Blue Green Algae

Barry Whitworth, DVM Area Food/Animal Quality and Health Specialist for Eastern Oklahoma

This year an Oklahoma cattle producer in Payne County found seven dead cows and one dead deer in close proximity to a pond.  The owner suspected something was wrong with the water.  An analysis of the water was performed. “The analysis revealed that it was positive for blue-green algae and was above the lethal threshold,” according to Oklahoma State University Payne County Ag Educator Nathan Anderson.  This year there have been reports of similar unexplained deaths by ponds.  This would not be much of a surprise if these had occurred in late summer, but this incident took place in early spring.  This should be a warning to livestock producers to inspect ponds for blue-green algae accumulation when conditions are right no matter what time of the year it is.

Blue-green algae is not really an algae but a bacterium, which is referred to as cyanobacterium. The most common species found in the Midwest are Microcystis, Oscillatoria and Anabaena (Morgan, 2011).  The bacterium is found in most bodies of water.  However, they become a problem during times of rapid growth fueled by high nitrogen and phosphorus content and warm sunny weather.  The overgrowth of the bacterium leads to the death of the organism, which then floats to the top and forms a “scum” on top of the water.  These “scum” layers can be moved about the pond by wind movement.  Sometimes this causes certain areas in the pond to be concentrated with the toxic levels of the dead bacterium.  Rain or wind disturbance can break up the “scum” and reduce the chance of toxicity, but this is not always the case.

All livestock, pets, wild animals, and humans are susceptible to blue-green algae toxicity.  The amount of water consumed needed to cause toxicity depends on the species of animal, concentration of the toxins in the water and how much water is ingested.  Ingestion of one quart of highly concentrated water is lethal to cattle (Meehan & Mostrum, 2015).

Most producers do not recognize a problem with blue-green algae until they find dead livestock in the pond or in close proximity to a body of water.  Most cattle that ingest contaminated water will die, but occasionally producers may find sick cattle.  The clinical signs of blue-green algae toxicity will depend on the type of toxin ingested.  The two types of toxins associated with blue-green algae are a neurotoxin (affect the nervous system) or a hepatotoxin (affect the liver).  If seen early, cattle affected by the neurotoxin will show muscle tremors, reluctance to move, and breathing problems.  This will lead to convulsions and death.  If cattle consume water with liver toxin bacteria, they will have weakness, pale mucous membranes, gastroenteritis, nervous signs, and death.  Animals that survive will lose weight and become poor doers.  These survivors may also develop photosensitization.  Animals with photosensitization are prone to sunburns on light colored skin areas. 

Typically, diagnosis is based upon exposure to blue-green algae along with clinical signs or sudden death.  If a producer suspects blue-green algae is the cause of death in his/her cattle, he/she should immediately collect a pint of water where large amounts of the algae exist.  The reason for quickly obtaining a sample is the toxin could be dispersed by the wind.  Then the producer should contact a veterinarian to conduct a necropsy.  A necropsy will rule out other causes of death.  A veterinarian will most likely take tissue samples for more testing and rumen contents may be taken to examine for presence of blue-green algae.  The water sample will need to be submitted for analysis.  More information for guidance about necropsy or water sampling may be found at the Oklahoma Animal Disease Diagnostic Laboratory at (405) 744-6623 or

Since there is no known antidote, treatment is usually unrewarding.  For this reason, producers need to focus on conditions that favor the development of blue-green algae.  Algae will bloom with sunshine and warm weather.  Excessive blooms are associated with ponds located in areas that catch runoff water high in nutrients.  Producers should be inspecting ponds anytime these conditions are present.  They should be prepared to provide alternative water sources in times of crisis. 

Blue-green algae toxicity is not a new problem for Oklahoma livestock producers but having problems with cyanobacterium early in the spring is new.  When weather conditions are right for algae build up, producers need to be constantly observing their ponds for any signs of the blue “scum” on the water, and if found, producers should take action to reduce the problem.  An excellent fact sheet is available from Oklahoma State University Extension Service on blue-green algae and how to best manage the problem.  The fact sheet can be found at  If a producer has questions about blue-green algae, they should contact their local veterinarian or an Oklahoma State University County Extension Educator.


Morgan SE. Water quality for cattle. Veterinary Clinics of North America Food Animal Practice. 2011;27(2):285 

Meehan MA, Mostrum M. Cyanobacteria Poisoning (Blue-green Algae). Fact Sheet at

Read more great stories in the May 2020 issue of Oklahoma Farm & Ranch.