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What causes fever in a horse?
The word fever is usually taken to mean an abnormal rise in body temperature, which is not the direct result of diet, exercise or environment. This means, then, that the heightened temperature is usually due to toxins that accompany an infection. Fever may also result when the destruction of tissue causes toxins in the blood. This may happen, for instance, when large parts of a tumor are undergoing necrosis (tissue death), or when burned tissue is being sloughed.

What happens internally to bring on fever?
The temperature of the horse is (normally 99.0' to 100.5' F) or (37.5C to 38.5C) - temperature is controlled by the hypothalamus, which is at the base of the brain. The hypothalamus causes fever by bringing into action a cold defence mechanism and inhibiting the heat loss mechanism. This occurs when it is stimulated by endopyrogen, a substance released from infection-fighting cells of the body. This entire chain may be set into motion by endotoxins, which are produced by infectious bacteria. Endoyrogens may also be released without the presence of infection, such as when
fever accompanies surgical recovery.

What are the external signs of fever?
In the initial stages of fever, constriction of the vessels near the surface of the skin brings on 'chills' and the skin feels cold. In spite of this, body temperature, taken through the rectum, is higher, and the pulse is faster (than the normal 44 beats per minute). The activity of shivering caused by the chills results in a further increase in temperature ' and it is at this stage that sweating begins. The vessels in the skin dilate, and the skin itself feels warm to the touch. At the peak of the fever, sweating may stop entirely. After the fever has 'broken,' muscles go limp, sweating begins again and body temperature falls. During this process, the horse may feel the same sort of discomfort common to fever in humans. General weakness, sensitivity to touch and irritability may be expected.

Is fever harmful?
Beyond an increase in temperature of about 10', the body is not capable of sustaining life. Even before this point is reached, there may be damage to tissue, and severe loss of fluids through sweating can lead to dehydration. However, many organisms that are dangerous to the health can only grow slowly and with great difficulty where a fever exists. Phagocytes and antibodies, the cells that fight infection in the body, can carry on their activities at a stepped-up pace during periods of fever. In other words, a fever is actually a beneficial body process.

How is a fever treated?
Since fever can accompany most infections, a veterinarian should be called to determine the exact cause. When this is done, the fever is best controlled by removing the source of the inflammation. Antibacterial drugs may be used against infection, or if dead tissue is the cause, it should be removed. If the temperature becomes high enough to be dangerous to the horse, drugs are available, through the veterinarian, which act directly on the temperature control mechanism. (Aspirin has this effect on humans.) These are generally not administered, though, until it is established that all sources of inflammation are removed.


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