Equis ISSN 2398-2977

Toxicity: blue-green algae

Synonym(s): Cyanobacteria

Contributor(s): Birgit Puschner, Nicola Bates


  • Cause: exposure to toxins produced by blue-green algae (cyanobacteria) can lead to an acute intoxication affecting either the liver or the central nervous system.
  • Signs: acute hepatotoxicosis with clinical signs of anorexia, depression, diarrhea, colic, weakness, pale mucous membranes and shock. Rapid onset of rigidity and muscle tremors, followed by paralysis, cyanosis and death.
  • Diagnosis: clinical signs, clinical pathology, post mortem findings, and analysis of water, supplements, or stomach contents.
  • Treatment: clean water, stall rest, and supportive care.
  • Prognosis: guarded to poor.



  • Cyanobacteria (blue-green algae), are bacteria found in fresh, brackish and marine water bodies. They often have a blue-green color but can be red, brown or black.
  • Under certain environmental conditions blue-green algae can quickly form extensive and often visible growths or blooms. These most commonly occur in warm weather and affect the color, odor and taste of the water.
  • Not all blooms are toxic; it is not possible to identify species by eye.
  • Many cyanobacteria contain or produce a variety of toxic substances (cyanotoxins); many produce more than one type of toxin.
  • These toxins have high acute toxicity and exposures frequently result in fatality, which usually occurs very rapidly.
  • Cases in horses appear to be less commonly reported, compared to cattle, sheep, pigs, birds and dogs.

Predisposing factors


  • Access to contaminated water.
  • Bloom formation is dependent on a number of factors including species, strain and nutrient concentrations. Eutrophication from high fertilizer levels, sewage, animal waste and field run off is a common cause of algal blooms, and water bodies next to farmland are often affected.
  • Dietary supplements containing the blue-green algae Spirulina platensis and Aphanizomenon flos aquae (A. flos aquae) can be contaminated with microcystins.


  • Steady winds that concentrate toxic blooms to shore allow for ingestion by drinking animals.
  • Destruction of cyanobacteria proliferations (either naturally or through the application of herbicides or algicides) can result in a pulse of toxin release following destruction of the individual cells.


  • Many blue-green algae contain or produce a variety of toxic substances (cyanotoxins).


  • Microcystins: potent inhibitors of protein phosphatases 1 and 2A leading to progressive centrilobular hepatocyte rounding, dissociation, and necrosis.
  • Cylindrospermopsin: a protein synthesis inhibitor; genotoxic through DNA fragmentation.


  • Anatoxin-a: potent cholinergic agonist at nicotinic acetylcholine receptors (nAChRs) in neurons and at neuromuscular junctions.
  • Anatoxin-as: irreversible acetylcholinesterase (AChE) inhibitor leading to cholinergic stimulation.
  • Saxitoxin and neosaxitoxin: fast acting sodium channel blockers.

Other toxic compounds

  • Many blue-green algae produce endotoxins that are irritant to skin and mucous membranes.
  • Some also produce dermatotoxic compounds such as lyngbyatoxin, aplysiatoxin and debromoaplysiatoxin.


  • Depends on the type of toxin involved and it is possible for more than one type of blue-green algae or toxin to be involved in the same incident, although there is usually a dominant type.


  • Evidence of liver damage generally occurs within 24 h of ingestion, but death can occur within a few hours of days after exposure.


  • Signs of neurotoxin exposure can occur as early as 5-15 min after ingestion but may take up to one hour.
  • Death is also rapid, often within 10-30 min of the onset of clinical effects.

Other toxic compounds

  • Gastrointestinal signs can occur within 24 h.
  • Few cases of dermatotoxic effects are reported in animals but signs may occur within 24 h.


  • Water contamination during warm environmental temperatures can result in farm outbreaks.


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Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Nolen R S (2018) A One-Health solution to the toxic algae problem. JAVMA 252 (8), 906-908.
  • Mittelman N S, Engiles J B, Murphy L et al (2016) Presumptive iatrogenic microcystin‚Äźassociated liver failure and encephalopathy in a Holsteiner gelding. J Vet Int Med 30, 1747-1751 PubMed.
  • McGorum B C, Pirie R S et al (2015) Grazing livestock are exposed to terrestrial cyanobacteria. Vet Res 46, 16 PubMed.
  • Rankin K A, Alroy K A, Kudela R M, Oates S C, Murray M J & Miller M A (2013) Treatment of cyanobacterial (microcystin) toxicosis using oral cholestyramine: case report of a dog from Montana. Toxins (Basel) 5 (6), 1051-1063 PubMed.
  • Stewart I, Seawright A A & Shaw G R (2008) Cyanobacterial poisoning in livestock, wild mammals and birds - an overview. Adv Exp Med Biol 619, 613-637 PubMed.
  • Lopez-Rodas V, Maneiro E, Lanzarot M P, Perdigones N & Costas E (2008) Mass wildlife mortality due to cyanobacteria in the Donana National Park, Spain. Vet Rec 162 (10), 317-318 PubMed.
  • Hall J O (2006) Blue-green algae poisoning in large animals. 78th Western Veterinary Conference. 15-19 February, Las Vegas, Nevada.
  • Puschner B, Galey F D, Johnson B et al (1998) Blue-green algae toxicosis in cattle. JAVMA 213, 1605-1607 PubMed.
  • Beasley V R, Dahlem A M et al (1989) Diagnostic and clinically important aspects of cyanobacterial (blue green algae) toxicoses. J Vet Diagn Invest 1, 359-365 PubMed.
  • Dahlem A M, Hassan A S, Swanson S P, Carmichael W W & Beasley V R (1989) A model system for studying the bioavailability of intestinally administered microcystin-LR, a hepatotoxic peptide from the cyanobacterium Microcystis aeruginosa. Pharmacol Toxicol 64 (2), 177-181 PubMed.
  • Galey F D, Beasley V R, Carmichael W W, Kleppe G, Hooser S B & Haschek W M (1987) Blue-green algae (Microcystis aeruginosa) hepatotoxicosis in dairy cows. Am J Vet Res 48 (9), 1415-1420 PubMed.

Other sources of information

  • Puschner B (2018) Cyanobacterial (Blue-Green Algae) Toxins. In: Veterinary Toxicology - Basic and Clinical Principles. 3rd edn. Ed: Gupta R C. Elsevier, USA. pp 763-777.


  • ASPCA Animal Poison Control Center. Tel: +1 (888) 426-4435; Website: www.aspca.org
  • Veterinary Poisons Information Service (VPIS). Tel: + 44(0) 2073 055 055; Website: www.vpisglobal.com.