Harmful Algal Blooms: Hazard for Dogs

Harmful algal blooms (HABs) can cause serious illness or death in pets and livestock that ingest water or mats containing high concentrations of cyanotoxins. Dogs can become extremely ill, and even die, within minutes to hours of exposure to cyanotoxins from drinking water, licking their wet fur or eating floating mats or dried crust along the shore. 

What to Watch For

A harmful algal bloom can look foamy, scummy, thick like paint, and pea-green or blue-green in color. It can look like a thick green or brownish-red mat, or bright green clumps suspended in the water. If you find thick, brightly colored foam, scum or mats in a lake, pond, river or stream—especially in shallow, marshy areas where cyanobacteria blooms can form—don’t let your pet drink or swim in the water.

Benthic algal mats have been identified in the Columbia River, which led to the deaths of two dogs in 2024. Cyanobacteria that predominantly grow on the bottom (benthos) of waterbodies are referred to as “benthic cyanobacteria.” The color of the material is diverse, ranging from vibrant yellow-green and drab olive, to burgundy and dusky brown or black. The bloom material may also include non-toxic algae as many groups of cyanobacteria grow with algae—forming algal mats. The risk of exposure to benthic blooms increases significantly when the cyanobacteria mats dislodge from the bottom, become buoyant, and accumulate along shorelines, backwater channels, or eddies via wind or water current.

Owners should be aware of the potential exposure to their pets while at these areas, especially shallow, marshy areas where cyanobacteria blooms can form. Cyanobacteria can be present on green algae, growing from the sediment or on rocks. Pets, livestock and people should avoid contact with the affected water. Toxins cannot be removed by boiling, filtering or treating water with camping-style filters.

This resource can help you tell the difference between normal algae and toxic algae, but when in doubt, stay out!

Symptoms

Children and pets are particularly susceptible to toxic algae. Exposure can result in:

• Weakness or collapse
• Nausea, vomiting
• Excessive drooling
• Weakness, fatigue, stumbling
• Abdominal pain
• Diarrhea
• Difficulty breathing
• Shaking, trembling
• Seizure like activity—tremors, rigidity, paralysis
• Liver failure

If Your Dog Does Go in the Water

• Don’t let your pet lick its fur.
• Wash your pet with clean water as soon as possible.
• If your dog shows the symptoms noted above after being in water, seek immediate veterinary care. Acute, life-threatening symptoms can develop rapidly. Death can occur within minutes to hours after exposure.

Treatment

Treatment depends on the type of toxin and how much damage has occurred, and may include aggressive decontamination, labs to evaluate liver health, fluids to help flush the toxins out of the body, activated charcoal. A dog may require aggressive care for shock, respiratory distress, or acute liver failure.