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When it comes to blue-green algae: when in doubt, stay out!

The Dunn County Health Department has received its first report of a local person becoming ill after exposure to a harmful algal bloom (HAB) in a local lake. A HAB occurs when certain types of microscopic algae grow quickly in water, typically forming visible patches that may harm the health of the environment, plants, animals or humans. HABs can deplete the oxygen and block the sunlight that other organisms need to live, and some HABs produce toxins that are dangerous to animals and people.

Blue-green algae, also known as Cyanobacteria, are a group of bacteria that many people refer to as “pond scum.” Bluegreen algae are most often blue-green in color, but can also be blue, green, reddishpurple, or brown. Blue-green algae generally grow in lakes, ponds, and slow-moving streams when the water is warm and enriched with nutrients like phosphorus or nitrogen. When environmental conditions are just right, blue-green algae can grow very quickly in number. Most species are buoyant and will float to the surface, where they form scum layers or floating mats. When this happens, we call this a “blue-green algae bloom.” In Wisconsin, blue-green algae blooms often occur between mid-June and late September.

The toxins produced from the bloom may cause drinking water and recreational water to become dangerous. People who drink or swim in water that contains high concentrations of cyanobacteria or cyanobacterial toxins may experience many health concerns including: difficulty breathing, headache, fever, stomach issues such as nausea, vomiting and stomach cramps, loss of appetite, skin irritation, and allergic reactions. These symptoms can show up minutes to hours after exposure. Pets, especially dogs, can experience symptoms such as fatigue, difficulty breathing, vomiting, convulsions, and even death following exposure to blue-green algae. Health officials recommend if you or your pets have been exposed to blue-green algae and are experiencing any of these symptoms to seek medical or veterinary attention.

“Algal blooms can appear and disappear within hours, so it is important to always be aware of the potential risk. When you see an algal bloom, avoid the water. Do not enter the water for any purpose and remember to make sure to keep kids and pets away from the water when an algal bloom is present” says KT Gallagher, Dunn County Public Health Director.

The Wisconsin Department of Natural Resources offers tips to protect you and your family:

• Do not swim in water that looks like green or blue paint, or that has a scum layer or puffy blobs floating on the surface.

• Do not boat, water ski, etc. over such water (people can be exposed through inhalation of aerosolized water droplets).

• Do not let children play with scum layers.

• Do not let pets or livestock swim in, or drink, waters experiencing blue-green algae blooms.

• Do not treat surface waters that are experiencing blue-green algae blooms with any herbicide or algaecide– toxins are released into the water when blue-green algae cells die.

• In general, avoid swimming in areas where you cannot see your feet in knee-deep water.

• Always take a shower after coming into contact with any surface water (whether or not a blue-green algae bloom appears to be present; surface waters may contain other species of potentially harmful bacteria and viruses).

If you think you, a family member, or pet developed an illness after exposure to blue-green algae, please report the illness to the Wisconsin Division of Public Health by filling out an online survey at or calling (608) 266-1120. Healthcare providers should report any suspected human cases of Cyanobacteria and Cyanotoxin Poisoning to local public health. 

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