Posted: 5th December 2024 | Back to news feed
Atypical Myopathy (AM) is a life-threatening acute muscle disease that occurs mainly in horses in autumn. In recent years, research has been conducted on the cause of AM and the pathophysiology of this muscle disease. One cause of atypical myopathy in horses is ingesting sycamore leaves, seeds, or sprouts. Since leaves and seeds end up in pastures mainly during autumn, AM is often seasonal and occurs primarily in autumn. Therefore, this muscle disease often occurs in horses with year-round pasture access.
The Cause of Atypical Myopathy
The first reports of Atypical Myopathy muscle disease were first made in 1939 (1). The first major outbreak occurred in the autumn of 1995 (1).
The muscle disease progresses rapidly and affects skeletal muscles and respiratory muscles. Skeletal muscles are important for supporting the skeleton and keeping the body in balance.
Muscles that are part of the respiratory system are important for oxygen uptake. Both types of muscles have important functions. In addition, research also suggests that the heart is affected by this disease (2).
Due to the acute breakdown of (important) muscles, about 70% of horses diagnosed with AM will be euthanised (3).
In more recent years, research has therefore been conducted to investigate possible causes causing this muscle disease. Results show that AM can be caused by a virus, bacteria or fungus, toxins derived from plants or nutritional deficiencies (4).
However, the most common cause of AM is related to the ingestion of sycamore seeds, leaves or sprouts (5).
The sycamore tree contains the toxin hypoglycin A (HGA) (6).
Results show that the ingestion of this substance causes the development of AM in the body. Seeds, leaves and sprouts are all three toxic but studies show that ingesting a kilogram of sprouts is more dangerous in comparison to a kilogram of maple seeds or leaves (1).
Yet AM is more common during autumn when seeds and leaves are more accessible to the horse and only a few sprouts. On the contrary, during spring there are more maple sprouts compared to leaves and seeds. An explanation for this may be that having access to nutrient-rich spring grass means that horses do not need to seek out other types of feed (1).
The number of horses diagnosed with AM varies every year. This is thought to be related to the amount of seeds available, the life stage of sycamore trees and the climate.
The component HGA, derived from the sycamore tree, is thus the most common cause of AM and, once absorbed into the body, adversely affects metabolic processes (3).
This causes multiple acyl-CoA dehydrogenase deficiency (MADD) which negatively affects muscle cells and mainly energy metabolism in muscles (7).
Therefore, this also causes the horse to become lethargic. Studies show that the degenerative process of AM primarily affects type I muscle cells (8).
In affected muscle cells, increased storage of fatty acids has been found, showing that the energy metabolism of fats in these cells is negatively affected (9).
Even though research has since been done on AM and the role of hypoglycin A in the progression of the disease, some things still remain unknown.
The clinical signs of AM in horses are: Dark-coloured urine, sweating, trembling muscles, sore and hard muscles, not wanting to move and lying down a lot (10). These are also symptoms similar to a work-related myopathy such as tying-up.
Toxic Sycamore Trees
There are as many as 100 different species of sycamore trees, not every species of which contains the substance hypoglycine A.
Many of them also do not grow in the UK. However, in the samples taken from Acer pseudoplatanus (Common Sycamore), a common maple tree in the UK, HGA was observed in the leaves, seeds and sprouts of the tree.
The concentration of hypoglycin A in the sprouts, leaves and seeds of the sycamore tree depends on several factors which can vary from tree to tree. Even in the different seeds that end up in pasture, the concentration of HGA can vary.
So if a sycamore tree grows near a pasture or paddock, this does not necessarily mean that it will make the horses in the pasture sick. However, it is important to prevent the development of AM and it cannot be estimated, without botanical examination, whether and how high the concentration of HGA is in the sycamore tree in question.
Treating Atypical Myopathy
Due to the rapid progression of this disease, it is important that action is taken immediately upon observing symptoms consistent with AM. As described earlier, the death rate of horses with AM is high. The faster the horse can be treated, the higher the chance of survival.
There is currently no specific treatment for curing AM. Treatments of AM consist of preventing further muscle damage, ensuring the horse stays hydrated and electrolyte balanced, providing the horse with sufficient energy mainly to support muscles that are affected, ensuring toxins are removed from the body and providing the horse pain relief (10).
Thus, the current treatment is supportive and symptomatic. Research has also shown that the administration of Carnitine, antioxidants and vitamins B, C and E and Selenium have a positive influence on horses with AM (11). Primarily riboflavin, or vitamin B2, has a positive influence on horses with AM (11). Myocare-E contains ingredients such as Vitamin E and Selenium to provide muscle support.
Prevention
Because AM is a disease that progresses rapidly and the risk of death is high, it is necessary to prevent the development of AM. It is important to avoid ingestion of seeds, leaves and sprouts of the sycamore tree.
Screen pastures regularly during autumn for sycamore leaves and seeds that have entered the pasture and remove them. Make sure the horse has access to sufficient forage throughout the day, especially when the quality of grass decreases in autumn (12).
In case of vitamin and mineral deficiencies from the diet, it is recommended to add a vitamin and mineral supplement to the diet. Synovium Prefit contains essential vitamins and minerals so adding this supplement ensures a balanced diet.
Check the drinking troughs regularly as sycamore leaves or seeds can also end up in these. If possible, it is advisable to turn horses out on pastures or paddocks near sycamore trees as little as possible.
References
1. Westermann, C.M., van Leeuwen, R., Van Raamsdonk, L.W.D., Mol, H.G.J. (2016) Hypoglycin A Concentrations in Maple Tree Species in the Netherlands and the Occurrence of Atypical Myopathy in Horses. Journal of Veterinary Internal Medicine, 20:880-884.
2. Verheyen, T., Decloedt, A., De Clercq, D., van Loon, G. (2012) Cardiac Changes in Horses with Atypical Myopathy. Journal of Veterinary Internal Medicine, 26:1019-1026.
3. Fabius, L.S., Westermann, C.M. (2018) Evidence-based therapy for atypical myopathy in horses. Equine Veterinary Education, 30(11):616-622.
4. Harris, P., Whitwell, K. (1990) Atypical myoglobinuria alert. The Veterinary Record, 127(24):603.
5. Votion, D.M., Francois, A.C., Kruse, C., Renaud B., Farinelle, A., Bouquieaux, M.C.,Marcillaud Pitel, C., Gustin, P. (2020) Answers to the Frequently Asked Questions Regarding Horse Feeding and Management Practices to Reduce the Risk of Atypical Myopathy. Animals, 10(365):1-15.
6. Bochnia, M., Scheidemann, W., Ziegler, J., Sander, J., Vollstedt, S., Glatter, N., Janzen., Terhardt, M., Zeyner, A. (2016) Predictive value of hypoglycin A and methylencyclopropylacetic acid conjugates in a horse with atypical myopathy in comparison to its cograzing partners. Equine Veterinary Education, 30(1):24-28.
7. Lemieux, H., Boemer, F., van Galen, G., Serteyn, D., Amory, H., Baise, E., Cassart, D., van Loon, G., Marcillaud-Pitel, C., Votion, D.M.(2016) Mitochondrial function is altered in horse atypical myopathy. Mitochondrion, 30:35-41.
8. Cassart, D., Baise, E., Delguste, C., Antoine, N., Votion, D., Amory, H., Rollin, F., Linden, A., Coignoul, F., Desmecht, D. (2007) Morphological alterations in oxidative muscles and mitochondrial structure associated with equine atypical myopathy. Equine Veterinary Journal, 39(1):26-32.
9. Votion, D.M. (2012) The Story of Equine Atypical Myopathy: A Review from the Beginning to a Possible End. ISRN Veterinary Science, 1-14.
10. Votion, D.M., Serteyn, D. (2008) Equine atypical myopathy: A review. The Veterinary Journal, 178(2):185-190.
11.Votion, D.M. (2016) Atypical myopathy: an update. In Practice, 38(5):241-246.
12.Votion, D.M., Linden, A., Delguste, C., Amory, H., Thiry, E., Engels, P., van Galen, G., Navet, R., Sluse, F., Serteyn, D., Saegerman, C. (2009) Atypical myopathy in grazing horses: a first exploratory data analysis. Veterinary Journal, 180(1):77-87.
For more information visit https://www.synovium.co.uk/
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