Every summer, many dogs enjoy swimming and cooling off in Lake Geneva. Around our practice, in Villeneuve, Veytaux, Montreux and across the Swiss Riviera, these swims are generally safe. However, it is important to know how to recognise a cyanobacterial bloom.
Sometimes referred to as blue-green algae, cyanobacteria are naturally occurring microorganisms found in lakes and rivers. Some species can produce toxins capable of causing severe, and sometimes fatal, poisoning in dogs.
The Villeneuve area has already been affected. In July 2024, a dog died suddenly after swimming in Lake Geneva. Cyanobacterial poisoning was suspected but could not be formally confirmed. However, water samples collected in the area identified a potentially toxic species of cyanobacteria, particularly around the mouth of the Eau Froide River. [1, 2]
How Can You Recognise Cyanobacteria in the Water?
Despite their nickname of’blue-green algae, they can come in a wide variety of colours: green, blue, yellow, brown, red or almost black.
Not all cyanobacteria produce toxins. However, it is impossible to tell with the naked eye whether a bloom is dangerous. If in doubt, it is safest to avoid swimming. [3]
Visible cyanobacterial blooms can take several forms:
- Unusually coloured or cloudy water;
- Streaks or surface scums;
- Flakes, clumps or foamy scum;
- Floating mats resembling wet wool;
- Unusual deposits along the shoreline.
The wind and currents can quickly move these clumps and concentrate them in certain areas of the lake.
Some cyanobacteria also grow at the bottom of the water, on stones, branches or aquatic plants. They then form «benthic mats», sometimes brown, black, dark green or dark red.
These mats can detach, float to the surface, and then accumulate on the banks. Once dried, they may resemble mud or vegetation debris.
Important point: benthic mats can form in clear water. Transparent water, free of green discolouration, therefore does not entirely rule out the presence of cyanobacteria. [3]
Why are dogs particularly at risk?
Dogs can be exposed to the toxins by:
- Drinking the water directly;
- Swallowing water while swimming;
- Ingesting clumps or fragments of cyanobacterial mats;
- Licking their coat after leaving the water;
- Chewing on contaminated sticks, stones or aquatic plants.
Some benthic cyanobacterial mats may contain highly potent neurotoxins. Their concentration can be very high within the mats themselves, even when the surrounding water appears perfectly normal. In such cases, ingesting only a small amount may be enough to cause severe poisoning. [3]
Key points to remember
Cyanobacteria do not always turn the water green. They can also form brown, black or dark green mats on the bottom of lakes and rivers or along the shoreline.
Why should you be particularly vigilant in Villeneuve and along the Riviera?
The risk associated with cyanobacteria is not limited to Villeneuve. Blooms can occur in different parts of Lake Geneva and in other bodies of water.
However, the area around the mouth of the Eau Froide River has a particular local history.
Following the suspected death of a dog that had swum in Lake Geneva at Villeneuve in July 2024, the General Directorate for the Environment of the Canton of Vaud carried out water sampling in this area of the lake.
The analyses confirmed the presence of a potentially toxic species of cyanobacteria. The Canton subsequently advised restricting swimming in the area around the mouth of the Eau Froide River. [1, 2]
The dog developed a very rapid deterioration after swimming, with loss of balance, seizures, excessive salivation and vomiting. Sadly, she did not survive.
As no post-mortem examination was performed, it was not possible to formally confirm that cyanobacteria were responsible for the dog's death. It should therefore be regarded as a suspected poisoning rather than a confirmed case.
More broadly, dog owners walking their pets along the shores of Lake Geneva should remain vigilant, particularly in Villeneuve, Veytaux, Montreux, La Tour-de-Peilz and Vevey. The Municipality of Montreux has also relayed the cantonal recommendations regarding the risks to animals. [4]
What are the symptoms in dogs?
Symptoms vary depending on the type of toxin involved and the amount ingested. Some cyanotoxins primarily affect the nervous system, while others cause acute liver damage.
In severe cases, the first signs may appear within minutes to a few hours.
Possible symptoms include:
- Excessive salivation;
- Vomiting or diarrhoea;
- Sudden weakness;
- An unsteady gait;
- Tremors or muscle twitching;
- Disorientation;
- Seizures;
- Breathing difficulties ;
- Collapse or loss of consciousness;
- Pale, bluish or, at a later stage, yellowish gums.
Some neurotoxins can cause paralysis of the respiratory muscles. Hepatotoxins may lead to severe liver damage, hypoglycaemia, blood clotting disorders and haemorrhage. [5–8]
Do not wait for several symptoms to appear.
Any unusual behaviour after swimming or after ingesting suspicious water warrants an immediate call to a veterinarian.
What should you do if your dog has been exposed?
Suspected cyanobacterial poisoning is a veterinary emergency. The toxins can act rapidly, and there is no specific antidote.
1. Remove your dog from the water immediately.
Move your dog away from the area and prevent them from drinking, returning to the water, or licking any deposits along the shoreline.
2. Prevent your dog from licking its coat
A dog can continue to absorb toxins by licking its paws, belly or chest after swimming.
3. Rinse your dog thoroughly with clean water
Rinse the entire coat thoroughly, paying particular attention to the paws, belly, chest, and the areas beneath the collar or harness.
Also remove the collar, harness or any wet dog coat. If possible, wear gloves to avoid direct contact with the deposits yourself.
4. Contact a veterinarian immediately
If your dog has drunk the water, swallowed a suspicious mat or shows any symptoms, contact a veterinarian without delay.
Veterinary treatment is based on decontamination as early as possible, together with supportive care, including seizure control, respiratory support if required, intravenous fluid therapy, and monitoring of liver function, blood glucose and blood clotting.
The prognosis depends on the type of toxin involved, the amount ingested and, above all, how quickly treatment is started. [5-8]
How can cyanobacterial poisoning be prevented?
Before allowing your dog to swim, carefully inspect the water and the shoreline.
Avoid letting your dog swim if you notice:
- Discoloured or very cloudy water;
- Streaks or surface scum;
- Floating clumps;
- Brown, black or greenish mats or fragments;
- Unusual deposits on rocks or along the shoreline.
Keep your dog on a lead in suspicious areas to prevent it from drinking, licking deposits or chewing on contaminated pieces of wood.
Bring fresh drinking water with you on walks. A thirsty dog is more likely to drink from a lake, river or puddle.
Always follow local warning signs and restrictions. However, the absence of a warning does not mean there is no risk. Cyanobacterial blooms can develop and move rapidly, so it is important to assess the water before every swim.
General information is available from the Canton of Vaud, the relevant local authorities, and the Swiss citizen science platform Cyano Community. [3, 4, 9]
Frequently Asked Questions
Can a dog be poisoned without drinking the water?
Yes. A dog can swallow water while swimming, ingest a fragment of a cyanobacterial mat, or absorb toxins by licking its coat.
Is clear water always safe?
No. Cyanobacterial mats can develop on the bottom of clear water, then detach and accumulate along the shoreline.
Is cyanobacterial poisoning always fatal?
No. Not every exposure results in severe poisoning. However, the prognosis can be poor in serious cases, particularly when neurological or respiratory signs develop rapidly.
References
- General Directorate for the Environment, Canton of Vaud. 2024 Annual Management Report. Cyanobacteria in Lake Geneva and measures taken in the Villeneuve area.
- Canton of Vaud. The Canton Recommends Vigilance Regarding Cyanobacteria. Press release, 7 August 2024.
- Eawag, Swiss Federal Institute of Aquatic Science and Technology. Cyanobacteria / Blue-Green Algae. Blooms in water bodies and benthic cyanobacterial mats.
- Municipality of Montreux and Canton of Vaud. The Canton Urges Caution Regarding Cyanobacterial Blooms. 26 May 2026.
- Foss AJ, Aubel MT, Gallagher B, et al. Diagnosing Microcystin Poisoning in Dogs. Toxins. 2019;11(8):456.
- Peterson ME, Talcott PA. Small Animal Toxicology. 3rd ed. Saunders; 2013.
- Cohn LA, Côté E. Côté’s Clinical Veterinary Advisor: Dogs and Cats. 4th ed. Elsevier; 2020.
- Merck Veterinary Manual. Algal Poisoning of Animals.
- Cyano Community. Swiss Citizen Science Project for Cyanobacteria Monitoring.
Need a vet in Villeneuve or on the Riviera?
Vétéo provides home veterinary consultations in Montreux, Villeneuve, Veytaux, Clarens, La Tour-de-Peilz, Blonay, Saint-Légier, Vevey and many other municipalities across the Vaud Riviera.
You can visit our page dedicated to home veterinary consultations or contact us to book an appointment.
Article written by Dr. Vet Med Théo Decalf, in July 2026.
The photographs and illustrations are reproduced with the kind permission of Eawag. [3]