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Spring and autumn can present as scary times for horse owners, especially if the paddocks are surrounded by trees or back onto the forest. Even before the fields and trees are coming into sprouting, the maple (sycamore) seedlings are already growing on the pasture – there is a high risk of the horses devouring the first ‘greens’ and subsequently fall ill with atypical pasture myopathy. In autumn, there is also a risk of excessive seed intake, with winds carrying the seeds up to 200m, which can also lead to subsequent disease.

Caution with sycamore and ash maple

The good news is that not all trees from the maple family (Acer) contain the toxin hypoglycin A. Only the sycamore maple (Acer pseudoplatanus) and the ash maple (Acer negundo) propose a thread. The field maple (Acer campestre) and the Norway maple (Acer platanoides), on the other hand, are harmless as their seedlings and seeds do not contain the poison hypoglycin A. So, no need to panic every time you see a maple tree near a paddock. It can be tricky to identify the seedlings and seeds of the different maple tree species, but looking at the leaves, there is a clear distinction.

Sycamore tree leaf
© Elke Malenke
Field maple tree leaf
© Elke Malenke
Norway maple tree leaf
© Elke Malenke

The Norway maple seems most common in a lot of regions, luckily this type is harmless to horses. In case of the sycamore and ash maple: only the seeds, inflorescences and small seedlings are poisonous. The leaves of mature trees, which when growing over the fence line can be picked on or fall into the paddock in the autumn are unproblematic as long as the tree is not affected by tar spot disease. Nothing too worry, even if the horses get hold of an entire branch full of leaves that might fall into the paddock after a heavy storm.

It is suspected that the tar spot disease in sycamore maples possible further stimulate the formation of hypoglycin A. Norway maple can also be affected by tar spot disease, but does not produce hypoglycin A.

If the paddock is directly adjacent to a garden, you should also be aware that the ornamental garden plants fan maple (acer palmatum), silver maple (acer saccharinum) and velvet maple (acer velutinum) contain hypoglycin A.

Maple leaves with tar spot disease
Maple affected by tar spot disease © Sanoanimal

Utilising endangered pastures in summer

In order to avoid poisoning, grazing pasture with sycamore or ash maple trees should only be used in midsummer. At this time, the seedlings are already larger and thus lose their toxicity. In addition, no new seeds are (yet) falling to the ground, which could be ingested by horses. Most importantly, there is plenty of grass to graze on at this time, so maple – in whatever form – doesn’t attract much attention.

If the pasture has little vegetation or is bare, then it is essential to offer additional hay in the pasture to prevent the horses from ingesting too much toxic maple. It is even better to remove the horses from the pasture, as soon as it has been eaten down, and move to a paddock or a new field where there is plenty of grass. The ingestion of maple seedlings or seeds is therefore minimised.

Clean water should be provided in the pasture. It is of upmost importance to check and if necessary clean the water troughs out when the first seeds start to fall again in late summer/autumn. As hypoglycin A is water-soluble, therefore it is not enough to just collect the seeds out of the water trough – it must be completely changed every day.

Intake quantity and symptoms

As with all poisonings, this includes maple toxicity: the dosage is decisive. Symptoms of poisoning occur after ingesting around 20g of seeds, 50 seedlings, 150g of inflorescences or 2 litres of water that has been in contact with seeds.
No need to panic right away if the horse has eaten one or two seeds but be mindful eating maple seeds instead of grass frequently can quickly turn into a problem.

The ingestion of hypoglycin A leads to the aerobic energy metabolism in the cells no longer functioning, i.e. the production of energy using oxygen. In response, the cells switch to anaerobic metabolism. This ultimately leads to hyperacidity and thus to the death of the cells. Type I muscle cells of the skeletal muscles, the heart muscle and the respiratory muscles are particularly affected.

Around 75% of horses die about one to three days after the first symptoms appear.

These include general weakness, sweating, increased respiratory rate, red-brown urine, trembling, heart problems and, in the end, lying still. Young horses usually fall more seriously ill and have a lower chance of survival than older horses.

In the blood test, muscle enzymes such as CK and LDH are massively elevated, as well as an elevated blood sugar level (hyperglycaemia) and a low calcium level (hypocalcaemia). Hypoglycin A and its metabolites are also detectable in the blood and urine, but this has no influence on the prognosis, just helps to confirm the diagnosis.

Quick and prudent action is required

The most important measure to prevent further absorption of hypoglycin A via the intestine is the administration of activated charcoal by the vet. As soon as one horse in the herd shows symptoms that indicate atypical pasture myopathy, all horses (!) in the herd that were on the same pasture should be treated prophylactically with activated charcoal, regardless of whether they also show symptoms or not.

A measure to further support may be infusion therapy to support and stabilise the energy and fluid balance. Be aware that hypoglycin A can reach a growing foetus through the placenta in pregnant mares as well as pass through the colostrum or milk if a mare has a foal at foot. This can lead to the death of the foetus and abortion or to illness of the new-born foal, which is why extra attention should be paid to pasture management for broodmares.

Maple seedlings in the pasture
Maple seedlings in the pasture © Elke Malenke

Precaution is better than hindsight

Most cases of poisoning occur when horses are left in the field without an alternative feed source such as hay or plenty of grass, to be proactive and avoid any such poisoning it’s relatively simple: ensure that the horses always have sufficient hay or grass available. If the pasture has been eaten down to about 10cm of vegetation, the horses must be moved to a pasture with sufficient growth. In the case of maple trees on or around turnouts or paddock pathways, hay must be available 24/7 with sufficient feeding places so that no horse eats maple seeds or seedlings out of desperation.

If you have horses in the herd that eat absolutely everything that crosses their path, then the horses should only use this field in the summer months when there are no seeds flying and no (more) seedlings are being released by the adult tree. As a precaution, best to remove any small trees that have grown, from the paddock as they will eventually mature and produce seeds of their own that will further spread into the paddock.

Only Field maples or Norway maples don’t pose a thread, if they are sighted near the grazing pasture, then there is nothing to worry about.