A foal has been euthanized after becoming poisoned by sycamore maple (Acer pseudoplatanus) seed toxins in its dam’s milk. This marks the second documented case of a newborn foal contracting atypical myopathy and the first to link the condition to toxins passed through the mare’s milk.

The eight-day-old foal and its dam, living at pasture in southwest Germany, were both “weak and depressed,” and diagnostic testing revealed they ingested the toxins responsible for atypical myopathy. Samples of the mare’s milk two days later continued to show traces of the toxins, indicating the foal had ingested them by suckling, said Johannes Sander, Dr. med., of the Screening-Labor Hannover laboratory in Ronnenberg, Germany.

Atypical myopathy, also known as pasture myopathy, is caused by hypoglycin A (HGA) and methylenecyclopropylglycine (MCPG), two toxic amino acids found in certain sycamore maple and box elder tree (Acer negundo) seeds. Ingestion is often fatal.

The mare recovered with treatment, but the foal’s condition was so severe, veterinarians decided to euthanize it, Sander said.

Further testing of other samples of commercially available mare’s milk (for human consumption) also revealed the presence of HGA.

It’s Official: Hypoglycin A Passes Through Mares’ Milk, Foals Susceptible

The findings confirm that sycamore maple seed toxins can pass through mares’ milk to their foals, he said. In fact, it might happen more frequently than people realize.

“Foals are certainly not less sensitive than adult horses,” Sander explained. “On the contrary, among the victims of maple poisoning, very young horses are particularly frequent.”

It’s something medical doctors have already noted in young humans, he added. Children are more frequently affected than adults by poisoning with HGA- and MCPG-containing fruits like litchis.

Sycamore maple seed poisoning in young foals probably goes underdiagnosed, according to Sander. “There are certainly several reasons for this, and one could be that the mare in such a case may not necessarily show the symptoms of atypical myopathy,” he said.

Sampling the Dam’s Milk and Random Mares’ Milk

Sander and his fellow researchers ran analyses on a sample of the study mare’s milk two days after she was diagnosed with atypical myopathy. Immediately after the diagnosis, the foal was euthanized, and the mare was kept on a diet without maple seeds. Even though HGA metabolizes quickly—meaning it’s harder to trace in samples—the scientists found enough quantities of it two days after the mare had stopped eating maple seeds to raise concern.

They also found detectable levels of HGA and MCPG in one of six samples of mare’s milk sold frozen by breeders for human consumption, he said. This milk is produced from a blend of milk coming from several mares, so it’s likely the toxins were diluted. However, their presence raises concern and suggests milk from mares—and even other mammals—could benefit from HGA/MCPG testing before being sold.

“Sycamore maple toxins are metabolized rapidly at first shortly after ingestion, but as concentrations fall, the rate decreases,” he said. “After ingestion of a higher dose, undegraded toxin may well still be detectable after 24 or 48 hours. The degradation, which takes place over a longer period of time, means that repeated ingestion leads to an accumulation of the toxins. If certain limits are exceeded, the energy metabolism collapses.”

It’s the point about limits that’s important to keep in mind, he added. At low doses, the toxins are harmless to both humans and horses. However, as concentrations increase, so does the risk. “It all depends on the dose,” he said.

Breeders Beware: Keep Pregnant and Lactating Mares Away From Dangerous Trees

Owners might not realize that sycamore maple trees present a danger to their breeding stock, said Sander. Problems might arise without them knowing it—until one day it’s too late. That’s even more likely when forage is scarce.

“Breeders usually make the observation that in pastures where sycamore maple trees are present, horses are kept for many years without any incidents,” he said. “Therefore, the danger posed by the seeds or seedlings is often underestimated. The danger becomes greater the less well fed the horses are. Ensuring adequate supply through supplemental feeding can reduce the danger.”

The owner and treating veterinarian did not report the breed, age, or other details about the mare whose milk they analyzed, Sander said. And unfortunately, they could not provide milk samples earlier than two days after the foal’s death.