English
Atypical Myopathy (AM) and Seasonal Pasture Myopathy (SPM) are highly fatal muscle disorders in horses caused by the ingestion of seeds of the box elder or sycamore maple trees.
Seeds of these species contain a compound called hypoglycin A that converts into a toxin once ingested by horses, leading to rapid muscle degeneration.
Clinical signs appear within 12-24 hours of ingestion and include lethargy, muscle weakness, tremors, stiffness, rapid heart rate, and recumbency.
Treatment options are limited as no antidote is currently available. Even with prompt veterinary intervention most horses do not recover. Both box elder and sycamore poisoning have mortality rates ranging from 75 to 90%.
Preventive strategies include removing or fencing off box elder and sycamore maple trees, avoiding overgrazing, limiting turnout during high-risk seasons, and ensuring horses receive a balanced diet. If exposure is suspected, early veterinary intervention provides the best chance of survival.
Causes of Atypical and Seasonal Pasture Myopathies
Horses are at risk of developing Atypical or Seasonal Pasture Myopathies if they consume seeds from box elder or sycamore maple trees. Both species of trees produce seeds that contain hypoglycin A, a protoxin that converts into a toxin after ingestion by horses. [1]
Atypical Myopathy (AM) affects horses in Europe, Australia, and New Zealand and is caused by ingestion of seeds from Acer pseudoplatanus trees, more commonly known as the sycamore maple. [1]
Seasonal Pasture Myopathy (SPM) occurs in North America and is the result of horses feeding on Acer negundo seeds. These trees are commonly referred to as box elders. [1][2]
These conditions are similar in presentation and etiology, both resulting in severe muscle damage with a poor prognosis and high fatality rate.
Box Elder
Box elder, scientifically known as Acer negundo, is a small perennial species of maple tree native to North America. It grows in the southern prairies in western Canada and it can be found across the United States, especially in the north east. [3]
Box elder is dioecious, meaning that male and female flowers are produced on separate trees. The male flowers are long, drooping clusters, while female flowers are shorter and produce winged seeds known as samaras, which are toxic to horses.
The seeds of the box elder maple drop from September to March in most regions. [3]
Key characteristics of this tree species include: [2][3][4][5]
- Habitat: primarily grows close to water sources, including floodplains, near lakes or river shores, and occasionally, along streams in hardwood forests
- Size: mature trees reach up to 50-65 feet (15-20 meters) in height and up to 3.3 feet (1 meter) in diameter
- Bark: young trees and branches have smooth, olive-green bark that develops into gray-brown with shallow cracks as the tree matures
- Leaves: compound, with three to seven leaflets that are ovate at the bottom and pointed at the tip
- Seeds: box elders produce winged seeds (samaras) in clusters. Each seed is 1-2 inches (2.5-5 cm) long
Sycamore Maple
Acer pseudoplatanus, commonly known as the sycamore maple, is a large species of maple native to Central Europe from the Aceraceae family. It is found throughout central and eastern Europe and the mountainous regions of southern Europe. [6]
Note that the sycamore maple is a distinct species from the North America sycamore tree (Platanus occidentalis), which is not toxic to horses.
The tree is dioecious, with male and female flowers on separate trees. Flowering in spring, the sycamore maple produces small, green-yellow flowers in hanging clusters that release seeds that are toxic to horses. Each seed has a wing that helps it to be carried away by the wind, aiding in dispersal.
The seeds of the sycamore maple drop from September to October in most regions. [6]
Other characteristics of this species include: [6]
- Habitat: maple sycamores grow easily in many soil types, making them a popular choice for an urban park or street ornamental
- Size: mature trees reach up to 100-115 feet (30-35 m) in height and up to 24-31 inches (60-80 cm) in diameter
- Bark: young trees and branches have smooth, gray bark that develops into deep gray with deep cracks as the tree matures
- Leaves: large with five pointed lobes, dark green color, up to 7 x 10 inches (18 x 26 cm) in size
- Seeds: paired samaras, each with a wing; samaras grow to 0.2-0.4 inches (5-10 mm) diameter, 1.2-2 inches 3-5 cm (3-5 cm) long
Toxicology
Both atypical and seasonal pasture myopathy are highly fatal acute muscle disorders in horses.
- Atypical Myopathy (AM) is caused by ingesting sycamore maple seeds and occurs primarily in Europe and New Zealand
- Seasonal Pasture Myopathy (SPM) is caused by ingestion of box elder maple seeds and generally affects horses in North America
The two conditions tend to have varied prevalence based on geography. This is due to the range and distribution of the associated trees. [1]
Both species of seeds contain the protoxin hypoglycin A. Hypoglycin A is a naturally occurring amino acid which is not toxic on its own. However, when ingested by horses, it is broken down into the toxic metabolite methylene cyclopropyl acetic acid (MCPA). [1][7]
Once formed, MCPA inhibits cellular enzymes called mitochondrial dehydrogenases that break down fatty acids and aid in energy production of cells. The inhibition of these enzymes causes a condition known as acquired multiple acyl-CoA dehydrogenase deficiency (MADD). [1][7]
MADD disrupts the normal breakdown of fatty acids for energy, leading to lipid (fat) accumulation in the muscles. This results in rapid muscle degeneration. [1][7][8][9][10]
The onset of clinical signs typically occurs 12 to 24 hours after the ingestion of a toxic amount of seeds, and progresses rapidly. Both conditions have an extremely high mortality rate that ranges between 75 and 90%. The majority of fatalities occur within 72 hours from the onset of symptoms. [1]