updated 9/25/2011 5:18:29 PM ET 2011-09-25T21:18:29

Endurance swimmer Diana Nyad called it quits on Sunday on her attempt to swim from Cuba to Florida. The reason this time was stinging pain from multiple stings from Portuguese man o' war stings.

Nyad had swum 40 hours and made it about half way through -- about 67 nautical miles miles -- before abandoning the attempt. She told the Associated Press the stings temporarily paralyzed her spine and the pain had become unbearable.

It was Nyad's third try to swim the course, after failed bids in 1978 and in August when shoulder pain, asthma and ocean swells forced her to abort her attempt after 29 hours.

Medics warned toxins from the stings were accumulating and another sting could be life-threatening.

Nyad made it farther than her last attempt to swim from Cuba to Florida. Nyad began her most recent attempt at 6:04 pm Friday from the Ernest Hemingway International Nautical Club, just west of Havana. She had hoped to reach Florida's coast by Monday morning.

NEWS: Diana Nyad Forced to Abort Swim From Cuba

Earlier Saturday, handlers saw barracudas in the area, and she was also visited by a curious shark.

"An Oceanic white tipped shark was spotted near Diana in the midst of the three boat flotilla. ... Rob MacDonald, one of (her safety) divers, swam towards the shark, where they faced off within 10 feet of one another," said a post on Nyad's website.

"I guess he thought I was more aggressive than him, and he turned in the other direction," MacDonald was quoted as saying.

Nyad had hoped to become the first person to swim the stretch of ocean without a shark cage.

Australian swimmer Susie Maroney, at 22, became the first person to swim from Cuba to the United States in 1997, though she used a shark cage.

Nyad tried to swim across the shark-infested straights in 1978, but gave up after 42 hours in the water due to bad weather.

NEWS: Invisible Shark Shield Put to the Test

The swimmer set an open sea record by swimming from the Bahamas to the Florida Keys -- a journey that is the same distance as the Cuba-Florida swim, but a feat she described as much less dangerous.

And she clinched a record for circling the island of Manhattan at the age of 50, clocking in at seven hours and 57 minutes.

- AFP contributed to this report.

© 2012 Discovery Channel

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