Cases of fatal algal poisoning in dogs have been reported in several states in recent weeks. Toxins produced by blue-green algae can lead to seizures, skin rashes, liver damage, or respiratory paralysis and can cause death within an hour of exposure. Dogs are more susceptible than people, because they may be more likely to swallow water while swimming or may lick their fur after. The algae can be found in both fresh and salt water.
Read more about recent incidents of algal poisoning in dogs in the New York Times.
More information on the toxicity of blue-green algae can be found in the Merck Veterinary Manual.