ATLANTA — A bat on a flight from Wisconsin to Atlanta last week has sparked a national search for passengers to protect them against possible rabies.

No one knows if the bat had rabies because it escaped. But the Centers for Disease Control and Prevention said today it wants to talk to people to make sure they didn’t have close contact with it, putting them at risk.

CDC officials are trying to reach all 50 passengers who were on the Aug. 5 Delta flight 5121, which departed Madison, Wis., at 6:45 a.m. for Atlanta.

The jet was in the air when the winged intruder emerged and repeatedly flew back and forth the length of cabin — as shown in a video posted on YouTube. The flight immediately returned to Madison.

Some who have watched the video have speculated the creature may have been a bird. But it was labeled a bat by the passenger who shot the video, the CDC rabies expert who studied the video and by a spokesman for Atlantic Southeast Airlines — the Atlanta company that operates the Delta connecting flight.

Employees at the Madison airport who helped get it out of the terminal also said it was a bat, based on how it flew and behaved, said Brent McHenry, spokesman for the Dane County Regional Airport. He said there are bats around other buildings at the airport. “We see them all the time.”

Passengers trapped the animal in a jet bathroom, but it later escaped out the passenger bridge, into the terminal and reportedly out some terminal doors to the open air.

As a precautionary step, health officials want to talk to each of the passengers and ask about possible exposure. Finding them has been complicated. The airline only could provide the names of the 15 who reboarded the plane after its return and completed the flight to the Atlanta. The other 35 were shifted to other flights.

The CDC is urging passengers to call 1-866-613-2683.

So far, health investigators have talked to 13 of the 15 passengers who stayed on the flight, and none was bitten or had close enough contact to cause concern.

Bats are the primary source of rabies deaths in the United States; only about two to three people die of rabies each year on average. But thousands of people get shots to prevent rabies after exposure to cats, dogs and other rabid animals.

If any people on the plane were infected, they wouldn’t know it yet because it generally takes more than two months for rabies to fully develop. The early symptoms are unremarkable — fever, headache and general weakness or discomfort. As the disease progresses, more telling symptoms appear like insomnia, anxiety, confusion, paralysis, hallucinations, agitation, an increase in salivation and a fear of water.

Dr. Charles Rupprecht, chief of the CDC’s rabies program, said agency officials don’t recall any similar such investigation in the past.