Red Maple (Acer rubrum) leaf toxicity is a relatively common cause of “oxidant-induced” anaemia.
The toxicity occurs through ingestion of dried leaves, typically in late summer and autumn. The dried wilted leaves can remain toxic to horses for up to 30 days. Although less common the toxicity can also occur through ingestion of bark from the tree. The precise toxic principle is not known, but the primary result is the formation of methaemoglobin. Iron in normal haemoglobin is in the reduced ferrous state (Fe2+), critical for binding oxygen. When the iron in haemoglobin is oxidised to the ferric state (Fe3+), it is no longer able to bind and carry oxygen – this is called methaemoglobin. There is a natural system in place to reduce methaemoglobin back to the healthy ferrous state. This is a cytochromeb5-based enzymatic system in red blood cells. There is a normally a low amount of methaemoglobin in normal animals. Unfortunately this system is easily overwhelmed leading to a proportion of haemoglobin circulating as methaemoglobin.
The methaemoglobin is subsequently oxidized causing the red blood cells to be destroyed within the circulation, so-called intravascular haemolysis. A characteristic finding of oxidant-induced damage to red blood cells is the formation of Heinz-Ehrlich bodies attached to the border of damaged red cells. The bodies are clumps of denatured haemoglobin. A special stain, new methylene blue, is needed to identify the Heinz-Ehrlich bodies. A similar toxicity occurs after ingestion of wild or domestic onions, kale, or rape. This is due to the toxic metabolite allyl propyl disulphide.
Signs of toxicity
Signs can appear within a couple of days of ingestion (range 18 hours to 5 days) and are dependent on the amount consumed. Affected animals become depressed and lethargic. They have a high heart rate and cardiac murmur due to the developing anaemia. If severe, the animal will become profoundly weak and collapse. Characteristically, the visible mucous membranes will become discoloured, changing from pale pink to chocolate brown. This is due to the methaemoglobin. This circulating pigment can cause the urine to turn brown and cause damage to the kidneys – pigment nephropathy.
Treatment and outcome
Affected (and non-affected) animals should have their access removed from red maple trees and dried leaves. Veterinary treatment is essential for a positive outcome, although cannot be guaranteed. New methylene blue can be given intravenously. Vitamin C and dexamethasone can also be helpful, along with careful fluid therapy. Many badly affected horses will require a blood transfusion.
Tags: Toxicities; Haemopoietic