Why is it dangerous to pets?
Cyanobacteria in warm, still, nutrient-rich water produce microcystins (liver toxins) and anatoxin-a (a neurotoxin). Microcystins destroy the liver within hours; anatoxin-a paralyses muscles, including the muscles used to breathe.
Pets are exposed by drinking the water or licking algae off their fur after swimming — not only by drinking. Blooms look like green paint, scum or pea soup, and can appear within days of hot weather.
Why speed matters
Delaying care significantly worsens outcomes. Some signs appear hours or days later, so "looks fine right now" is not a reason to wait.
Symptoms of blue-green-algae poisoning
- Vomiting, diarrhoea, drooling within minutes to hours
- Weakness, collapse, muscle rigidity
- Tremors, seizures, paralysis
- Difficulty breathing, blue-tinged gums
- Jaundice, dark urine and liver failure within 24 hours
What to do right now
- Go to an emergency vet immediately and call ahead — say “suspected blue-green algae” so they prepare.
- Wash the pet with clean water and mild soap so it cannot lick more toxin off its coat.
- Do not wait for symptoms; by the time they start, liver damage is already underway.
- Keep pets out of any water with visible scum, and never let them drink from ponds or lakes in hot weather.
If your pet shows any of the signs above, go to the nearest emergency vet now and call ahead.
Sources & further reading
- ASPCA Animal Poison Control — pet poison database & dosage references
- Pet Poison Helpline — toxicology triage guidelines
- Merck Veterinary Manual — toxicology monographs
- Vets Now (UK) — emergency toxicosis guidance