Blue-Green Algae Poisoning in Horses

Key Takeaways

Blue-green algae poisoning occurs when horses ingest a toxic dose of cyanobacteria. Cyanobacteria, commonly referred to as blue-green algae, are a group of bacteria that live in aquatic ecosystems (e.g., ponds, lakes, and creeks). 

• Cyanobacteria can produce two distinct kinds of toxins: hepatotoxins and neurotoxins

• Symptoms of cyanobacteria toxicosis include diarrhea, appetite loss, tremors, recumbency, and sudden death

• Diagnosis is based on history of exposure and clinical signs

• There is no antidote for algae toxicosis; treatment focuses on early gastrointestinal decontamination and symptomatic and supportive care

• Prognosis is poor, with many horses dying shortly after ingestion, and surviving animals are at a high risk of chronic liver damage

A Closer Look: What is Blue-Green Algae Poisoning in Horses?

Blue-green algae commonly grows on the surface of standing bodies of water, especially during warm seasons. The toxins produced by cyanobacteria are toxic to all mammals, including dogs, horses, and humans. Bodies of water that are turgid (appear swollen), cloudy, or have an obvious algae bloom on the surface should not be used for swimming or drinking.

Symptoms of blue-green algae toxicosis depend on the specific toxin ingested. Some toxins target the liver (hepatotoxins), while others target the nervous system (neurotoxins). 

Given the absence of an antidote, prevention is of the utmost importance.

Risk Factors

Blue-green algae toxicosis is a potentially fatal condition and must be treated as an emergency. The only way to increase the chances of survival is immediate veterinary attention.

Blue-green algae toxicosis is uncommon in horses. Blue-green algae toxicosis is more prevalent during the summer months. Water ecosystems near farmland and industrial areas are more likely to have a high concentration of nitrogen and phosphorus, causing algae blooms.

Horses living in proximity to farmland and industrial areas are more likely to enter into contact with contaminated waters.

Possible Causes

The ingestion of cyanobacteria causes blue-green algae toxicosis. 

Ingestion can occur when horses swim in, or drink from waterways contaminated with toxic algae. The algae produce toxins that damage the liver or central nervous system.

Main Symptoms

Main symptoms of blue-green algae toxicosis include:

Decreased appetite  • Abdominal pain (colic) • Tremors 

• Weakness • Excessive salivation • Diarrhea 

More specifically, symptoms can be divided depending on which type of toxin was ingested with the algae. 

Hepatotoxin symptoms

• Weakness • Diarrhea • Blood in stool 

Yellow gums or eyes • Weakness • Pale gums

Neurotoxin symptoms

• Seizures • Tremors • Recumbency • Blue gums

• Excessive salivation • Excessive urination 

• Excessive tearing • Difficulty breathing

Testing and Diagnosis

Diagnosis is based on history of exposure and clinical signs. Diagnostic tools include:

• Bloodwork

• Examination of water sources

• Examination of stomach contents

• Algae sample tests

Steps to Recovery

Treatment options include: 

• Gastrointestinal decontamination: in the case of early detection 

• Activated charcoal

• Access to clean water 

• Liver protectant drugs 

• Anti-seizure medications

• IV fluids

Note: always consult a veterinarian before administering activated charcoal. 

Once treated, ongoing monitoring is required to ensure proper recovery. Horses showing signs of liver damage must be kept away from direct sunlight for several days while the liver recovers. 

Prognosis for algae toxicosis is generally poor. Horses can die prior to the onset of symptoms, and many die shortly after symptoms develop.

If treatment is delayed, the animal has a very low chance of survival. 

Prognosis for hepatotoxins is poor and depends on the severity of liver damage. Horses that survive often require repeated bloodwork to monitor liver function.


Blue-green algae toxicosis is not contagious, however several horses drinking from the same water source may be affected at the same time.

Horses that do not have contact with cyanobacteria-contaminated waters will not develop algae toxicosis. Prevention strategies include:

• Fencing off ponds and other waterways 

• Providing abundant clean water sources 

• Making sure that manure does not enter into waterways

• Reducing fertilizer run-off into drinking water sources

• Using algicides to prevent or control algae levels 

• Immediately contacting a veterinarian if contact is suspected

Is Blue-Green Algae Poisoning Common in Horses?

Blue-green algae toxicosis is more prevalent in summer months and hot climates.

Typical Treatment

• Gastrointestinal decontamination

• Activated charcoal 

• Access to clean water 

• Liver protectant drugs 

• Anti-seizure medications

• IV fluids

Want to speak to a vet now?

Book an appointment

Health concern with your pet?

Start a video chat with a licensed veterinary professional right now on Vetster!

Book an online vet

Online veterinarian and virtual pet care services available on-demand.

Available now on Apple and Play stores.

Vet on phone