AURORA, Colo. (CBS4)– Parks officials have determined that botulism has been killing ducks in a pond in Aurora.

The naturally-occurring bacteria is spread through insects. Visitors to Exposition Park started seeing the dead birds last week.

They are concerned about the level of pollution in the ponds and how it affects the health of residents living near the park.

The City of Aurora periodically tests the water in the park and said there has not been an increase in toxins.