FRIDAY, Nov. 6 (HealthDay News) -- Poor cleaning of restrooms aboard cruise ships raises the risk of norovirus-caused gastrointestinal illness outbreaks, a new study finds.

Norovirus causes about 95 percent of acute gastroenteritis outbreaks that occur on cruise ships. Between 2003 and 2008, norovirus outbreaks occurred on 66 ships monitored by the U.S. Centers for Disease Control and Prevention.

In this study, U.S. researchers checked 273 cruise-ship restrooms on 1,546 occasions and found that only 37 percent of them were cleaned daily. Overall, toilet seats were the best-cleaned objects in the restrooms, while baby changing tables were the least thoroughly cleaned objects. On three ships, none of the baby changing tables were cleaned during the study period.

The researchers also found that 19 restroom objects in 13 ships weren't cleaned at all during the entire five- to-seven-day monitoring period. Toilet area handholds were largely neglected and accounted for more than half of the uncleaned objects on 11 ships.

The thoroughness of restroom cleaning didn't differ by cruise line, the study said.

The findings are important because five of the six evaluated restroom objects are easily contaminated by pathogens during regular use.

"Although hand hygiene with soap after toileting may diminish the transmission of enteric pathogens via bathroom door knobs or pulls, hand washing is unlikely to mitigate the potential for any of the other toilet area contact surfaces to serve as a source of transmission of enteric pathogens," study author Dr. Philip Carling, a professor of clinical medicine at the Boston University School of Medicine, said in a school news release.

"Furthermore, there was substantial potential for washed hands to become contaminated while the passenger was exiting the restroom, given that only 35 percent of restroom exit knobs or pulls were cleaned daily. Only disinfection cleaning by cruise ship staff can reasonably be expected to mitigate these risks," he said.

The Cruise Lines International Association on Friday issued a response to the findings.

"The cruising industry takes the sanitation of its vessels and the mitigation of all gastrointestinal illnesses, including norovirus, very seriously," the statement read. "Our comprehensive public health and sanitation procedures -- which go above and beyond this study's focus on public restrooms -- are highly effective in maintaining healthy settings for families on vacation. In fact, according to the Centers for Disease Control and Prevention, the vast majority of norovirus outbreaks take place at land-based locations, such as schools, daycare centers, hospitals and nursing homes."

The study appears in the Nov. 1 issue of the journal Clinical Infectious Diseases.

More information

The U.S. National Institute of Allergy and Infectious Diseases has more about norovirus.