Health officials ran tests Monday to try to determine what caused at least 36 people to complain of flu-like symptoms while on a cruise down the Ohio River.

If the preliminary tests don't confirm the cause, cultures will be sent to the federal Centers for Disease Control and Prevention for further investigation, said Dr. William Hacker, commissioner of the Kentucky Department of Public Health.

Hacker said Sunday that the illness appeared to be similar to Norwalk virus, a contagious but normally not life-threatening ailment common aboard cruise ships and in nursing homes and schools.

The 36 passengers aboard the Mississippi Queen who complained of being sick were examined during the weekend by medical staff at Methodist Hospital in Henderson, said Pam Moran, spokeswoman for the hospital.

Federal and state health officials were monitoring food and hygiene practices aboard the paddlewheel riverboat, which was en route from Cincinnati to St. Louis, where it is due on Tuesday. The CDC sent a team to travel aboard the boat from Paducah to St. Louis, Hacker said.

Norwalk virus causes mild to moderate gastrointestinal problems including vomiting and diarrhea, and usually "burns itself out" in 12 to 24 hours. Treatment usually includes hydration and anti-diarrheal or nausea medication.

The majority of the Mississippi Queen's 525 passengers remained on the boat, said April Matson, spokeswoman for the boat's owner, Majestic America Line.