Water contamination by blue-green algae is becoming more prevalent with climate change. Heartbreaking reports of dogs dying soon after swimming in contaminated waterways appear frequently in the news. This situation is not unique to dogs; horses, livestock, and people are all subject to the same toxicity.
The most common condition that promotes rapid cyanobacteria growth in waterways is an influx of certain nutrients found in fertilizer, sewage, animal waste runoff, and decaying organic debris, which are all rich in phosphorus and nitrogen. Slow-moving, calm, or shallow water and warm temperatures above 64 degrees Fahrenheit further contribute to this growth. Toxicity is most common during summer and early fall when conditions are warm enough for algae growth; springtime can also be dangerous in warmer climates. Algae blooms form a green scum on the water’s surface, but the blooms can be forced beneath the surface by rain, wind, or currents, only to reappear when the water calms.
It doesn’t take a large water source for algae blooms to threaten horse health. Cyanobacteria can grow in even the smallest water containers, including watering troughs, automatic waterers, birdbaths, and garden pots. Routine and regular cleaning of all horse watering devices is important to limit this risk, especially during warm months.
How Is Blue-Green Algae Toxic?
At least 80 of the more than 2,000 species of blue-green algae produce toxins that are poisonous to humans and animals. Blue-green algae produces two types of toxins—microcystins and anatoxins—that affect many systems. Specifically, microcystins cause liver damage, while anatoxins affect the neurologic system.
Contact with blue-green algae, either by drinking algae-contaminated water or licking contaminated wet skin, causes poisoning. Consumption of dried algal mats is similarly dangerous. Damaged, digested, and dead algal cells release toxins, and water can remain toxic for up to five months after algal blooms disappear from sight.
The only way to confirm if a water source is infected with cyanobacteria is to test it in the lab. Not all infected water is tinted blue-green, and not all blue-green discolored water harbors cyanobacteria. Cyanobacteria can survive beneath ice and in winter conditions, so contamination risk does not automatically disappear when summer ends.
Signs of Blue-Green Algae Toxicity in Horses
Signs of blue-green algae toxicity in horses are variable, ranging from liver disease—characterized by jaundice, unthriftiness, and skin photosensitization—to neurologic signs such as disorientation, muscle tremors or rigidity, staggers, collapse, seizures, or convulsions. Horses might also experience respiratory distress, excessive salivation and tearing, pale mucous membranes, colic, and diarrhea or bloody manure. The prognosis is grave once a horse shows clinical signs.
Toxicity affects horses rapidly, leading to death within 24 hours of exposure and sometimes within 20 minutes, depending on the amount of water consumed. If wind blows across the water, it can concentrate cyanobacteria in specific areas downwind. Just one quart of contaminated water is enough to kill a cow.
Preventing Blue-Green Algae Toxicity on Horse Farms
Fence horses away from ponds or slow-moving water. Provide a fresh water source, clean it routinely, and ensure it isn’t contaminated with manure or urine. Don’t allow horses to drink from unknown murky or scum-laden water during trail rides, events, or rest stops while traveling.
Prevent pasture or field fertilizer from running into waterways. Manure is best discarded and composted on flat areas at a far distance from water sources with no chance of drainage into waterways.
After treating a water source with an algae-killing compound like copper sulfate, completely restrict horses from drinking that water for at least a week or longer to allow time for toxins released into the treated water to degrade sufficiently.
Blue-Green Algae in Oral Equine Supplements
Horse owners often seek magic elixirs to makes their horses stronger, faster, and more robust with a competitive edge. Some of these supplements contain blue-green algae, which the manufacturers market as a “superfood.” These supplements are not necessarily benign and can, in fact, prove fatal.
A journal paper described an 8-year-old Holsteiner gelding that received a powdered blue-green algae supplement for two months for “purported hoof health benefits.” Three days before the horse became ill, a new container was used. He developed jaundice, a poor appetite, and a dull mental state and began yawning compulsively and showing mild signs of colic. Blood chemistry parameters indicative of liver disease were elevated.
The horse was treated for liver failure with medications for decreasing brain edema and high ammonia levels. He was supplemented with intravenous fluids and dextrose. Despite treatment, the horse developed mania (involuntary biting and compulsive circling) and was euthanized two days after admittance. Necropsy results identified a small, flaccid liver, which on histology was consistent with microcystin toxicity that causes hepatic failure and hepatoencephalopathy. Researchers tested five unopened containers of the supplement, and three of them contained microcystin, a component of blue-green algae.
Nutraceuticals (nutritional supplements) are not scrutinized by the Food and Drug Administration (FDA) as closely as pharmaceutical medications; in many cases, the FDA doesn’t evaluate them at all. It then falls onto horse owners to be discerning about their purchases.
Many holistic practitioners recommend blue-green algae, including spirulina, as a supplement for horses. Despite the lack of evidence-based documentation, some reports claim that blue-green algae has benefits for hooves and joints. There are far better and safer supplements, both oral and systemic, to address these and many other musculoskeletal issues that have minimal and preferably no adverse effects.
References
- Fritz SA, Chamas S, and Ensley S. Blue-Green Algae. Vet Clinics of North America: Equine 2024, vol. 40; pp. 121-132.
- Mittelman, NS et al, Presumptive Iatrogenic Microcystin-Associated Liver Failure and Encephalopathy in a Holsteiner Gelding, Journal of Veterinary Internal Medication, 2016; 30: 1747-1751
Related Reading
- Research Spotlight: Sudden Death in Horses
- Blue-Green Algae Supplementation for Horses
- Nutritional Management of the Competitive Equine Athlete
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