It wasn’t toxic blue-green algae that killed two dogs in Lake Mendota on Sunday, but health officials aren’t sure what did.

The dogs had been in the water at two different spots on the lake, one at Marshall Park and the other near the UW-Madison Memorial Union. Both died later in the day.

The Clean Lakes Alliance, a nonprofit organization that works toward cleaner water in the Yahara chain of lakes, including Lake Mendota, collected samples of lake water from both locations on Sunday and Monday, so Public Health Madison and Dane County could analyze the water.

Public Health spokeswoman Sara Mattes said the samples were tested for both blue-green algae and E.coli, and neither sample had elevated levels of the two.

Could something else in the water or on the beach have killed the dogs?

“We can’t speak to anything else in the water or on the shore,” Mattes said.

The city’s 11 public beaches — five on Lake Mendota, five on Lake Monona and one on Lake Wingra — were checked for bacteria on Monday and Tuesday, and all checked out OK.

Other dogs were seen playing in the two beach areas where the dogs that died had been, and there were no other reports of sick dogs from Marshall Park or the Union.

While blue-green algae blooms weren’t evident or believed to be the cause of the deaths, Mattes said pet owners should take precautions before letting dogs swim in the lakes.

“Follow our standard advice to do a visual inspection of the water before letting a dog enter, provide a fresh water source for the dog to minimize the amount of lake water it drinks, and hose down the dog after swimming in the lake,” she said.

At UW-Madison, spokeswoman Meredith McGlone said the water at the Memorial Union and Hoofers are tested for bacteria weekly.

If limits are exceeded, she said, the areas are closed to swimmers and signs are posted to alert the community.

State Journal reporter Chris Aadland contributed to this report.

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