YOKOHAMA, Japan — In the passenger decks of the quarantined Diamond Princess cruise ship, more than 2,500 travelers are carefully isolated. Meals are delivered to their cabins. They have permission to walk on deck, six feet apart, for a few minutes a day.
Down below, more than 1,000 crew members live and work elbow-to-elbow, preparing the passengers’ dishes and eating simple buffet-style meals together, with as many as four sharing a bathroom — and sharing the risk of possible infection from the coronavirus.
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The risks to all aboard were reinforced on Monday as Japan’s health ministry said that an additional 65 people had tested positive for the virus, nearly doubling the total to 135. According to Princess Cruises, which operates the ship, at least 20 of them are American. The ship already has the largest number of coronavirus cases outside the epicenter in China, where more than 40,000 have been infected.
While the quarantine slapped on the Diamond Princess was meant to contain the virus, the conditions facing the crew could end up doing the opposite and help spread the illness, disease experts say.
At least 10 crew members have been infected, with five cases announced on Sunday and five more on Monday. And according to employees, the infected crew members identified on Sunday had been eating in the mess hall alongside their co-workers.
It is not even clear how many people on the ship have the coronavirus because the authorities in Japan have only tested a fraction of all aboard, asserting they currently lack the resources to test them all.
Under a two-week quarantine in the port of Yokohama since Feb. 4, the Diamond Princess is now a floating, mini-version of Wuhan, China, the city of 11 million that is the epicenter of the outbreak and has been subjected to a lockdown for weeks.
The Diamond Princess is like a “small city,” Binay Kumar Sarkar, an Indian citizen who prepares meals and washes dishes in the galley, said Monday in a Facebook chat, making it “very easy” to spread the virus.
In a video posted to Facebook, he asked the Indian government to help get him and his co-workers off the ship before the virus spread further. The crew, overwhelmingly from developing countries in Asia, includes 132 Indians.
Unlike the passengers they serve, most from wealthy nations, the ship’s employees have not received the same global attention as passengers from countries like the United States, Australia and Britain, whose social media posts have been widely read.
In response to emailed questions, a representative of Princess Cruises, which operates the Diamond Princess, said that all crew members “are highly trained in safety and public health standards.”
Without offering specifics, the representative added that the company was “implementing processes developed in coordination with public health officials to support the elevated requirements of this situation.”
Some experts said that keeping all of the passengers and crew members on board could exacerbate the rate of infection. In the city of Wuhan and the surrounding province, Hubei, the authorities have barred close to 50 million people from leaving, and cases there are still rising as family members infect one another.
“Similar to the situation in Wuhan, but at a smaller scale, by quarantining the ship, the crew members are being forced to stay together, which increases the likelihood of transmission,” said John B. Lynch, an associate professor of infectious diseases at the University of Washington. “We have to remember that quarantines protect those outside the quarantine, not those within.”
Other experts said supervisors on the ship needed to enforce strict hygiene policies, including frequent hand-washing. Both passengers and crew members should also be “keeping distance from others and avoiding congregating,” said Dr. Peter Rabinowitz, who is co-director of the University of Washington MetaCenter for Pandemic Preparedness and Global Health Security.
Crew members, who spoke on the condition of anonymity for fear of losing their jobs, said they had been provided with masks, gloves and hand sanitizer, but given little training on how to reduce their chances of infection in a situation of this magnitude.
Like passengers, they have been given thermometers and told to monitor their own temperatures and report back if they develop a fever. They have received no new guidance since the quarantine began a week ago, according to one employee.
Passengers said they were grateful to the crew but also worried that the employees, despite wearing protective gear when they enter cabins, might be passing the infection to people isolated inside.
On Monday, passengers were given new masks designed to filter out 95 percent of airborne particles, as well as packages of alcohol wipes. The Centers for Disease Control and Prevention in Atlanta also sent a letter to passengers advising them to wear face masks if they shared cabins with other passengers and to avoid sharing personal household items.
Japan’s health ministry said on Monday that so far it had tested 439 people on the ship for the coronavirus. That leaves more than 3,000 who have not been tested, receiving only initial health checks.
Japanese officials have said they do not have the capacity to test everyone on the ship. On Sunday, the health minister, Katsunobu Kato, said his ministry needed to consider how it would respond to the challenge of administering thousands of tests.
In Hong Kong, where another cruise ship, the World Dream, has been held at port, about 1,800 crew members aboard were tested for the coronavirus after the authorities said that infected passengers had disembarked on Jan. 24 in Guangzhou Province, China.
When the ship arrived in Hong Kong last Wednesday on a subsequent journey, the health authorities first tested those who had fevers or showed symptoms of the virus. All of those initial tests came back negative, but out of an abundance of caution, the Hong Kong health authorities decided to test all crew members.
Experts said the authorities should also test everyone on board the Diamond Princess in Yokohama.
“It is extremely possible that the infection has been transmitted on the ship,” said Harue Okada, a professor of public health at Hakuoh University in Tochigi Prefecture. She added that it was difficult to identify who had been exposed to infected people, including those who came into contact with other people during shore excursions.
“Furthermore, as it is assumed that there are asymptomatic but infected people, the virus test is necessary,” Dr. Okada said.
The cruise ship terminal where the Diamond Princess is docked has been closed to the public. On Monday, a sort of war room had been set up where around a dozen people sat at computers and on phones.
Some of them wore jackets that identified them as members of a psychological support team. The room’s walls were plastered with long strips of butcher paper, where information about the patients and a timeline had been scribbled in thick black marker.
At the port, the daughter of a passenger tried to deliver food and water to her elderly mother, who she said had a fever and was having trouble getting attention from the medical staff.
“She feels sick. I hope she can disembark soon,” the woman, Etsuko Takashima, said through tears as she spoke about her mother, Ayako Jinnai, 84. “At least, I hope she can get a drip infusion in the medical room on the ship. I don’t think her current condition is known to the staff.”
Ben Dooley reported from Yokohama, and Motoko Rich from Tokyo. Makiko Inoue, Eimi Yamamitsu and Hisako Ueno contributed reporting from Tokyo.