cruise ship with 2,000 people sailing through the warm tropical waters of the Caribbean. There’s an all-you-can-eat self serve buffet. Conditions are perfect. If you’re a norovirus.
A quick search of recent news can find case after case of gastro outbreaks on cruise ships. There’s a current class action case being brought against a company in Australia that has had back to back outbreaks of norovirus between 2016 and 2017.
Cruises provide the perfect conditions for an outbreak
Cruising is a $100 billion a year industry, with about 25 million passengers annually. The problem is that cruise ships are a “perfect storm” for brewing norovirus “gastro” outbreaks, with the semi-closed system of the ship, the shared living and activity spaces and communal eating areas perfect for spreading disease.
Despite the huge economic and health costs of norovirus, estimated at $500 million a year in the US, there’s still not a commercially available vaccine or specific drug treatment. Prevention remains the best medicine, through good sanitation and hygiene.
Norovirus spreads from person to person
A team of mathematicians from Arizona State University used data from a cruise ship outbreak in the early 2000’s that sickened almost 600 people to model how norovirus spreads. There were two cruises, but the crew remained the same between cruises. Norovirus outbreaks, caused by the same strain of virus, occurred in both cruises.
“Our model indicates that person-to-person contact is the primary mode of transmission,” says Sherry Towers, the study’s lead author.
Related: How to avoid norovirus
The model also showed that the crew were acting as a reservoir for infection, triggering new outbreaks when a fresh batch of passengers embarked. The authors point out that the model suggests that only 57% of the crew affected seek treatment for norovirus, for fear of missing out on wages if they report feeling ill and then being isolated.
Isolating infected people can help reduce the size of an outbreak, but is not enough to stop one completely. In reality, there is usually some time before people start feeling ill and actually going into a quarantine situation.
Similarly, keeping things clean is important, but for norovirus it’s just not enough. It only takes a few virus particles to cause an infection. Wiping surfaces with detergent can just serve to move virus particles around rather than killing or removing them. The model used in this study showed that even in the perfect case scenario where you could clean everything to the point it did actually eliminate all the virus from the environment you’d reduce the rate of infection from the first to subsequent cruises by 3-59%.
Wash your hands!
The solution is a simple idea – just wash your hands. The most disturbing part of the faecal-oral route, apart from it’s name, is the fact that it’s usually your hands that get the bugs from the one location to the other.
The studies authors point out that “passengers with a lackadaisical attitude towards hand hygiene were more likely to be infected with norovirus during an outbreak.”
Previous norovirus outbreaks at scout jamborees where they implement strict handwashing protocols reduced the infection transmission by 85%.
“However, unlike children at a jamboree, who can be forced by adult authorities to wash their hands before eating, overcoming long-standing poor hygiene habits among some adults on a cruise ship can be quite a challenge” write the authors.
So don’t be that person that doesn’t wash their hands after using the bathroom. Previous research has shown that the public are terrible at handwashing hygiene, for example one study finding that only 61% of women and 37% of men washed their hands with soap after using the toilet.
The authors suggest you scrub up with warm soapy water for at least 20 seconds before eating food, as well as after using the toilet or touching a potentially contaminated surface.
“When your mother told you to always wash your hands before coming to the dinner table,” says Towers, “she was right.”
This research appeared in Royal Society Open Science.
