Freshwater Harmful Algal Blooms: Symptoms by Toxin and Exposure

Key points

• Where: harmful blooms of cyanobacteria (blue-green algae) occur mostly in fresh water — lakes, rivers and streams.
• What they cause: signs and symptoms depend on the toxin and the type of exposure.
• How bad: illness ranges from mild to life-threatening.

Swallowing

People can swallow cyanobacteria or their toxins in contaminated water, in fish carrying the toxins, or in contaminated blue-green algae supplements.

Liver and kidney toxins (hepatotoxins and nephrotoxins):

Nerve toxins (neurotoxins): anatoxins, guanitoxin (formerly anatoxin-a(S)) and saxitoxins can all cause muscle twitches that progress. High doses of saxitoxin may cause progressive paralysis. Anatoxin-a can also cause tingling, numbness, burning, drowsiness, salivation and speech problems.

Skin and eye contact

Swimming or boating can bring skin into contact with contaminated water, and eyes can be exposed to the water or spray. Skin toxins can cause:

• allergic dermatitis: rash, itching or blisters;
• conjunctivitis.

Breathing

As blooms die off, they give off gases such as hydrogen sulfide and methane, which can irritate the airways.

Dialysis

Rarely, dialysis patients have been exposed to microcystin in contaminated dialysis water, causing liver damage, liver failure and death.

What the evidence rests on

Knowledge of human health effects comes mostly from:

• a few epidemiology studies of recreational exposure;
• laboratory animal studies;
• reports of extreme exposures, such as contaminated dialysis water;
• poisonings of cattle and dogs.

The long-term health effects of cyanobacterial blooms are still unclear.

Resources

• CDC: Physician Reference for Cyanobacterial Blooms and Facts about Cyanobacterial Blooms for Poison Center Professionals
• ASTHO: clinician training module
• EPA: health effects from cyanotoxins
• FDA: blue-green algae products and microcystins

Sources

Based on "Clinical Signs and Symptoms Caused by Freshwater Harmful Algal Blooms," Centers for Disease Control and Prevention; a work of the United States government in the public domain.