Blue-Green Algae Poisoning in Horses

Many animals that ingest blue-green algae toxins die suddenly, but some can recover.

No account yet? Register


Blue-Green Algae Poisoning in Horses
It is impossible to tell visually if a water source contains blue-green algae or to determine which species are present without laboratory analysis. | Photo: Thinkstock
Blue-green algae, or cyanobacteria, poisoning is a condition caused by the ingestion of water containing excessive growths of toxin-producing blue-green algae species. Of the more than 2,000 species of blue-green algae identified, at least 80 are known to produce toxins poisonous to animals and humans. Many more species and toxins have yet to be identified. Heavy blue-green algae growth or blooms occur when water sources are contaminated with excessive nutrients, especially nitrogen and phosphorus, and weather conditions are hot and dry. In farm settings, stagnant ponds contaminated with fertilizer run-off or direct manure and urine contamination are prime places for blue-green algae blooms to occur.

The most common species of blue-green algae in North America associated with poisoning are Anabaena, Aphanizomenon, Oscillatoria, and Microcystis. Not all strains of these genera are capable of producing toxins, and in those that do toxin production is sporadic. Environmental factors such as water temperature, sunlight, water pH, and nutrient concentration affect when toxins are produced. Intoxications are most common in the summer and early fall when water temperatures are warmest.

These algae can produce several major toxins. Some algae produce potent neurotoxins that cause clinical signs such as muscle tremors, respiratory distress, seizures, profuse salivation, diarrhea, and rapid death within minutes to hours. Other algae can produce hepatotoxins (toxins affecting the liver) that can cause acute death or a more delayed death after signs of acute liver failure occur. Photosensitization, a skin condition affecting nonpigmented areas of skin, can occur in animals that survive the acute stages of liver damage. Other types of algal toxins occur in other regions of the world. Blue-green algae toxins are released when algal cells are damaged and die in the water (e.g., after water is treated with an algaecide such as copper sulfate), or when ingested water reaches the animal’s digestive tract and disrupts cells, releasing the toxins.

Most animals exposed to blue-green algae toxins die acutely. Treatment is supportive and symptomatic. In some cases animals can recover, but death typically occurs so quickly that the animals are found dead near the water source

Create a free account with to view this content. is home to thousands of free articles about horse health care. In order to access some of our exclusive free content, you must be signed into

Start your free account today!

Already have an account?
and continue reading.


Written by:

Related Articles

Stay on top of the most recent Horse Health news with

FREE weekly newsletters from

Sponsored Content

Weekly Poll

sponsored by:

Where do you go to find information on pituitary pars intermedia dysfunction (PPID)? Select all that apply.
78 votes · 136 answers

Readers’ Most Popular

Sign In

Don’t have an account? Register for a FREE account here.

Need to update your account?

You need to be logged in to fill out this form

Create a free account with!