Briefly mentioned in our article on leptospirosis and water-related diseases, cyanobacteria deserve a dedicated article: every summer, dogs die within hours after simply swimming in a seemingly harmless body of water.

What is a cyanobacterium

Despite being nicknamed "blue-green algae," cyanobacteria are not algae but bacteria capable of photosynthesis, naturally present in most freshwater bodies. The problem arises when specific conditions — heat, stagnant water, strong sunlight, excess nutrients (often linked to agricultural runoff or garden fertilizers) — trigger a massive proliferation: the so-called "algal bloom."

Certain strains then produce some of the most potent toxins known in nature — more toxic, quantity for quantity, than many synthetic poisons.

Why dogs are particularly at risk

  • They drink the water directly while swimming, unlike humans who generally avoid swallowing it
  • They lick their coat after swimming, ingesting toxins deposited on their fur
  • Their smaller body size makes a given toxin dose proportionally far more dangerous than for an adult human
  • Their exploratory instinct draws them toward calm water at the edges — precisely where blooms concentrate

Recognizing risky water

No single sign is 100% reliable, but certain clues should raise alarm:

  • Unusual coloring: bright green, blue-green, sometimes reddish or brownish
  • A "spilled paint" appearance or thick foam on the surface
  • A distinctive smell, sometimes earthy or foul
  • Accumulation at the edges, in shallow, calm areas, especially during hot weather

The absolute rule of caution: stagnant water of dubious appearance should never be used for swimming or drinking, even in summer, even if the water looks clear in some spots — a bloom can be localized and invisible just a few meters away.

Symptoms depending on toxin type

Neurotoxins (the fastest-acting and often most lethal)

Onset within minutes to a few hours: excessive salivation, tremors, muscle weakness, breathing difficulties, seizures, paralysis. The progression can be devastatingly fast — a dog dying less than an hour after swimming is not a rare scenario.

Hepatotoxins (acting on the liver)

Symptoms sometimes delayed by several hours: vomiting, diarrhea (sometimes bloody), extreme lethargy, jaundice, abdominal pain.

What to do: an absolute emergency

1. Rinse the dog thoroughly with clean water as soon as it leaves the suspect water, before it can lick itself

2. Prevent it from licking its coat during the drive to the vet

3. Rush to the vet without waiting for symptoms to appear if the water was suspect — every minute counts

4. Never induce vomiting without veterinary advice

5. Report the exact swimming location

Prevention: the only real protection

  • Check locally for algal bloom warnings
  • Systematically avoid stagnant water during hot weather
  • Bring clean water for hydration rather than relying on natural water sources
  • Respect posted swimming bans
  • Be extra vigilant in late summer

A risk not to be underestimated

This is one of the rare cases where prevention isn't just the best option — it's the only genuinely effective one.

This article is one of the first fully translated into English on our blog — also available: the Dutch Shepherd, the Bernedoodle and choosing a dog club. If in doubt, or to prepare to react effectively, our Canine First Aid course remains the best prevention.