(4:35 p.m. EDT) -- A growing number of Middle East Respiratory Syndrome cases in South Korea has forced Royal Caribbean to cancel cruise ship port calls at affected cities.

Royal Caribbean's Mariner of the Seas, Voyager of the Seas and Quantum of the Seas will skip visits to Seoul until at least August 2015. Mariner of the Seas also will skip Jeju Island. SkySea Golden Era, which is jointly operated by Royal Caribbean Cruises Ltd., has followed suit.

The ships will replace canceled ports with either sea days or calls at Kagoshima, Fukuoka, Okinawa or Nagasaki, Japan.

Royal Caribbean is also screening passengers and crew before they board, providing cruise passengers with letters breaking down MERS symptoms and offering hand sanitizers throughout the ships, said Cynthia Martinez, director of global corporate communications for Royal Caribbean Cruises Ltd.

If passengers or crewmembers show symptoms, such as a fever, cough and shortness of breath, the ships' medical staff will isolate and treat patients using anti-viral medications onboard.

"We've been closely monitoring all developments regarding the increase in the number of cases of Middle East Respiratory Syndrome, also referred to as MERS in South Korea," she said. "Health authorities from a variety of nations are investigating the situation, and we remain keenly attuned to their progress."

Princess Cruises' Diamond Princess also has scheduled stops in South Korea. The line has not yet made any changes.

The Centers for Disease Control and Prevention defines MERS as a viral respiratory illness. It currently lists the Republic of Korea as a Level 1 watch. The baseline-risk watch reminds travelers to use usual precautions, such as washing hands with soap and water often, avoiding contact with sick people and not touching their eyes, noses and mouths.

--By Gina Kramer, Associate Editor