Why cruise ships rarely have rats despite hantavirus fears

• Officials are investigating a hantavirus cluster linked to a cruise ship, with three confirmed and five suspected cases.
• Hantavirus is typically spread by rodents, but cruise lines have strict sanitation and pest control measures.
• Health officials believe the initial infections likely occurred on land before the cruise or during island stops.

Officials are working to determine the source of a hantavirus cluster aboard a cruise ship that has resulted in three lab confirmed cases, with five more suspected.

Three people who were traveling aboard Oceanwide Expeditions’ MV Hondius have died, one of whom was confirmed to have a hantavirus variant. 

Hantavirus is primarily spread through exposure to the urine, droppings or saliva of infected rodents. World Health Organization official Maria Van Kerkhove said during a May 5 news briefing that the cruise operator told the organization there were no rats on board.

That’s standard, according to Walt Nadolny, professor emeritus of marine transportation and global business at the State University of New York Maritime College, and a former environmental officer for Carnival Cruise Line and Norwegian Cruise Line. Much remains unknown about where the passengers were infected, but seeing rats, mice or other rodents aboard cruise ships is a rarity.

Here’s why.

Are there rats or mice on cruise ships?

Generally, no. “In my time on cruise ships, inspecting everything from the bilge to the galleys to passenger cabins, I've never seen a rodent or evidence of rodent droppings,” he said.

There are traps “all over the place” as a precaution, he said, but no rodents are typically caught. Cruise lines usually appoint a designated crew member onboard to be in charge of those procedures.

The Centers for Disease Control and Prevention’s Vessel Sanitation Program also routinely inspects ships that stop at U.S. ports across various areas, including pest and insect management. Inspectors interview “the person in charge of the plan to manage pests on the ship,” according to the health agency’s website, and confirm that employees are knowledgeable about pest control.

There are equivalent authorities in other jurisdictions, as well.

Van Kerkhove said the WHO believes the first guest to show symptoms and his wife – both of whom died – were infected on land. “Our assumption is that they were infected off the boat, and then joined the cruise,” she said.

The expedition sailing also stopped at multiple islands off the coast of Africa, she added, some of which have many rodents. “So, there could be some source of infection on the islands as well for some of the other suspect cases,” Van Kerkhove said.

There may also be some human-to-human transmission happening among "really” close contacts.

Van Kerkhove emphasized that the health threat to the general public is low. “This is not a virus that spreads like flu or like COVID,” she said. “It’s quite different.”

The risk of seeing a rat or mouse on your next cruise is, too. If a CDC inspector saw evidence of rodent droppings, Nadolny said, “that ship would be shut down in port for a thorough cleaning.”

“Shipboard sanitation is taken so seriously by the cruise industry.”

This story was updated to refresh headlines.

Nathan Diller is a consumer travel reporter for USA TODAY based in Nashville. You can reach him at [email protected].