Sycamore seed poisoning

The sycamore tree (Acer pseudoplatanus) is now regarded as a threat to horse health. During a wet summer in 2014 the sycamore seed toxin was linked to over 200 horse deaths.

Equine atypical myopathy, also known as atypical myogloninuria or seasonal pasture myopathy (SPM), a highly fatal muscle disease, is a condition which can affect grazing horses caused by consumption of sycamore tree seeds or seedlings. Disease incidence peaks in Autumn and Spring, resulting from ingestion of seeds and seedlings containing the toxin hypoglycin A (HGA) resulting in rhabdomyolysis (destruction of muscle cells).

A typical story is of a horse demonstrating initially stiffness and a reluctance to move. The muscles suddenly become weak to the point the horse can no longer remain standing. Then, as quickly as clinical signs set in, the horse dies. Just 48 hours earlier the horse grazed happily at pasture—an overgrazed field full of seed heads and dead leaves.

Disease incidence peaks in Autumn and Spring. Weather conditions impact the degree of pasture contamination; warm summers the year before flowering have been associated with particularly strong flowering and fruiting intensity. Stormy weather in autumn is a contributor to dispersal. Humidity is also implicated with regions receiving high levels of rainfall found to have increased incidence.

It has been proposed that ingestion of seedlings is more likely on over-grazed pasture where seedlings protrude above the level of the grass, facilitating their ingestion. Similar may be true when climatic conditions lead to slow grass growth in early spring despite the rapid appearance of Acer seedlings.

Individual susceptibility to HGA is suspected to vary as clinical cases can originate from the same contaminated pasture as unaffected co-grazers. There is generally a higher incidence in younger horses, though older animals can be affected.

Equines that develop Atypical Myoglobinuria are often kept on sparse pastures with an accumulation of dead leaves, dead wood, and trees in or around the pastures. Chronic low-level exposure e.g. through ingestion of hay or haylage made from contaminated pasture containing seeds, seedlings or saplings is also possible. Water sources that have come in contact with plant material containing HGA are another potential source

Clinical signs

Unlike other muscle disorders, the disease’s clinical signs are not associated with exercise and include:

Reluctance to move, progressive stiffness and muscle tremors
Increased periods of recumbency (unable to rise after lying down)
Tachycardia (irregular and overly rapid heartbeat)
Myoglobinuria (dark red-brown coloured urine)
Respiratory distress
Hypothermia
Occasionally choke (oesophageal obstruction)

These clinical signs progress quickly with rapid respiratory rate within 48 hours and difficulty breathing and death within 72 hours in at least 75% of cases. The cause of death is a very specific metabolic block in the muscle’s ability to burn fat for fuel.

Diagnosis is based on a combination of history (i.e. grazing on pasture proximal to sycamore trees), clinical signs, and laboratory findings (Hypoglycin A detected in blood and urine).

Mortality rates are high (up to 75%) and many animals succumb within 2-3 days. Treatment consists of intensive supportive care, including nutritional support to provide alternative energy substrates and vitamin and mineral supplementation. There is no specific antidote or treatment for atypical myopathy, so prevention is key.

Protecting your equines

Identify trees within and close to grazed fields, bearing in mind that seeds can disperse a significant distance from the tree.
test for HGA in seeds or seedlings. Consult with the Irish Equine Centre.
Prevent access to affected areas during peak risk periods.
Removal of sycamore trees from affected pastures can be one approach, although, this might not always be feasible. Be aware that stormy Autumn weather or felling of trees may result in heavy contamination of pasture with seeds.
Avoid over-grazing and provide supplementary feeding to pastured horses, particularly
during autumn and spring.
Where possible, provide access to mains water rather than natural sources of water and ensure that any stationary water source is not situated under a Sycamore canopy. Pastures with a natural water source, such as a river or stream, should not be used during the high-risk seasons and an alternative water supply should be provided³.
In cases where the trees can’t be removed, decrease or discontinue turnout time on affected pastures from October through mid-December and in the early spring. Remember that even paddocks free of sycamore trees may still be at risk from seeds being blown on to the pasture so remain vigilant in this regard.

Other important preventive measures could include providing additional forage, preventing over grazing of pastures through rotational grazing, and limiting turnout to less than six hours per day during autumn and early spring³. Remember that feeding supplemented forage from the ground or close to sycamore trees can increase the risk of ingesting seeds.

Also, if mowing such pastures then collect and dispose of the cut grass and seedlings. Pastures contaminated with sycamore material should not be used to produce hay, haylage or silage as research has found that seeds and seedlings present in bales after six to eight months storage still contained appreciable amounts of HGA².

The condition has progressed from a sporadic condition to more frequent large outbreaks throughout Europe. Climate change may have increased sycamore tree seed and seedling production, seed dispersal by weather or toxin concentrations in seeds. It is also possible that increasing awareness of the disease has resulted in increasing reporting rather than an increased incidence.

References

Votion, D.-M. et al (2014). Identification of methylenecyclopropyl acetic acid in serum of European horses with atypical myopathy Equine Veterinary Journal
González-Medina S et al. (2019). Atypical myopathy-associated hypoglycin A toxin remains in sycamore seedlings despite mowing, herbicidal spraying or storage in hay and silage. Equine Veterinary Journal
Votion, D.-M. et al (2020). Answers to frequently asked questions regarding horse feeding and management practices to reduce the risk of atypical myopathy. Animals 10