Is Lake Water Making Dogs Sick? Vets Advise Caution During Summer Swims

Concern Grows After Reports of a Sick Dog in Saint-Fargeau

In Saint-Fargeau, Burgundy, several social media users recently raised alarm over Lake Bourdon, claiming the water is “dangerous for animals” and blaming toxic cyanobacteria for recent dog illnesses—even death.

A widely shared post on Facebook read: “Another dog died yesterday at Lake Bourdon. Be careful with your pups.” These claims have sparked concern among dog owners who frequent the lake during the summer months.

However, the local recreation center has not confirmed any such cases. When contacted by journalists, staff insisted the water is safe: “The water quality is good. We’ve had people asking constantly,” a representative told France 3. “But we always advise dog owners to avoid letting pets swim during periods of extreme heat.”

Water Quality: Safe for Humans, Risky for Dogs?

According to France’s official Eaux de baignade website, which tracks national water quality, tests conducted on June 16 and July 7, 2025, show that the water at Lake Bourdon is classified as “good.” Levels of Escherichia coli and intestinal enterococci bacteria were recorded at under 15 CFU per 100 mL—well below the 100 CFU threshold for safe swimming.

So why the concern?

Veterinarians explain that dogs are not exposed the same way humans are. While people avoid swallowing lake water, dogs often drink it while swimming—putting them at higher risk from bacteria, particularly cyanobacteria.

Cyanobacteria and Leptospirosis: Rare But Serious

“Cyanobacteria aren’t toxic through contact—but they are through ingestion,” said Dr. Jean-Jacques Dentz, a veterinarian based in Dijon and regular expert on France 3. “That’s why dogs, who tend to drink while swimming, can get sick.”

He adds that illness from cyanobacteria is still very rare compared to the risk of heatstroke during summer heatwaves. “It’s safer to let dogs cool off in the water than risk overheating,” Dentz said. Still, he encourages owners to discourage their dogs from drinking while in the water.

If your dog swims in a lake and then experiences vomiting, diarrhea, lethargy, or loss of appetite in the hours or days that follow, you should consult a vet. Immediate vomiting post-swim, however, may just mean your dog drank too much water too quickly—not necessarily a sign of poisoning.

Another threat Dr. Dentz flags is leptospirosis, a bacterial disease spread through rodent urine in stagnant warm water. “Some strains are 100% fatal for animals,” he warned. “It can infect humans too, entering through the skin.” He reports regular cases from places like Lake Kir in Dijon.

The best protection? Yearly vaccination against leptospirosis, which is highly recommended by veterinarians.

Summer Safety Tips for Dog Owners

As temperatures soar, it’s natural to want to let your dog cool off with a dip. But experts urge caution:

• Let your dog swim—but try to stop them from drinking the water
• Avoid small, stagnant ponds where bacteria thrive
• Keep an eye out for warning signs like vomiting or lethargy
• Ask your vet about the leptospirosis vaccine

Being informed can help keep your dog safe and cool this summer.