Five of the world's largest cruise lines, including Royal Caribbean, have cancelled stops at ports in Mexico as the country's death toll from swine flu passes 150.

Other cruise lines are continuing to stop in Mexico but are assuring passengers that ships are clean and risk is low because most reported illnesses are inland. No cases of swine flu have been reported on any cruise ships that have docked in Mexico.

Travel agents say customers are still booking trips that include visits to destinations such as Puerto Vallarta and Mazatlan, but those itineraries are likely to change if the outbreak worsens.

Carnival Corp. and Royal Caribbean Cruises, the world’s two largest cruise lines, made the announcement following recommendations from the U.S. Centers for Disease Control and Prevention to avoid non-essential travel to Mexico. Princess Cruises, which is owned by Carnival Corp., along with Holland America Line and Norwegian Cruise Line also suspended all stops to the country.

The cruise lines will instead route ships to alternative ports, such as San Diego or Santa Catalina Island, or will spend additional time at sea. Passengers already on the ships are unlikely to get a refund because of the change in itineraries, but travelers who are scheduled to take a cruise to a Mexican port may be offered a chance to switch to a ship with a different route, such as cruises to Alaska.

"We hope people will understand that this is beyond our control," said Karen Candy, a spokeswoman for Princess Cruises. "We understand there will be some disappointment."

The Mexican cruise business has experienced an unprecedented boom over the past decade, with passenger numbers more than doubling since 2000. Mexico is ranked in the world’s top 10 destinations for cruise lines, with nearly 6.5 million passengers in 2008, according to the Cruise Lines International Assn.