A recent government study of norovirus on cruise ships found that the number of cases has been declining, in part because of proactive steps taken by the industry when outbreaks occur.
The study, published by the U.S. Centers for Disease Control (CDC) in its weekly bulletin on Jan. 15, said the industry’s collaboration has helped reduce outbreaks and made them less severe during the six years studied.
The most recent example of stepped-up collaboration occurred last week when Royal Caribbean International’s Anthem of the Seas became the latest cruise ship to suffer an outbreak of the gastrointestinal illness.
According to a CDC investigative update on the outbreak, Royal took numerous steps to clean the Anthem and limit the spread of the virus among the ship’s 4,061 passengers.
Fay DeHaas, a Cruise Planners agent in West Hartford, Conn., who was taking a group of 16 on the Anthem’s next cruise on March 5, said Royal Caribbean emailed a letter explaining the outbreak to her clients.
“Royal Caribbean has been amazing,” DeHaas said.
She said the outbreak, which affected 132 passengers, has definitely created a buzz, adding, “Friends have been calling friends [saying], ‘What are you going to do? You can’t go.’ What I tell them is that the ship is always thoroughly sanitized, and there will be extra people onboard taking precautions to keep cleaning bathrooms, banisters, anything you touch.”
“We’re proud of the continual efforts of cruise line crews to clean and sanitize cruise ships to keep travelers healthy.” — Bud Darr, CLIA
The CDC study said measures such as those are having an impact. Of the 73.5 million passengers who sailed on a cruise ship subject to CDC reporting requirements from 2008 to 2014, only 129,678 cases of gastroenteritis were reported, a rate of 0.18%.
It said illness per 100,000 cruise ship travel days declined from 27.2 cases in 2008 to 22.3 cases in 2014. Although the report concluded that represented a “declining but not statistically significant trend,” it also noted that the rates were lower than those reported in an earlier 2001-04 study.
“Collaborative efforts with the cruise industry have allowed [the CDC’s Vessel Sanitation Program] to provide more rapid support to cruise lines and ships experiencing higher than expected levels of acute gastroenteritis,” the report, authored by three CDC health experts, stated.
It said fewer and less severe outbreaks are likely the result of earlier detection than in the past, “along with the cruise industry efforts to identify and control outbreaks.”
Increasingly, the study said, cruise officials have developed detailed outbreak plans, have used effective chemical disinfectants and have been proactive about seeking strategies to curb the virus’ spread.
In the case of the Anthem of the Seas, the CDC’s update on the ship said that Royal Caribbean had sent four hotel and housekeeping operations managers, three public health staffers and one nurse from the corporate office to oversee and assist with sanitation and outbreak response.
In addition, Royal sent 10 extra internal cleaning crew members and 10 contract cleaning staff plus equipment to the ship to assist in the execution of the Anthem’s outbreak response.
Royal Caribbean said it was able to contain the illness to about 10 cases a day, although by the end of the cruise enough passengers had fallen ill (3.3%) to qualify as an outbreak of gastroenteritis.
The Anthem of the Seas returned to Cape Liberty two days early from a 13-day Caribbean cruise, although Royal attributed that decision to the possibility of rough weather rather than to the illnesses.
Royal Caribbean tweeted that when the ship arrived in port, there were only a few active cases. “The ship will now go through an enhanced and thorough cleaning,” the tweet said.
The CDC study, titled “Acute Gastroenteritis on Cruise Ships — United States, 2008-2014,” drew praise from CLIA, the cruise industry trade group.
“We’re proud of the continual efforts of cruise line crews to clean and sanitize cruise ships to keep travelers healthy,” said Bud Darr, CLIA’s senior vice president for technical and regulatory affairs. “This study shows what a great job they’re doing.”
Darr said that norovirus cases at sea account for only a tiny fraction of the estimated 20 million total annual cases.
He said the public is more likely to hear about outbreaks on cruise ships because they report on gastrointestinal illnesses to a federal agency, and, “There is no similar federal program for hotels, airlines or restaurants.”
The CDC study noted that of the estimated 140 million norovirus cases in the U.S. between 2008 and 2014, verified cases on cruise ships accounted for 14,911, or about 0.01%.
However, while it was likely that many of the 172,810 cases of gastroenteritis on cruise ships during the period could be attributed to norovirus, public health workers were unable to verify the cause.
When clinical samples were available for testing, 92% of outbreaks were caused by norovirus, the study said. E. coli bacteria was the second-leading source of gastrointestinal illness.
The study said all of the E. coli outbreaks in the study period occurred on ships sailing back from Central or South America.
The highest rate of illness during the study period was in 2012 when a novel strain of norovirus emerged. The number of passenger outbreaks on cruise ships jumped to 27 that year from 15 in 2011 but dropped back to 17 outbreaks the following year.
Overall, passengers stand a less-than-1% chance of being on a cruise with gastrointestinal outbreak, the study showed. Of the 29,107 voyages during the study period, outbreaks occurred on 133 cruises, a rate of 0.5%.