Blue green algae: What it can do to animals

Farm Forum

I’ve often written in this space about the “One Health” concept. “One Health” refers to the interaction of animals, people and the environment – and how all of them can influence the health of the others. For many, that term primarily means the movement of germs from animals to cause illness in people. The environment component of “One Health” seems to take a back seat. But summers in South Dakota can bring a very important disease issue to the forefront – one with its main source not in animals or people, but out in the environment we all inhabit.

I’m speaking of the group of organisms called cyanobacteria, or “blue green algae”. Under the right conditions, these organisms can multiply in ponds and stock dams that serve as water sources for animals on pasture. And for animals that drink that water, the consequences could be deadly.

Blue green algae appear green while in the water, but may take on a bluish tint as they dry on the shoreline. It’s very important to understand that most of the “pond scum” that normally forms on water sources in the summertime is not of the blue-green variety. Blue-green algae does not form a mat that you can pick up—the individual organisms are dispersed throughout the water. They often create the appearance of “green paint” on the water, a color best described as “John Deere green.”

Also, it’s only under certain environmental conditions that blue-green algae form. Hot, dry conditions favor the growth of these organisms, which is why blue green algae poisonings are typically a summer and early fall phenomenon. When these organisms “bloom,” they outstrip their oxygen supply, die off, and float towards the surface. Winds that push the algae to the shore then make it more accessible for animals to reach the contaminated water.

Toxins produced by the algae are what cause death in cattle and other species—sheep and horses included. I’ve even heard reports of dogs dying after taking a swim and licking the algae off their coat. Some of these toxins affect the nervous system, resulting in muscle tremors and convulsions before death. Others cause liver damage, resulting in anemia, behavior changes and death. A quick onset is common: animals are usually just found dead, often near the water source. Animals that get a lower dose of the algae become chronic poor-doers. Diagnosis of these poisonings in cattle involves having a veterinarian doing a post-mortem exam to rule out other causes of death, along with evidence of exposure to a source. Blue green algae can also affect people – causing rashes and non-fatal illnesses.

How does one know whether the “scum” in your pond is blue-green algae, or something less harmful? First, consider whether the environmental conditions have been right for the bloom of algae, and whether the algae fits the description above. If necessary, water samples can be tested by having your veterinarian send them to the ADRDL at SDSU. Lab diagnosis is made by directly looking at the algae under a microscope. The desired sample is a pint of fresh water collected where there is a high concentration of suspect algae is sufficient. Collect it in a clean container, keep it cool, and get it to the lab as soon as feasible.

Dealing with a stock dam that has a bloom of blue-green algae can be difficult, and usually is best accomplished by just fencing livestock out of the area – at least the downwind areas where the algae may concentrate. In some cases, water sources can be treated with copper sulfate to help diminish the problem.

While it’s still a bit early yet, and reports so far are few, the drought conditions in some areas of our state this summer have set up the right conditions for blue-green algae blooms. In most years we see positive samples at SDSU, along with reports of associated death losses. Keep an eye on your animals’ water sources as our summer progresses, and contact your veterinarian or Extension field specialist to investigate if you suspect problems. With a bit of vigilance, this environmental “One Health” problem can be identified and managed before animal illness or death occurs.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University. He can be reached via email at or at 605-688-5171.