Iron is absolutely essential for life. The high affinity of iron for oxygen is what makes it so useful in trapping oxygen in haemoglobin for delivery to the body tissues. Iron is also incorporated in some very reactive enzyme systems. However, iron’s high affinity for oxygen and high reactivity also makes it dangerous.
Free, ionized iron will react with peroxide or water in the body, ending in the creation of OH*, hydroxy radicals. OH* damages lipids/fats in cell structures and membranes, in the process generating more radicals to produce a chain reaction of destruction.
Iron is absorbed by binding to specific metal transporters in the small intestine, and also passively via the junctions between cells. Work in other species has shown that the volatile fatty acids produced from fibre fermentation enhance iron absorption in the colon. Because the horse is a hindgut fermenter, this likely is a significant source of iron. Ironbound to metal transporters is taken inside the intestinal cells. From there, it may be bound to blood proteins and released, or continue to accumulate and eventually be lost into the manure when the cell dies. By regulating how much iron gets released into the blood, the body has some control. However, the other pathways for absorption cannot be regulated.
If you ask any veterinary pathologist they will tell you that finding black, iron-loaded livers at necropsy is common in horses. The colour comes from iron deposits called hemosiderin. This is so common it is considered “normal”. There are reports of iron causing liver disease, even death in foals, but chronic conditions related to iron overload were not recognized in horses.
Iron overload in humans is usually caused by genetic disorders resulting in overabsorption, or diseases that require frequent transfusions. However, overload related to diet is also possible. Liver damage or liver failure does occur but is a late stage. Earlier symptoms include fatigue, joint pain/arthritis and often metabolic syndrome/insulin resistance.
The same syndrome and consequences have been recognized in a variety of animal species, including several birds, black rhinoceros, tapir, lemur, and dolphins. It may also be linked to IR in other old-world primates but iron overload has not been studied in them.
Iron overload symptoms in horses are likely a combination of direct iron effects and induced secondary deficiencies of other minerals. These include coat changes of bleaching and red ends on dark manes and tails, often hoof issues such as laminitis and abscessing.Iron overload symptoms in horses include coat changes of bleaching and red ends on dark manes and tails, often hoof issues such as laminitis and abscessing.Click To Tweet
A recently released study, shown below, linked iron status to IR in horses:
Other areas for exploration to determine if there is a link with iron status include arthritis, raised liver enzymes and other issues, even Cushing’s disease because the changes seen in the brains of horses with Cushing’s are virtually identical to those seen in human brains with iron overload. There is a discussion of this and suggestions for further research in my Equine Congress paper here.
Diagnosis and Treatment of Iron Overload in Horses
The only way to accurately diagnose iron overload is with the correct blood work. Serum iron alone is not accurate. It reflects iron in the diet but not how much is stored.
Transferrin is the protein that carries iron in the blood. When transferrin and iron are both measured, the per cent transferrin saturation can be calculated by dividing serum iron by transferrin and multiplying by 100. That % is useful in interpreting the third test that is needed, ferritin.Ferritin is a measure of the body's total iron content. A high ferritin can mean iron overload but chronic disease involving inflammation or infection may also elevate ferritin.Click To Tweet With true iron overload, transferrin saturation is high normal or elevated. There is currently only one laboratory in the world that can measure equine ferritin, the comparative haematology laboratory at Kansas State Veterinary Diagnostic Laboratory.
Iron overload in humans is treated by phlebotomy – blood draws. Bloodletting sounds positively medieval but is a very effective way to reduce the body’s iron level because of how much iron is in red blood cells. Stored iron is then used to replace the lost red cells. It works in horses too and can have dramatic effects on IR.
A case in point was a gelding with PPID and IR that needed to have surgery to break up and remove a very large bladder stone. Despite diet and pergolide control, his insulins were always running around 120 and he was iron overloaded. The surgery took several hours and blood loss was substantial. His insulin dropped to 20 after surgery, despite a postop diet of senior feed.Very tight mineral balancing to forage: hay, haylage, grass-fed, will help iron overload in horses.Click To Tweet
An alternative to bloodletting is very tight mineral balancing to the forage, grass, hay or haylage fed. The forage must be tested through forage analysis. This also works but can take a year or longer. The ratio of iron:copper: zinc must be no higher than 4:1:3. In some cases, even higher copper and zinc intakes are needed.
I truly hope interest in researching possible health effects of iron in horses increases. It is a very fertile area for study and the best part about it is that iron overload is preventable.
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