Cyanobacteria Poisoning

By Renee Schmid, DVM, DABT, DABVT for Pet Poison Helpline; Ahna Brutlag, DVM, MS, DABT, DABVT; Lynn Buzhardt, DVM

Does the water in your local lake have a blue or green coating on the surface? If so, that pond water may have a floating layer of blue-green algae that can be deadly.

Although commonly referred to as blue-green algae, these organisms are not plants, they are bacteria – cyanobacteria, to be precise. Named for their bluish pigment, cyanobacteria are aquatic and photosynthetic, meaning they live in water and manufacture their own food.

Overgrowth of cyanobacteria can color the water green or blue-green, ranging from pea soup to iridescent hues, but colors may vary. Some bacterial colonies float on the surface and, on windy days, may be blown into concentrated algae mats along the shore where it is easy for livestock or pets to reach them.

Cyanobacteria are found in fresh water, brackish water, and salt water. They are most common in standing bodies of water rather than streams and rivers. However, if there are small puddles or areas of standing water along streams and rivers, cyanobacteria may be present. These microscopic bacteria can also grow in backyard fountains, pools, garden pots, birdbaths, or anywhere water is stagnant. Regardless of where they are found, cyanobacteria can be dangerous.

Where are cyanobacteria found?

Cyanobacteria are present across North America. Overgrowth of these bacteria, referred to as a bloom, typically occurs during late summer and fall when the water temperature rises and there is little rainfall, although their growing season is much longer in temperate climates. Nutrient-rich waters, such as areas of agricultural or municipal runoff, encourage cyanobacterial growth.

"Nutrient-rich waters, such as areas of agricultural or municipal runoff, encourage cyanobacterial growth."

It is important to note that not all blue-green algae blooms are poisonous. Of the 2,000-plus cyanobacteria species, only about 80 produce toxins. Unfortunately, it is impossible to determine whether a bloom contains toxins simply by looking at it. Therefore, all blue-green algae blooms should be considered toxic until proven otherwise. There are many harmless types of algae that appear like blue-green algae and differentiating them often requires professional testing.

What types of animals are affected by cyanobacteria?

All animals, including dogs, cats, horses, livestock, mammalian, fish, birds, and humans, are susceptible to the toxins produced by cyanobacteria. The toxin enters the body when the animal drinks contaminated water. Cats are susceptible to poisoning, but their risk of exposure is lower because they rarely swim and are picky about the water they drink.

What signs and symptoms can cyanobacteria cause?

The clinical signs of poisoning depend on which toxin is involved. Blue-green algae can produce toxins that affect the nervous system (neurotoxins), liver (hepatotoxins), or skin (dermatotoxins).

  • Neurotoxins have an onset within 60 minutes of exposure. Clinical signs include muscle rigidity, paralysis, tremors, seizures, excessive salivation, vomiting, diarrhea, fluid buildup in the lungs, and paralysis of the diaphragm, which can lead to death. Neurotoxins can cause death within minutes to hours post exposure. In fact, livestock that drink from contaminated ponds are often found dead right by the water’s edge.
  • Hepatotoxins have an onset within four hours of exposure. Clinical signs include vomiting, diarrhea (may be bloody or black), pale or yellow gums, low blood sugar, low protein levels, and bleeding. These signs occur because the liver is unable to produce important blood clotting components. Death can occur within a few hours to a few days of exposure. Those that live for several days can develop seizures.
  • Dermatoxins can cause itching, redness, and blistering of the skin within hours of contact. These signs are not fatal, but may take several days to weeks to resolve, and cause discomfort to the patient.

What is the treatment for cyanobacteria poisoning?

Prompt treatment is important in all cases of poisoning, but since blue-green algae attack so fast, immediate intervention is critical. If you know your dog consumed potentially toxic algae, seek immediate medical care.

"If you know your dog consumed potentially toxic algae, seek immediate medical care."

There is no antidote for cyanobacteria toxicosis. If ingestion is witnessed or suspected, decontamination is the first step. Other treatments are supportive and symptomatic based on clinical signs present or anticipated.

What is the prognosis?

If the patient becomes symptomatic from hepatotoxins or neurotoxins, the prognosis is poor to grave despite aggressive treatment. Some animals succumb before reaching a veterinarian.

Dermatotoxins typically have a good prognosis.

What steps can be taken to prevent poisoning by cyanobacteria?

First and foremost, you should block access to blue-green algae sources. Fence off contaminated ponds and lakes to prevent livestock from drinking affected water. Remove standing water from plant containers, birdbaths, fishponds, and fountains, and provide fresh water sources in clean troughs or dishes for all animals. Do not let dogs swim in contaminated water. Simply licking the water off their coat could be fatal.

Not all types of algae are deadly, but it is difficult to tell which are poisonous without specific analysis. It is safest to consider all algae blooms to be dangerous and avoid them altogether.

Pet Poison Helpline, an animal poison control center based out of Minneapolis, MN, is available 24/7 for pet owners and veterinary professionals that require assistance treating a potentially poisoned pet. The staff provides treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals, and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline’s per-incident fee includes follow-up consultations for the duration of the poison case. Pet Poison Helpline is available in North America by calling 800-213-6680. Additional information can be found online at 

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