All pastures with a sycamore tree in the vicinity heighten the risk of grazing equines developing the dangerous disease atypical myopathy, researchers say.
Equine atypical myopathy results from the ingestion of the seedlings or the distinctive winged seeds, known as samaras, released by trees from the Acer species.
The sycamore maple (Acer pseudoplatanus) is involved in European cases whereas the box elder (Acer negundo) is recognized as the cause in the Unites States.
Two compounds contained in the plant material, hypoglycin A and methylenecyclopropylglycine, trigger the disease.
Neither is toxic per se, but the metabolic byproducts that develop during their breakdown are, and it is these that inhibit crucial enzyme production that affects energy production from lipid metabolism.
The result is atypical myopathy, which has a death rate of 74%.
The typical sign is acute rhabdomyolysis (muscle breakdown) unrelated to exercise. Horses will be weak and may not be able to stand. Their urine is likely to be dark as a result of the muscle breakdown.
Animals may show stiffness, depression, muscle tremors or flickers, a reluctance to move, and sweating whilst showing a normal body temperature. Their mucus membranes may appear congested.
“In light of the high mortality rate and the absence of specific treatment, prevention is the key to avoid intoxication of animals,” Dominique‐Marie Votion and her colleagues wrote in the open-access journal Animals.
In their paper, which involved a review of scientific literature, the researchers addressed a series of questions frequently asked about the disease.
The cause of atypical myopathy was discovered and linked to Acer pseudoplatanus in Europe in 2014.
“Despite the cause of the intoxication being known, outbreaks have continued to occur.”
Owners often ask which maples are toxic. “The question about which maple trees are toxic is often associated with a request to identify trees on the pasture.”
Data collected by the Atypical Myopathy Alert Group in Belgium reveals that 99% of pastures contained or were directly bordered by a tree or trees.
“However, looking at the data from 2014 up to now, it is observed that 20% of atypical myopathy horse-owners could not answer if seeds and/or seedlings of sycamore trees were present in their meadow.
“Despite the educational material available on the internet, horse owners and veterinarians are still struggling to recognize the different maple species.”
A guide is available to help people differentiate the three Acer species commonly found in European pastures where cases have been declared.
The researchers note that the maple genus includes about 561 species, some of which are extensively planted as ornamental trees because of their autumnal color.
“As a result, there is not only a demand to distinguish non‐toxic trees (Acer platanoides (Norway maple) and Acer campestre (Field maple)) from Acer pseudoplatanus, but also many questions regarding the potential toxicity of other maple species.”
Not all maple tree species have been tested.
Among the tested species, the following species have tested positive for hypoglycin A and/or methylenecyclopropylglycine (non‐exhaustive list): Acer palmatum, Acer japonicum, Acer macrophyllum, Acer spicatum, Acer saccharinurn, and Acer saccharum.
“These exotics species may be found in ornamental gardens and may spread to the neighboring regions. Therefore, these Acer species might ultimately represent a risk of intoxication for equids.”
Atypical myopathy occurs seasonally, with outbreaks starting in autumn that may continue in early winter. There are also spring outbreaks that usually cease before summer. At pasture level, the risk can be decreased by avoiding contact with toxic plant material and by favoring low‐risk meadows for pasturing during autumn and spring.
Indeed, 94% of cases occur over two 3-month periods, starting in October and in March, for cases resulting from seeds and seedlings ingestion, respectively.
It is important to be able to recognize Acer pseudoplatanus, its samaras and seedlings, they say.
“When in doubt, professional expertise should be sought to identify the tree.”
“Depending on weather conditions, samaras of Acer species may be able to spread their seeds up to several hundred meters. Therefore, pasture contamination with seeds or seedlings is not necessarily linked to the presence of a tree on the pasture.”
In Europe, young and inactive animals with a thin to normal body condition and no feed supplementation, except for hay in autumn, are at higher risk.
The risk is also associated with full-time pasturing in a humid environment. Indeed, dead leaves piling up in autumn as well as the presence of trees and/or woods presumably exposes the horses to the sycamore maple.
Permanent pasturing of horses in at-risk pastures is not recommended, nor is the spread of manure in pastures with sycamores in the vicinity.
At a horse level, owners should reduce pasturing time according to weather conditions and to fewer than six hours a day during risk periods for horses on at-risk pasture. They should provide supplementary feeds, with preferences for feeds containing riboflavin, including toxin-free forage. This should not be placed on the ground, nor under sycamore trees.
“It is probably wise not to produce hay/haylage and/or silage from pasture areas in the vicinity of sycamore trees,” they added.
Water from the distribution network should be made available, as water from other sources may be contaminated by the toxins in at-risk areas.
They also recommended horses be supplemented with vitamins and have a salt block available.
“The risk of developing atypical myopathy results from the combinations of protective and risk factors.
“It is worth noting that atypical myopathy is an emerging disease and equids of any age and all pastures with a sycamore tree in the vicinity must be considered at risk.”
They say preventive measures should be implemented for a period of three months twice yearly, starting in March for “spring cases” then again in October to prevent “autumnal cases”.
“As mentioned before, these are the critical seasons and samaras or seedlings are likely to be present on the pasture.”
The study team comprised Votion, Anne‐Christine François, Caroline Kruse, Benoit Renaud,
Arnaud Farinelle, Marie‐Catherine Bouquieaux, Christel Marcillaud‐Pitel and Pascal Gustin, from a range of institutionsm, including the University of Liege in Belgium.
Votion, D.-M.; François, A.-C.; Kruse, C.; Renaud, B.; Farinelle, A.; Bouquieaux, M.-C.; Marcillaud-Pitel, C.; Gustin, P. Answers to the Frequently Asked Questions Regarding Horse Feeding and Management Practices to Reduce the Risk of Atypical Myopathy. Animals 2020, 10, 365.