Clinical Signs and Symptoms Caused by Freshwater Harmful Algal Blooms

Key points

  • Harmful blooms of cyanobacteria are most commonly found in fresh water such as lakes, rivers, and streams.
  • Signs and symptoms caused by these harmful algal blooms vary based on toxin and exposure.
  • Illnesses can be mild to life-threatening.
Tip of a metal boat in bright green, algae-covered water

Ingestion

People may ingest cyanobacteria or their toxins by:

  • Swallowing water contaminated with cyanobacteria or their toxins
  • Eating fish contaminated with toxins
  • Swallowing blue-green algae supplements contaminated with toxins

Toxins and symptoms

Hepatotoxins and nephrotoxins

Microcystins and nodularins are hepatotoxins and nephrotoxins made by cyanobacteria. Ingesting these toxins can cause:

  • Bad taste in mouth
  • Loss of appetite
  • Malaise or lethargy
  • Headache or fever
  • Nausea, vomiting, or diarrhea
  • Blood in urine or dark urine
  • Acute hepatitis or jaundice

Cylindrospermopsin is also a hepatotoxin and nephrotoxin made by cyanobacteria. Ingesting cylindrospermopsin can cause:

  • Headache
  • Fever
  • Vomiting
  • Bloody diarrhea

Neurotoxins

Anatoxins, guanitoxin (formerly known as anatoxin-a(S)), and saxitoxins are neurotoxins made by cyanobacteria. All three of these toxins can cause progression of muscle twitches when ingested. High doses of saxitoxin may cause progressive muscle paralysis.

Anatoxin-a can cause additional neurologic symptoms including:

  • Tingling
  • Numbness
  • Burning sensation
  • Drowsiness
  • Salivation
  • Speech disturbances

Skin or eye contact

People can have direct skin contact with water contaminated with cyanobacteria or their toxins during activities like swimming or boating. Their eyes may be exposed from contact with contaminated water or aerosols.

Contact with dermal toxins can cause:

  • Allergic dermatitis, including rash, itching, or blisters
  • Conjunctivitis

Inhalation

People may experience respiratory irritation as blooms of cyanobacteria die off and produce gases such as hydrogen sulfide and methane.

Dialysis

Rarely, dialysis patients have been exposed to the hepatotoxin microcystin through contaminated dialysis water.

Exposure to microcystin through dialysis water can cause:

  • Liver damage
  • Liver failure
  • Death

Health effects research

Information about human health effects from exposure to cyanobacteria and their toxins is primarily derived from:

  • A few epidemiology studies of recreational exposures
  • Studies with laboratory animals
  • Reports of extreme human exposure events, such as exposure to contaminated dialysis water
  • Animal (cattle and dog) exposures

The long-term health effects of harmful algal blooms of cyanobacteria are being studied but remain unclear.