Fangs pierced Leah Prudhomme's legs as she swam across the deep, dark rum-colored northern Minnesota lake. It could be anything, she thought -- muskrats, beavers, maybe a muskie. But it didn't let up.

In the middle of Island Lake near Duluth, the triathlete struggled as the animal sunk its needle-sharp teeth into her legs, feet and back, leaving 25 bite marks, some 2 inches deep.

"It just kept coming after me," said Prudhomme, 33, of Anoka. "You never knew where it was going to bite next."

In between peppering her with puncture marks, the animal's head popped up a few feet away. That's when Prudhomme noticed its distinctive long tapered tail, small beady eyes and gray head. An otter.

"I couldn't believe Duluth had an otter," she said Saturday before getting more rabies shots, her swollen ankles and bite marks still healing three days after the incident.

The rare lake attack baffled conservation experts and doctors, who could only surmise that the atypically aggressive otter had rabies or was a mother protecting young pups.

"I've never seen or heard of it before," said Mike Scott, a conservation officer with the Minnesota Department of Natural Resources in Duluth. "We've got otters everywhere ... lakes, streams. Most times, [swimmers] wouldn't even know it. Otters usually stay away."

As a Duluth native growing up on Pike Lake, Prudhomme was familiar with the trails, water and wildlife, and had swam Island Lake several times. Last Wednesday, she was visiting her father on the lake and left with a friend for an eight-mile road run and half-mile swim in preparation for her second Ironman Triathlon. Despite the sunny, 90-degree day, they donned wet suits and goggles over swimsuits before diving into the dark lake.

"You always wonder, 'oh my gosh what could happen out there,'" she said. "Already, you're trying to get over your fear of fish or whatnot. You can't even see your hands below."

They swam around an island and veered back to shore. As they passed a bog, Prudhomme felt a nip at her ankle. She stopped, frantically treading water. Up popped the otter's head before it dove back underwater and attacked "ferociously."

Her friend helped her father get his boat, driving it out to Prudhomme to pull the shaking woman to shore. At St. Luke's Hospital in Duluth, doctors cleaned the 25 bite marks and gave her rabies and tetanus shots. Once home in the Twin Cities, Prudhomme received more rabies shots Saturday.

She said she's thankful she wore the wet suit, which was shredded during the attack, but which likely saved her from more extensive injuries.

Prudhomme, who's done about 50 triathlon and road races, said she now needs to do what she's done after bike crashes or other mishaps: get back up.

Next month, she plans to do the Duluth Triathlon -- on the same lake.

"I'm scared, but it's one of those things you don't want to let get the best of you," she said. "It's not like I'll be bitten by another otter."

Kelly Smith • 612-673-4141; Twitter: @kellystrib