Red maple leaf poisoning of horses

Introduction

The native red maple (Acer rubrum), also called swamp or soft maple, is a potent killer of horses and ponies. Red maple is a tree native to the eastern half of North America.

Toxicity

The toxic ingredient in red maple leaves is believed to be gallic acid^{footnote 1[1]}. Gallic acid causes methemoglobinemia and is plentiful in both water and methanol extracts of red maple, sugar maple and silver maple, and in the extract fractions from these species that oxidize blood cells^{footnote 1[1]}. Ingestion of wilted or partially dried red maple leaves from fallen or pruned branches causes lysis of the red blood cells with the subsequent development of a hemolytic anemia, which can be deadly^{footnote 2[2]}. The problem can occur from June to October. Older wilted leaves (for example, those collected after September 15) cause faster poisoning than wilted leaves of early summer growth. This indicates that the amount of toxin increases in leaves during the summer. Wilted leaves remain toxic for a few weeks or more. Ingestion of fresh leaves does not appear to cause disease. The ingestion of 1.5–3 gm of leaves per kilogram of body weight (0.7–1.5 kg for the average 450-kg horse) will cause hemolytic disease.

Clinical syndrome

Horses often die within 18–24 hours of ingestion of wilted leaves. Horses that remain alive for 18–24 hours after ingestion of wilted leaves will be severely depressed and cyanotic and produce dark red or brown urine. The mucous membranes are blue to brown from poor oxygenation. They suffer intravascular and extravascular hemolysis (red blood cell breakdown). The percentage of red blood cells circulating in the blood (packed cell volume (PCV)) can drop as low as 8%–10% and the hemoglobin (Hb) concentration can be as low as 50 g/L. The normal PCV and Hb concentrations in horse blood are 28%–44% and 112–169 g/L respectively^{footnote 3[3]}. Death is due to a severe lack of oxygen delivery to vital cells from hemolysis of red blood cells, anemia and the oxidation of hemoglobin to methemoglobin, which is incapable of transporting oxygen. Of 32 horses, 19 (59%) died after ingesting wilted red maple leaves. The clinical signs observed included: colic, fever, followed by laminitis and disseminated intravascular coagulation. All horses had both gross and microscopic evidence of hemoglobin in their urine (hemoglobinuria)^{footnote 4[4]}.

Identifying red maples

The leaves of red maples are palmate (like the palm of your hand), 5–15 cm long and about as wide, with 3 to 5 lobes. The two sides of the centre lobe are almost parallel to the midvein^{footnote 5[5]}. Between the lobes, the leaf edge or leaf margin is serrated or jagged, while the leaf margin of sugar maple and Norway maple is smooth with no serrations. The underside of the red maple leaf is silver grey and the keys are red. Red maple can hybridize with silver maple, creating crosses of intermediate forms that should also be avoided near horse pastures. Silver maple is a soft maple with heavily indented lobes compared to red maple or sugar maple. In northern parts of Ontario, mountain maple with its small, heavily serrated 3- to 5-lobed palmate leaves could be confused with red maple. However, it only grows to 3–5 m or as a shrub. Red maple trees can grow up to 25 m high.

Maple trees commonly found in Ontario

Sugar maple

The sugar maple (Acer saccharum) has smooth leaf margins.

Silver maple

The silver maple (Acer saccharinum) has heavily indented leaf lobes with serrated margins.

Red maple

The Red maple (Acer rubrum) has serrated leaf margins and the margins of the centre lobe are almost parallel to the midvein. The underside of the leaf is silver grey and the keys are red.

Other maples and trees with similar leaves

Other maples and other trees have leaves similar to red maple.

Norway maple

The Norway maple (Acer platanoides) has smooth leaf margins.

Mountain maple

The Mountain maple (Acer spicatum) has 3-5 lobes with serrated margins.

Manitoba maple

London plane-tree

If in doubt, ensure that overhanging branches or leaves cannot fall into horse pastures OR remove the tree and plant a tree that is known to be safe for horses.

Not all trees that are red are red maple

The landscape industry sells a number of Norway maple tree types that have red and purple coloured leaves but are not the native "red maple" tree. It is not known if these specific "red" coloured maples will poison horses.

General comments

Native red maples should not be planted around pastures where horses or ponies may be kept. If red maple trees are already present, pasture owners might:

• Consider cutting them down and ensuring the stumps are completely dead to prevent sprouting of new shoots and leaves.
• Ensure leaves and branches are not placed on the manure pile where horses can reach them.
• Ensure that the fallen leaves and branches of uncut trees are not allowed to enter into horse paddocks by fencing the trees off to a distance that extends beyond the widest branches of red maples. Be aware that wind may blow detached leaves from nearby trees into paddocks.

Researchers have identified the presence of gallic acid in silver and sugar maple as well as red maple. However, no reports citing either of them as a cause in poisoning have been published.

As a general rule, horse owners should be on a constant vigil to ensure that horses do not browse on the leaves and branches of all trees.

J.L. Farrar's Trees in Canada^{footnote 5[5]} and Linda Kershaw's Trees of Ontario^{footnote 6[6]} are excellent references for identifying all trees.