At least 10 horses have died in northwest France from poisoning after consuming acorns,  according to a treating veterinarian.

The horses had consumed sufficient quantities of acorns to potentially result in tannin toxicity, said Hélène Lemoine, DVM, equine veterinarian in Saint Lô, Normandy, France. Tannin is present in the leaves, bark, and acorns of oak trees and affects the intestinal tract and the kidneys.

“None of the affected horses survived,” Lemoine told The Horse. “As soon as they showed signs of hemorrhagic diarrhea death followed very quickly, usually before we could even arrive on site to examine the horse.”

Strong winds and rain in the region have caused significant quantities of acorns—including unripe acorns, which are more toxic—to fall from the trees recently, often into pastures, Lemoine said. According to the Western Animal and Environmental Anti-Poison Center (CAPAE) in Nantes, France, it takes approximately 3 kg (6.6 lbs) of acorns to poison a 500 kilogram (1,100 pound) horse.

Although post-mortem examinations were not carried out, the cases were very “typical” for tannin poisoning, Lemoine said. All the horses—most of which were mares—had colic associated with bloody diarrhea, and all the owners reported that their horses had eaten acorns.

Different kinds of oak trees have different levels of tannin, according to CAPAE. The deciduous pedunculate oak and two varieties of sessile oak are considered to be the primary poisonous species in France which were likely responsible for the deaths of the horses in Normandy.

Howe