Sheep may have greater metabolic resistance than horses to a dangerous compound contained in the seeds, seedlings and leaves of some species of Acer trees.
Horses that ingest hypoglycin A can develop a distressing condition called atypical myopathy, in which muscles, including the heart, suffer damage.
It kills around three-quarters of affected horses.
Researchers have noticed that some horses appear more susceptible than others, perhaps because of underlying genetic differences.
In Britain and Northern Europe, the main source of hypoglycin A is sycamore trees (Acer pseudoplatanus), a species found commonly near pastures.
Hypoglycin A is known to be toxic in several species. In humans, unlike horses, it can cause brain and liver damage.
Researchers in England, writing in the Journal of Veterinary Internal Medicine, noted that hypoglycin A intoxication after ingestion of Acer tree material has never been confirmed in domesticated ruminants, despite their similar grazing habitats.
However, atypical myopathy was recently confirmed in exotic deer (Elaphurus davidianus) kept at a zoo in Germany. And in the 1970s, an unusual outbreak of disease involving severe muscle breakdown was encountered in cattle in Britain. Plant intoxication was suggested as the cause but was not confirmed.
“Nevertheless, the lack of additional or confirmed reports of domesticated ruminant intoxication, despite enhanced recognition and possible increased incidence of atypical myopathy in horses, suggests that ruminants might be less sensitive to the toxin.”
This, however, has not been established.
For their study, Sonia González‐Medina and her fellow researchers wondered whether the function of the rumen might break down hypoglycin A, thus reducing its bioavailability.
Stomach and rumen fluid samples were collected from five adult horses and five adult sheep.
Ground-up sycamore seeds were added to the different gastric fluids, with samples taken for testing after one hour, and two hours.
The study team found that neither the rumen fluid nor equine gastric fluid had any influence on the hypoglycin A content of the mixtures, with neither differing from water and methanol used as a control.
Indeed, both equine and sheep gastric juices enhanced the soluble hypoglycin A content of the mixture in comparison with water alone after 1 hour.
In the second phase of the study, serum samples were taken from three groups of healthy ewes, as well as grass samples from their pastures. Two of the pastures were contaminated with sycamore seedlings.
The third pasture was free of seedlings, and the sheep tested clear for hypoglycin A.
All the sheep in one of the groups on contaminated pasture return positive tests for hypoglycin A in their samples after 48 hours on the grass.
In the other group, half the animals, including two nursing lambs, tested positive.
The researchers said they found no evidence that sheep rumen fluids can break down hypoglycin A.
Indeed, both rumen and stomach fluids enhanced its release from seeds in comparison with water.
They also showed that hypoglycin A is absorbed systemically by sheep after eating Acer material.
Given that the sheep in the study had appreciable serum hypoglycin A concentrations within 48 hours of grazing contaminated pasture, rumen protection in this species seemed unlikely, the researchers said.
Indeed, serum hypoglycin A concentrations in these groups of sheep were similar to those in horses with subclinical disease (with mildly raised muscle enzyme activities) or slightly lower than those in horses without clinical signs that were co‐grazing with horses affected by atypical myopathy.
“We cannot rule out the possibility that disease would not have occurred if the animals had absorbed more toxin.”
They continued: “Despite ingestion of the compound, and despite evidence for its metabolism in at least one animal, we found no evidence for associated toxicosis.
“Whether disease develops in sheep that absorb higher amounts of hypoglycin A remains to be seen. Future study of the relative differences in susceptibility of horses and sheep to the toxin might best be made in vitro.
“Such work might help establish whether sheep can be used safely for Sycamore decontamination of pastures.”
They concluded: “Any reduced sensitivity to hypoglycin A intoxication in sheep seems unrelated to ruminal degradation.
“Any reduced sensitivity of sheep to hypoglycin A might be related to greater metabolic resistance rather than selective grazing habits or lower bioavailability.”
The detection of hypoglycin A in nursing lambs suggests it is excreted in milk, they said.
The study team comprised González‐Medina, Yu‐Mei Chang and Richard Piercy, all with the Comparative Neuromuscular Diseases Laboratory at the Royal Veterinary College London; and William Bevin and Rafael Alzola‐Domingo, with Oakham Veterinary Hospital.
González‐Medina, S, Bevin, W, Alzola‐Domingo, R, Chang, Y‐M, Piercy, RJ. Hypoglycin A absorption in sheep without concurrent clinical or biochemical evidence of disease. J Vet Intern Med. 2021; 35: 1170– 1176. https://doi.org/10.1111/jvim.16077