Blue-green algae, also known as cyanobacteria, are a diverse group of oxygenic photoautotrophic Gram negative bacteria, some of which are capable of producing deadly cyanotoxins. Cyanotoxins are secondary metabolites which are toxic to most of the eukaryotic organisms including algae, plants, animals and humans. Although there are several types of cyanotoxins, they primarily affect the animal’s liver (Microcystin, Nodularin, Cylindrospermopsin) or nervous system (Anatoxin-a, Saxitoxin). Cyanobacterial blooms occur worldwide in freshwater sources, usually nutrient-rich calm waters such as that found in ponds and dugouts. The occurrence of toxic cyanobacterial blooms has increased in frequency and severity. They are often associated with hot, dry weather.

Horses are poisoned through ingestion of water from these contaminated water sources. Symptoms of poisoning vary depending on the type of toxin ingested. Neurotoxic cyanotoxins (associated with the nervous system) will result in muscle tremors, decreased movement, difficulty breathing, convulsions, or in many cases sudden collapse and death. Hepatotoxic cyanotoxins (associated with the liver) will cause weakness, bloody diarrhea, pale colored mucous membranes, mental derangement, and eventually death. Those horses that survive may lose weight or develop photosensitization. Cyanobacteria in the intestinal micro flora may produce neurotoxins such as Beta-N-Methylamino-L-Alanine (BMAA) which may be related to development of Equine Motor Neuron Disease.