Freshwater Environments

In freshwater, a harmful algal bloom (HAB) is most commonly caused by small organisms called phytoplankton. The phytoplankton that commonly cause HABs are cyanobacteria, which use sunlight to create food. Some cyanobacteria produce toxins called cyanotoxins. Depending on the specific chemical structure, cyanotoxins can be neurotoxins that affect the nervous system, hepatotoxins that affect the liver, dermatoxins that affect the skin, or other toxins that affect the stomach or intestines 1. Some common cyanotoxins that are known to cause illnesses in humans and animals are microcystins, cylindrospermopsin, anatoxins, saxitoxins, nodularins, and lyngbyatoxins 2.

Human and animal illnesses and symptoms can vary depending on the how they were exposed, how long they were exposed, and the particular HAB toxin involved. No human deaths in the United States have been caused by cyanotoxins; however, companion animal, livestock, and wildlife deaths caused by cyanotoxins have been reported throughout the United States and the world 3.

Humans and Freshwater HAB-associated Illnesses

People or animals can be directly exposed to cyanotoxins in freshwater during recreational activities or by breathing in aerosolized toxins (toxins in water turned into tiny airborne droplets or mist). People or animals exposed to cyanotoxins through direct skin contact or inhalation may experience the following symptoms 1, 2:

  • Skin irritation
  • Eye irritation
  • Nose irritation
  • Throat irritation
  • Respiratory irritation
  1. Carmichael WW. Health effects of toxin-producing cyanobacteria: “The CyanoHABs”External. Hum Ecol Risk Assess. 2001;7(5):1393-407.
  2. Hilborn ED, Roberts VA, Backer L, DeConno E, Egan JS, Hyde JB, Nicholas DC, Wiegert EJ, Billing LM, DiOrio M. Algal bloom-associated disease outbreaks among users of freshwater lakes—United States, 2009–2010. MMWR Morb Mortal Wkly Rep. 2014;63(1):11-5.

People can be exposed to cyanotoxins by eating freshwater fish from cyanotoxin-contaminated lakes or ponds or by drinking cyanotoxin-contaminated water. Additionally, some toxins may be ingested when taking blue-green algae dietary supplements 1,2. People who ingest cyanotoxins may experience the following symptoms 3:

  • Abdominal pain
  • Headache
  • Neurological symptoms
  • Vomiting
  • Diarrhea
  • Liver damage
  • Kidney damage
  1. Gilroy DJ, Kauffman KW, Hall RA, Huang X, Chu FS. Assessing potential health risks from microcystin toxins in blue-green algae dietary supplements.External Environ Health Perspect. 2000;108(5):435-9.
  2. Heussner AH, Mazija L, Fastner J, Dietrich DR. Toxin content and cytotoxicity of algal dietary supplements.External Toxicol Appl Pharmacol. 2012;265(2):263-71.
  3. Bienfang P, DeFelice S, Laws E, Brand L, Bidigare R, Christensen S, Trapido-Rosenthal H, Hemscheidt T, McGillicuddy D, Anderson D. Prominent human health impacts from several marine microbes: history, ecology, and public health implications.External Int J Microbiol. 2010;2011.

Animals and Freshwater HAB-associated Illnesses

Pets, livestock, and wildlife can be poisoned through direct contact by swimming in waters with a HAB or by drinking cyanotoxin-contaminated water. Coyote deaths have been reported after they were believed to have eaten fish that washed up on the beach from a part of the Gulf of Mexico that was experiencing a HAB 1-3. Dogs are especially at risk to cyanotoxin poisoning due to their behaviors, which include swimming in contaminated waters, drinking the water, and licking algae or scum from their fur after swimming 2. Domestic pets may also be poisoned if they eat dietary supplements that contain algae contaminated with cyanotoxins 3.

A HAB may also make birds sick. Side effects of cyanotoxin poisoning in birds are sometimes confused with, or occur at the same time with, avian botulism, a disease that paralyzes birds. Birds can get avian botulism when they eat toxins produced by the bacteria Clostridium botulinum directly or eat insects containing the toxins 4. Avian botulism is becoming more common in the Great Lakes region since it was first reported in 1963, and there is some evidence that Clostridium botulinum can grow in the presence of algae commonly found in the Great Lakes region 4.

Animals who are exposed to cyanotoxins may experience the following symptoms 2, 5-7:

  • Excessive salivation
  • Vomiting
  • Fatigue
  • Staggered walking
  • Difficulty breathing
  • Convulsions
  • Liver Failure
  • Death

Death in animals can occur within hours to days of exposure 2.

  1. Castle KT, Flewelling LJ, Bryan J, Kramer A, Lindsay J, Nevada C, Stablein W, Wong D, Landsberg JH. Coyote (Canis latrans) and domestic dog (Canis familiaris) mortality and morbidity due to a Karenia brevis red tide in the gulf of Mexico.External J Wildl Dis. 2013;49(4):955-64.
  2. Backer LC, Landsberg JH, Miller M, Keel K, Taylor TK. Canine cyanotoxin poisonings in the United States (1920s–2012): Review of suspected and confirmed cases from three data sources.External Toxins. 2013;5(9):1597-628.
  3. Bautista AC, Moore CE, Lin Y, Cline MG, Benitah N, Puschner B. Hepatopathy following consumption of a commercially available blue-green algae dietary supplement in a dog.External BMC Vet Res. 2015;11(1):136.
  4. Chun CL, Ochsner U, Byappanahalli MN, Whitman RL, Tepp WH, Lin G, Johnson EA, Peller J, Sadowsky MJ. Association of toxin-producing Clostridium botulinum with the macroalga Cladophora in the Great Lakes.External Environ Sci technol. 2013;47(6):2587-94.
  5. Stewart I, Seawright AA, Shaw GR. Cyanobacterial poisoning in livestock, wild mammals and birds–an overviewExternal. Adv Exp Med Biol. 2008;619:613-37.
  6. Briand J-F, Jacquet S, Bernard C, Humbert J-F. Health hazards for terrestrial vertebrates from toxic cyanobacteria in surface water ecosystems.External Vet Res. 2003;34(4):361-77.
  7. Ferrão-Filho AdS, Kozlowsky-Suzuki B. Cyanotoxins: bioaccumulation and effects on aquatic animals.External Mar Drugs. 2011;9(12):2729-72.

When a HAB decomposes, it can use up the oxygen in a body of water. When this happens, fish may not have enough oxygen to breathe and may die. Fish and other aquatic animals (vertebrates or invertebrates that live in the water) may also eat cyanobacteria,storing the cyanotoxins in their bodies. When other animals eat these animals (for example, when small fish are eaten by larger fish), the toxins can build up and move up the food web. This process is called bioaccumulation 1. Some cyanotoxins can also kill fish by affecting their gills and preventing fish from breathing 2.

  1. Ferrão-Filho AdS, Kozlowsky-Suzuki B. Cyanotoxins: bioaccumulation and effects on aquatic animals.External Mar Drugs. 2011;9(12):2729-72.
  2. Zimba P, Khoo L, Gaunt P, Brittain S, Carmichael W. Confirmation of catfish, Ictalurus punctatus (Rafinesque), mortality from Microcystis toxins. J Fish Dis. 2001;24(1):41-47.


  1. Lopez CB, Jewett, EB, Dortch Q, Walton BT, Hudnell HK. Scientific assessment of freshwater harmful algal blooms. Cdc-pdf[PDF – 78 pages]External Washington DC: Interagency Working Group on Harmful Algal Blooms, Hypoxia, and Human Health of the Joint Subcommittee on Ocean Science and Technology. Washington DC; 2008.
  2. Carmichael WW. Health effects of toxin-producing cyanobacteria: “The CyanoHABs”.External Hum Ecol Risk Assess. 2001;7(5):1393-407.
  3. Backer LC, Manassaram-Baptiste D, LePrell R, Bolton B. Cyanobacteria and algae blooms: Review of health and environmental data from the harmful algal bloom-related illness surveillance system (HABISS) 2007–2011.External Toxins. 2015;7(4):1048-64.
Page last reviewed: December 14, 2017