12. Oxidative Stress in Horses

Like all multicelled animals, horses face the constant challenge of controlling oxidation to maintain proper metabolism and cellular defenses. A number of factors can impact the horse's capacity to defend against free radicals and excessive oxidation. Understanding these conditions can help the horseman to design horse care programs that enhance antioxidant support and avoid unnecessary oxidative stress.

Infections
The progress of the typical viral or bacterial infection can be seen as one inter-related free radical cascade, one that ends (we hope) in a return to oxidative balance, or health. Horses like people, are more vulnerable to infection when oxidative stress is high. Travel, heavy exercise, poor nutrition, toxicity and many other factors can contribute to a horse's oxidative stress. The relationship between antioxidant activity and immune function is close, and highly interdependent.

Trailering
Veterinary researchers are discovering more about the stresses of trailering, but we'll probably never understand what the horse experiences it until we stand, upright and unassisted, in the bed of a moving truck for several hours. The oxidative threat for horses is generated by a combination of psychological stress and prolonged aerobic output in restricted conditions.

Inflammation
Inflammation is a natural process that creates high levels of free radical activity through many different pathways. Successful resolution of inflammation depends on the body's antioxidant defenses to restore balance and avoid degenerative changes.

Rapid Growth
Growth is an energy-intensive process. During rapid development, oxidative stresses can contribute to the disruption of enzyme-dependent growth processes. This can result in inflammation and/or tissue deformation, creating new, potentially permanent sources of oxidative stress.

Environmental Toxins
Toxicologists are familiar with the central role free radicals in the toxicity of hazardous chemicals, drugs, and pollution. Exposure to insecticides, organic solvents, ozone, smoke, and other toxins in the air, feed and water puts horses at risk for damaging levels of oxidative stress.

Psychological Stress
Biologists have long recognized the importance of radicals in generating the physical effects of emotional stress. The demand for antioxidant support is likely to be higher during periods of psychological stress.

Protozoa and Mycotoxins
Microscopic protozoa generate damaging levels of free radicals. In horses afflicted with EPM, the protozoal radicals "eat" through the conductive myelin sheath of the nerves. The fungal mycotoxins and aflatoxins found in contaminated pastures, hay, and grains also due much of their damage through free radical attack.

Exercise
Exercise requires high levels of oxidation as fuel is consumed to meet the demand for work. Both aerobic and anaerobic exercise significantly increase oxidative stress, through somewhat different pathways. Exercise-induced oxidative stress is especially high inside the muscle cells, the heart, blood, and the linings of the lungs.

Trauma and Burns
When horses suffer cuts, injuries, or burns, free radicals are extremely active at the damage site. Antioxidants help by countering the radicals and restoring balance so healing can proceed.

About Selenium and GSH
Horses need selenium to make GPX (GSH peroxidase), a vital antioxidant enzyme. Selenium and GSH have a mutual affinity, and can combine spontaneously to form GPX. Selenium should never be over-fed.