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Health Concerns Related to Blue-Green Algae

Are all blue-green algae dangerous?

There are many species of blue-green algae found in Wisconsin lakes and rivers, but only certain species can produce the algal toxins that cause illness. Not all algal blooms produce toxin; a blue-green algae bloom may not be producing toxins or producing enough toxin to cause a health concern. Larger algal blooms have the potential for higher concentrations of toxin. Unfortunately, there is no immediate way to know if an algal bloom is dangerous or not. It takes at least several days for a laboratory to analyze a water sample to see if toxins are present. During that time, a harmless algal bloom may become more toxic, a toxic algal bloom may become harmless, or the bloom could blow away or dissipate all together.

Algal blooms can appear and disappear within hours, which makes it difficult to decide when to close a beach. By the time water quality analysis is done, water conditions may have changed from the time a sample was collected. This is why it is so important to follow a common-sense approach. When you see an algal bloom, avoid the water and remember that you should not allow your pets in water where an algal bloom is present! For photos of algal blooms in Wisconsin, see the images page.

What are the common symptoms of algae-related illness?

Each algal toxin can affect your health differently: neurotoxins affect your central nervous system, hepatotoxins cause liver damage. Exposure to algal bloom material can also affect skin and the gastrointestinal system. Symptoms depend on which toxin and how much of it you are exposed to, and how you are exposed (drinking, swimming, etc.).

Common Human Symptoms:

Respiratory Symptoms Dermatological Symptoms Other Symptoms

Sore throat

Itchy skin Earache
Congestion Red skin Agitation
Cough Blisters Headache
Wheezing Hives Abdominal pain
Difficulty breathing   Diarrhea
Eye irritation   Vomiting

Common Animal Symptoms:



Difficulty Breathing

Pet Information

Pets are especially susceptible to blue-green algae because they don't naturally avoid smelly, green water. Because of their relatively small size, animals do not need to ingest very much tainted water to become ill. Many dogs have gotten sick and some have died as a result of drinking water experiencing an algal bloom or licking their fur after swimming in algae-filled waters.

If you have a pet that enjoys swimming in the lakes and rivers of Wisconsin, review our fact sheet Keeping Your Pet Safe from Harmful Algal Blooms P-00089 (PDF, 39 KB). The Merck Veterinary Manual has additional information about algal poisoning in domestic animals.


Please report algae-related illness to the Wisconsin Harmful Algal Blooms Program by calling 608-266-1120


Last revised August 7, 2023