The next day, the city of Austin advised pet owners to keep their animals out of the lake because of the potential presence of harmful algae. On Friday, it said the advisory remained in effect after tests confirmed a neurotoxin from algae had been found in one area called Red Bud Isle.

Morgan and Patrick Fleming of Marietta, Ga., took their Border collie, Arya, to Lake Allatoona, about 35 minutes north of Atlanta, on Saturday , a local television station reported on Monday. The animal became ill and died from what a veterinarian said was “most likely” an algal toxin, it reported.

“It happens every single year in the U.S. and around the world,” Val Beasley , a professor of veterinary, wildlife and ecological toxicology sciences at Pennsylvania State University, said on Monday.

“A lot of times, the neurotoxins will kill the animal before they can get to the veterinarian,” he said. “This time of year is when you have the most numbers of cases and people are out and about with their animals and the conditions are ripe for the cyanobacteria to grow.”

He said that there were no nationwide figures of dog deaths from the poisoning.

Melissa Martin, the owner of the three dogs in North Carolina, said Harpo jumped into a pond in Wilmington, N.C., on Thursday.

“He just splashed around in it a little bit,” she said. A few times, he put his face under the water as it he were “bobbing for apples.”

When he got to shore, he apparently got Abby and Izzy, who had stayed out but were muddy, wet with the pond water, she said. When they went home, Ms. Martin started to give Harpo a bath when she heard her wife shriek from the yard.