Nine dogs have been reported dead after bathing in the Loire river, which was found to contain a toxic bacteria dangerous to animals and people.
The Maine-et-Loire prefecture on Tuesday issued a "call for vigilance", noting that authorities had taken several precautions to protect the public from the bacteria, including closing the swimming area at Rochefort sur Loire and banning fishing in the Loire river.
"Thirteen dogs have been poisoned, eight of them fatally," authorities wrote in mid-August, explaining that each of the affected dogs had swum in the river before "succumbing very quickly" to neurological, digestive, and respiratory disorders.
Two kinds of toxic cyanobacteria - oscillatoria and phormidium - were found in the river when authorities carried out analyses.
Both types of bacteria are known to grow on pebbles and stones, generally remaining on riverbanks but moving to the surface when water levels are low, as has been the case in central France following weeks of extreme heat and unusually low rainfall.
In a statement, the Maine-et-Loire prefecture reminded people to observe the swimming ban, which is often ignored by holidaymakers during the summer months, and also discouraged swimming in other bodies of water in the area - particularly places where the dark clusters of algae were visible.
Members of the public were also advised to supervise children closely and carefully clean any objects which had been in contact with the water before use.