Coronavirus: are cruise ships really ‘floating Petri dishes’?

People fishing in front of the Diamond Princess cruise ship - Tomohiro Ohsumi/Getty Images
People fishing in front of the Diamond Princess cruise ship - Tomohiro Ohsumi/Getty Images

Often dubbed “floating Petri dishes” cruise ships are notorious as breeding grounds for bugs. And the Diamond Princess cruise ship has played up to this stereotype with more than 540 people – out of around 3,600 passengers – testing positive for the coronavirus, now officially known as Covid-19.

Among the passengers on the ship are British holidaymakers David and Sally Abel, who have been documenting their increasingly miserable quarantine on social media and slammed what they see as the UK government's neglect of its citizens trapped on the vessel.

But why has the disease proved so virulent on board the ship? Kentaro Iwata, a professor of infectious diseases at Kobe University, took to YouTube to reveal his fear of contracting the virus amid “chaotic” conditions when he inspected the ship this week.

He said the ship was “completely inadequate in terms of infection control”.

“There was no distinction between the green zone which is free of infection and the red zone which is potentially contaminated,” he said. Professor Iwata said he had handled outbreaks of Ebola in Africa, severe acute respiratory syndrome in China and cholera around the world and had never been so afraid for his safety.

“But inside Diamond Princess I was so scared of getting Covid-19 because there was no way to tell where the virus was, no green zone, no red zone,” he said.

Coronavirus live map
Coronavirus live map

The United States Centers for Disease Control and Prevention (CDC) was also critical of the quarantine measures, saying while they may have slowed down the spread of the disease they “may not have been sufficient to prevent transmission among individuals on the ship”.

Cruise ships are well known for outbreaks of the vomiting bug, the norovirus, with 227 passengers and crew on board the Royal Caribbean International’s Oasis of the Seas – one of the world’s largest cruise ships – struck down by the illness last year.

Former medical director of P&O Cruises Dr Kate Bunyan says the reason for disease outbreaks on the high seas is simple – people spend a lot of time cooped up with each other.

“One of the challenges of a cruise ship is that it is an enclosed environment – like a prison or a hospital – where people come together and associate with each other for prolonged periods of time,” she said.

“People come from a diverse set of backgrounds, geography and age groups. There is an increased risk of illness being able to be spread,” she said.

“If you look back at the beginning of the outbreak [on the Diamond Princess] it was related to someone who had been on ship and, on leaving, was tested and found to be positive. That person would have been spreading the virus unknowingly,” she said.

Dr Nathalie MacDermott, clinical lecturer at King’s College London, said the failure of the quarantine measures on board the ship was unusual and warranted a full investigation.

Sally and David Abel have been documenting their quarantine via Facebook  - David Abel/PA
Sally and David Abel have been documenting their quarantine via Facebook - David Abel/PA

“While cruise ships have previously been associated with outbreaks of norovirus infections, once affected individuals are isolated in their cabins until symptoms resolve the outbreak is usually contained,” she said.

Dr MacDermott questioned whether there had been some kind of environmental contamination through the recirculation of air between cabins or the mixing of waste materials from different cabins.

Dr Bunyan, now medical director of Doctor Care Anywhere, said it would be unlikely for the disease to spread through ventilation or air conditioning as most systems were highly sophisticated with fine mesh that would prevent droplets entering cabins.

She also questioned the stereotype that cruise ships are a breeding ground for bugs, saying that liners are governed by strict guidelines with many tightening up their infection control policies in recent years.

“In the cruise industry you start measuring outbreaks when one per cent of the population on board are infected – you don’t need a lot of people to be ill for the disease to be taken incredibly seriously,” she said.

If a norovirus outbreak does occur the crew would introduce a range of infection control measures such as removing self-service buffets, ensuring food is only consumed in restaurants and implementing quarantine for affected passengers.

Other measures include removing all printed literature – as paper can’t be sanitised effectively – and throwing away playing cards after every game, through to cleaning handrails continuously and monitoring the use of handwashing facilities after people use the bathroom.

“The most important thing when you have an outbreak is good hand hygiene but when you have people that don’t take that as seriously that can be a problem,” said Dr Bunyan.

She also said that because Japanese health officials were actively looking for cases, it was not surprising that many passengers were testing positive for the virus.

Dr Mike Ryan, the World Health Organization's executive director of the health emergency programme, urged calm.

"It's an unfortunate event," he said. "A good number of these passengers have very mild symptoms so we need to put this in perspective," he said.

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