Nutritional Secondary Hyperparathyroidism (NSH)

Nutritional Secondary Hyperparathyroidism is a bone diseasae in horses.

Anyone who owns horses and knows the tiniest bit about horse nutrition knows that the calcium:phosphorus ratio is important to growing horses.

The main reason is that an imbalanced Ca:P ratio can cause bone diseases in growing horses... of which is Nutritional Secondary Hyperparathyroidism, commonly abbreviated NSH.

NSH is caused by diets that contain too much phosphorus or not enough calcium.

This can be caused simply by a calcium deficiency, or it can be caused by excess phosphorus in the diet.

Not only does excess phosphorus in the diet change the Ca:P ratio, but an excess in phosphorus can actually create a calcium deficiency by hindering calcium absorption.

Symptoms of NSH

Nutritional Secondary Hyperparathyroidism can easily go unnoticed in the early stages, due to its general symptoms.

The two earliest signs of this disease are a shifting lameness and tender joints. However, since this disease often affects horses anywhere from weaning to 7 years of age, these first two symptoms can easily be attributed to the horse's training or exercise regimen.

Because of this, it is sometimes hard to catch this disease in its early stages.

However, the next two symptoms, reluctance to move and a stiff gait are usually serious enough symptoms to warrant a call to the vet in the eyes of most owners. Therefore, when these symptoms appear, the horse is more likely to get early treatment.

All the symptoms of NSH are caused by the bone losing its mineral content, and as a result, tiny fractures occurring in the bone.

This eventually leads to a loss of bone integrity, and a cartilage disruption, which eventually leads to tearing and/or detachment of tendons and ligaments.

As the minerals in the bone are replaced with fibrous tissue, the bones thicken, and a physical distortion occurs. Because the facial bones are most often affected with this disease, it has been termed "big head" in the past.

In the early 1900's when it was most prevalent, Nutritional Secondary Hyperparathyroidism was often called "bran disease" or "miller's disease" because horses that consumed large amounts of bran by-products, particularly that of wheat bran, were most often afflicted.

When the facial bones are affected, the horse often has trouble breathing, which is what usually warrants the veterinary call that leads to eventual diagnosis.

As NSH progresses in the facial bones, the horse will eventually have trouble chewing as well as dental pain. This will lead to a decreased intake of food, which will eventually result in weight loss and a poor body condition.

What kind of diet causes it?

The most common equine diet that results in Nutritional Secondary Hyperparathyroidism is the diet that consists of mature grass forage (hay or pasture) and large amounts of cereal grain-based supplements.

These diets are often borderline or low in calcium and borderline or high in phosphorus. This is because most grasses, though they vary in calcium content, often have a Ca:P ratio around 2:1, compared to 4:1 for legume forages.

Compounding the problem in these diets are the cereal grains, which are extremely low in calcium, while being very high in phosphorus. Most cereal grains have a Ca:P ratio of 1:6 or higher!

Unfortunately for most horses, this diet is still very common in today's horse population. Many horses receive grass hay as their forage and then corn or oats, or some combination of both as their grain.

Fortified commercial products usually do not have this issue, as they contain added calcium for the most part, but it is still something to keep in mind if you are feeding large amounts of cereal grains (more than 4-5 pounds/day).

Diets that are heavily supplemented with wheat bran or rice bran can also cause NSH. However, there are fortified rice brans available on the market today that contain added calcium to keep the ratio where it should be.

How is it treated and prevented?

The good news about NSH is that it is easy to treat. The diet must simply be altered to provide more calcium while reducing the phosphorus so that a correct Ca:P ratio is reached.

For early treatment of the disease, a diet with Ca:P ratios ranging from 3:1 to 6:1 has been advocated.

In older horses that contract the disease, the prognosis is very good. The bony changes will take a few months to reverse, and some slight facial distortion may be permanent, but they will usually not have lifelong problems resulting from the disease.

However, in younger horses, this disease is much more serious, because their bones are still growing. This disease can cause serious problems to the long bones, and how serious the problems are depends on if the horse will suffer lifelong problems as a result of the disease.

The easiest and most effective way to prevent this disease is to ensure that your horses receive a diet that has a balanced Ca:P ratio, particularly if there are growing horses in your herd.

Just as essential is to ensure that the horses have enough calcium, as that can cause problems even if the Ca:P ratio is correct.

Nutritional Secondary Hyperparathyroidism is a disease that can be problematic in horses. Thankfully it is fairly rare in today's horse world, due mostly in part to the large amount of fortified feeds available to today's horse owner.

However, it is still around, despite is easy prevention, so horse owners should be aware of the symptoms of this disease and how to prevent it.

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