As people head out to enjoy the outdoors this summer, the Missouri Department of Natural Resources wants everyone to stay safe by being aware of possible harmful algae blooms in Missouri waterways. It is important to know what harmful algae blooms are, how to identify them and understand the potential health risks both to people and their pets.

Harmful algae blooms are clusters of cyanobacteria, often referred to as blue-green algae, that can grow in lakes, ponds and slow-moving or pooled streams. Cyanobacteria are capable of producing dangerous toxins that can cause illness and even death in people and animals. While they typically appear during summer and early fall, harmful algae blooms can occur any time of year.

Cyanobacteria are naturally occurring, but can form blooms when there is an abundance of nutrients, such as nitrogen and phosphorus, in a waterbody. Blooms are commonly described as looking like pea soup or spilled green paint. They might form a thick foam or scum on the water’s surface, though cyanobacteria can be present without forming surface scum.

Exposure can be through direct skin contact, incidental ingestion or inhalation of contaminated water while swimming, boating or other fresh-water activities. Skin irritation and gastrointestinal distress are the most commonly reported symptoms. More severe reactions can occur when large amounts of contaminated water are swallowed, potentially leading to liver or brain damage. Inhalation of aerosolized toxins might result in allergy- or asthma-like symptoms. Anyone who thinks they have been exposed to cyanobacteria and who is experiencing any of these symptoms should seek medical attention immediately and let health care personnel know that they have been in an area possibly affected by a harmful algae bloom.

Pets are exceptionally susceptible to cyanobacteria because of their smaller size and increased likelihood of ingesting toxins while playing, swimming, licking their fur or eating dried algae along the shore. In pets, severe reactions such as seizures and even death might occur within hours of cyanotoxin exposure. If a pet comes into contact with water containing cyanobacteria or algae scum, wash it off with fresh water immediately. Anyone who is concerned that their pet might have been exposed should contact a veterinarian immediately.

To report a suspected harmful algae bloom, complete an online reporting form at the link below or call the department’s Environmental Emergency Response 24-hour hotline at (573) 634-2436. People can also call the Missouri Department of Health and Senior Services’ 24-hour Public Health Emergency Hotline at (800) 392-0272.

To report a harmful algae bloom and to find more information about cyanobacteria, including photos of blooms, go to

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