As warmer weather arrives, walks along lakes, ponds and rivers become particularly enjoyable. Many dogs take advantage of these outings to swim, drink or play with sticks found in the water.

However, some freshwater environments may contain cyanobacteria capable of producing extremely dangerous toxins. Every year in France, these microorganisms are associated with severe poisoning and deaths among dogs.

What Are Cyanobacteria?

Cyanobacteria are microorganisms that occur naturally in aquatic environments. They are sometimes incorrectly referred to as “blue algae” or “blue-green algae”.

They can develop in:

• lakes;
• ponds;
• reservoirs;
• rivers;
• shallow areas or bodies of water with a low inflow.

When conditions are favourable—high temperatures, strong sunlight, calm water and high levels of phosphorus or nitrogen—they can multiply rapidly. This is known as a proliferation or a cyanobacterial “bloom”.

Not all cyanobacteria are toxic. However, some species can produce substances known as cyanotoxins, which may affect the nervous system, liver or digestive system.

Visible cyanobacteria do not necessarily mean that toxins are present. Conversely, water that appears clean does not guarantee that it is safe. Their distribution can change rapidly depending on the wind, current, depth or time of day. Only laboratory testing can accurately assess the risk.

How Can a Cyanobacterial Bloom Be Recognised?

Cyanobacteria can appear in many different forms.

In lakes, ponds and reservoirs, they may cause:

• unusual green, turquoise, brown, yellow or reddish discolouration of the water;
• a film on the surface;
• foam or floating accumulations;
• an appearance resembling paint, soup or vegetable purée;
• an unpleasant smell;
• deposits blown by the wind and accumulated near the banks.

In rivers, some cyanobacteria grow on the riverbed, covering pebbles, rocks, branches or aquatic plants. They form mats or biofilms that may detach and become brownish, greenish or blackish clumps. These accumulations may then collect in calm areas and along the banks.

These so-called benthic cyanobacteria are particularly dangerous to dogs. ANSES, the French Agency for Food, Environmental and Occupational Health and Safety, reports that canine deaths are mainly associated with the ingestion of cyanobacterial clumps or biofilms found in bathing areas.

Why Are Dogs Particularly Vulnerable?

Dogs generally do much more than simply put their paws in the water. They may:

• drink directly from a river, lake or pond;
• swallow water while swimming;
• ingest deposits found along the banks;
• chew a contaminated stick or pebble;
• lick their coat after swimming;
• eat accumulations of cyanobacteria because of their smell.

They may therefore absorb a large quantity of toxins relative to their body weight. Furthermore, the health thresholds established to protect human swimmers are not designed to assess the risks faced by animals.

An area that is officially open for human swimming should therefore never automatically be considered safe for a dog.

Which Symptoms Should Raise Concern?

Symptoms may appear very quickly, sometimes within minutes of exposure, or during the following hours. Their nature depends on the type of toxin involved and the quantity ingested.

Possible signs include:

Digestive Symptoms

• excessive salivation or unusual drooling;
• nausea;
• vomiting;
• diarrhoea;
• abdominal pain.

Neurological Symptoms

• anxiety or unusual behaviour;
• sudden weakness;
• tremors;
• difficulty standing;
• uncoordinated movement;
• loss of balance;
• seizures;
• paralysis.

Other Possible Symptoms

• abnormally bulging eyes;
• breathing difficulties;
• extreme tiredness;
• loss of consciousness;
• irritation of the skin or mucous membranes.

Poisoning may progress extremely quickly and can result in the animal’s death.

What Should You Do If Exposure Is Suspected?

Suspected cyanobacterial poisoning is an absolute veterinary emergency.

If your dog has just drunk suspicious water, eaten an accumulation found along the bank or developed symptoms after swimming:

1. Move your dog away from the water immediately.
2. Prevent your dog from drinking any more water or licking deposits.
3. Contact a veterinarian or an emergency veterinary service immediately.
4. Provide the precise location and time of exposure, together with the symptoms observed.
5. If it can be done safely, take photographs of the water or deposits.
6. If possible, collect any vomit in a clean container or bag to show the veterinarian.

Never wait to see whether the symptoms disappear on their own. The sooner treatment is provided, the greater the chances of saving the dog.

Health authorities recommend seeking immediate veterinary assistance whenever an exposed animal develops vomiting, diarrhoea, tremors, difficulty walking or unusual behaviour.

Essential Prevention Measures

The best protection is to prevent all exposure.

Before allowing your dog to approach the water:

• check warning signs and displayed restrictions;
• consult information issued by the local council, prefecture or Regional Health Agency;
• examine the water, rocks and banks;
• avoid stagnant or shallow areas and places covered with deposits;
• never allow your dog to drink water of unknown quality;
• carry a bottle and bowl containing clean drinking water;
• do not allow your dog to play with sticks or pebbles taken from the water;
• keep your dog on a lead in suspicious areas or places subject to official warnings;
• strictly observe all restrictions, even when the water appears clear.

When accumulations, foam, unusual discolouration or deposits are visible, do not allow your dog to enter the water, drink, sniff or lick the banks.

Vigilance Is Necessary Beyond the Summer

Cyanobacterial blooms are often associated with high temperatures and low water levels. They are therefore particularly common during the summer.

However, they may persist into autumn or occur at other times of the year when conditions remain mild and sunny. Vigilance should therefore not be limited to periods of extreme heat.

Key Points to Remember

Cyanobacteria may be present in lakes, ponds and rivers. Some produce toxins capable of causing digestive, neurological or liver poisoning in dogs, which can sometimes be fatal.

A dog may be poisoned by drinking the water, swimming, licking its coat or playing with a contaminated stick.

At the slightest doubt, move your dog away from the water and contact a veterinarian immediately.

A few minutes can make all the difference.