Horse Dies in Decomposing Seaweed; Toxic Gas Blamed

A horse died and its owner fell unconscious within seconds of falling into decomposing green seaweed along the northern coast of Brittany, France, last week.

Vincent Petit, DVM, PhD, said he was hand-leading his 15-year-old English Thoroughbred gelding along a beachside road after riding a long stretch of gallop when both he and the horse slipped into an algae-filled sludge nearly up to the horse's withers.

"I cried for a man on a tractor to throw a rope, and then I looked at my horse and saw that his nose was falling into the sludge," Petit said. "I held his head up for him, but a few seconds later he went into respiratory arrest, without even a fight. It was incredibly fast."

Petit said that he became unconscious immediately after that and did not recover until he had been moved out of the sludge by passersby who prevented him from drowning after he lost consciousness.

"The rapid onset of death and loss of consciousness in this case strongly suggest that we are dealing with a poisonous gas emitted from the decomposing algae," said Pierre Philippe, MD, an emergency room physician at the hospital where Petit was transferred.

Although initial veterinary reports cited the cause of the horse's death as asphyxiation, the autopsy results show no signs of drowning. However, they tend to support the theory of gas poisoning, particularly the acute pulmonary edema, with the horse's lungs "filled with blood," Petit said.

Petit and his horse might have broken through a sort of white "crust" that can form over decomposing green algae, suddenly releasing dangerous levels of hydrogen sulfide, according to Claude Lesné, MD, CNRS, researcher and toxicologist specializing in airborne pollutants at the Department of Public Health of the University of Rennes Medical School.

The horse might have succumbed to the gases faster because he had been galloping, as physical effort significantly increases exposure to toxic gases, Lesné said. "He could have even been already breathing lower doses of the gas before the accident. But research also suggests that generally humans are more resistant to toxicity than animals," he said. He added that two dogs died in a similar accident in the same location last year.

Green algae-covered beaches are becoming more common worldwide, according to Lesné. "If the beach smells of rotten eggs, stay clear of it," he said, "especially if physical effort is involved."

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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