Osteoporosis – the silent epidemic

Because of difficulties associated with diagnosing bone density loss, osteoporosis has been called a silent epidemic. Dr Jennifer Stewart explains.

Horses are prone to a range of musculoskeletal injuries with a range of signs and symptoms. In some there is swelling, pain or lameness, in others the only signs may be decreased performance, behavioural problems, reluctance to stop or turn hard, dropping leads, shortened stride, flat-footed or toe-first landing, stumbling, back pain (which most often arises secondary to front or hind limb pain), sweating or an elevated heart rate after exercise. In young growing horses there may be changes in conformation, leg abnormalities or growth anomalies.

Although swelling and pain are usually self-evident, lamenesses can be difficult to detect: they may affect more than one leg; there may be more than one problem; there are difficulties associated with observing lameness in short, high-speed events; or the footing may be soft and the lameness less obvious than on a hard surface. Riders can often feel that something is ‘not right’ but it’s difficult to pinpoint exactly where the problem lies. Often the reasons are obscure and a definitive diagnosis difficult.

Loss of bone density is one condition that is difficult to diagnose. Reduced bone mineral density (BMD) means the bones don’t contain as much mineral (primarily calcium) as they should, they are weaker and the risk of fractures is increased.

Osteoporosis is defined as a severe loss in BMD, predisposing the individual to spontaneous fractures; osteopenia, on the other hand, describes a decrease in BMD that is not associated with spontaneous fractures. Osteopenia is halfway between healthy bones and osteoporosis (Figure 1). Osteoporosis literally means ‘porous bones’ and describes bones that have lost sufficient calcium and have become weak, brittle. and prone to fractures. Tests on the tensile strength of normal and osteoporotic horse bone has shown that 31% less weight is required to cause fractures in horses with osteoporosis.

Recognised in humans as a silent epidemic, osteoporosis in horses is similarly difficult to diagnose – until a fracture occurs. However, there are many symptoms of osteoporosis that alert owners can pick up on – including dental or chewing problems; intermittent shifting lameness; tendon or ligament pain; general tenderness of the joints or joint pain; a stiff, stilted gait; a mild shortening of stride in front and/or hind limbs and a preference to canter rather than trot.

Causes of osteoporosis

Similar to humans and other species, osteoporosis is linked to low calcium intake, lack of exercise and certain diseases. A low calcium and/or high phosphorus intake in horses is caused by excess phosphorus and/or inadequate calcium or poor calcium availability in the diet. Demand for calcium may also exceed supply, during pregnancy and lactation for example. The amounts of calcium and phosphorus and the ratio of calcium to phosphorus in the diet needs to match that found in the bones and needed by the body.

However, diets low in calcium and high in phosphorus are not uncommon. Cereal, grassy/meadow and teff hays are often low in calcium and high in phosphorus and diets based on these are almost always calcium-deficient. Similarly for pastures: the amounts of calcium and phosphorus vary with the stage of growth and soil levels. During certain stages, rye grass has more phosphorus than calcium and studies in Victoria found even good quality pastures, including suitable grasses and legumes, don’t meet the calcium requirements of horses in hard work, aged horses, pregnant/lactating mares, or horses with dental issues. Other minerals can also be deficient. If you’d like to know more, Don’t guess; test! is an interesting read.

In addition, fertilising paddocks with manure and stable bedding can easily lead to excess phosphorus – especially during droughts or in dry climates where rain/irrigation are insufficient to leach the excess out of the soil. Published figures for mineral levels in pastures and hay can be very different to what is actually in the plants – a roughage analysis combined with a whole diet analysis is the only accurate way to assess calcium intake.

Poor calcium availability can occur on any pasture, but grasses high in oxalate further compromise calcium availability. All plants contain soluble and insoluble oxalates and both forms are capable of reducing calcium absorption by horses. Insoluble calcium-oxalate doesn’t dissolve in the gut and the calcium is not available to the horse. Soluble oxalates (mainly magnesium, potassium and sodium oxalates) do dissolve in the gut. The minerals are released from the oxalate and become available for absorption. However, the oxalates are now free to bind to any free calcium in the gut, reducing its availability. They can also be absorbed into the blood from whence they are filtered through the kidneys. Oxalate deposits in the kidney are a not infrequent cause of kidney failure in older horses.

Osteoporosis occurs in all horses grazing pastures dominated by high-oxalate grasses. Called nutritional secondary hyperparathyroidism, it causes enlarged facial bones and is often misnamed Big Head. These enlarged bones occur in around 10-15% of affected horses (Figures 3 & 4) and the risk is that if a horse doesn’t have signs of Big Head, it is natural to assume it doesn’t have osteoporosis. However, the most common symptoms are a vague, shifting lameness; joint tenderness; a stiff, stilted gait; weight loss; a rough coat; less voluntary movement; more time spent lying down; frequent shifting of weight; difficulty rising; pica (dirt eating); chewing problems (due to loosening of the teeth); tendinitis; bursitis; spontaneous fractures and avulsion of ligaments. All bones are affected – and variation in severity is thought to be due to external influences and mechanical loading.

Signs of osteoporosis

Osteoporosis is often confused with arthritis, bursitis, ringbone, joint sprain, spavin or an undetermined lameness. It may be mild or severe, acute or chronic and involves all the bones of the body. Osteoporosis should be suspected when lameness, shifting or intermittent and without apparent cause, is present – even if other conditions are concurrent. Pain and lameness result from tearing of the bone surface, fractures, strain in tendons and ligaments, thinning and/or hairline fractures in the vertebrae and discs, cannon bones or hock and loss of bone underlying the joints.

Diagnosing osteoporosis

Our eyes are a blunt instrument – but even X-rays won’t show any changes in bone density until the bones have has lost 30% of their calcium (Figure 2). Detecting changes in bone during growth, training, spelling or disease requires imaging techniques that have a high level of accuracy and precision. Currently, most imaging techniques used in horses do not possess such characteristics and are more suitable for detecting end-stage disease rather than subtle changes.

Other contributing factors

Other factors that can contribute to osteoporosis include lack of sprinting exercise, stabling, antacid ulcer treatments, foot trimming and PPID.

Conditioning and training involves many factors such as behaviour modification, cardio-vascular fitness, muscular fitness and skeletal strength. Of these, the most difficult to assess is skeletal strength. While proper nutrition is critical for bone health, it does not guarantee it without appropriate exercise. A decrease in bone mineral content is associated with stabling and exercise restriction – it is also affected by paddock size. Relatively short sprints, between 50 and 82 metres are necessary to maintain bone strength and as little as one sprint per week is enough to prevent mineral loss.

Omeprazole has been commonly provided to aid in healing or preventing

gastric ulcers in horses, but there have been concerns as to whether the suppression of gastric acid may inhibit absorption of calcium and impair skeletal health. Long term use at higher dose rates could pose a risk.

Foot trimming has a significant impact on distribution of load through the feet – and hence bone density. Both incorrect and infrequent trimming are associated with pedal osteitis – a form of osteoporosis. And again, if you’d like to know more, read Osteoporotic Coffin Bones, and Understanding Laminitis: How We View “Normal” Function.

Horses with Cushings (PPID) are easy to identify – they have long hair coats that don’t shed normally. But this isn’t the only side effect of PPID with which our horses must cope. They are also at risk of fractures, which are caused by loss of bone mineral density. Most common in non-weight-bearing bones (mandible, pedal bone, pelvis, and ribs), it is thought to be linked to lack of exercise and loss of muscle mass and strength.

Gender and age

Mares do not experience menopause and their bone microstructure is equivalent to that of geldings and stallions. Neither is there any scientific evidence of age-related osteoporosis, and younger and older horses have a similar incidence of fractures. In horses, bone strength peaks at about four-and-a half years of age, and bone mineral density increases until around six years of age and then remains unchanged.

In osteoporosis, bone loss is generalised but the bones are not uniformly affected. The hierarchy of bone loss is, in decreasing order, the jaw bones, especially the alveolar bone; other skull bones; ribs; vertebrae; and finally, long bones.

The silent disease

Osteoporosis is often called a silent, insidious disease because bone loss occurs without symptoms – and breaking a bone may be the first clue of osteoporosis. It is obscure in the initial stages, slow to develop, and easily confused with, and intimately associated with other conditions (arthritis, bursitis, ringbone, joint sprain, spavin or undetermined lameness) that result in disturbed locomotion. To add to its complexity, it is difficult to assess the strength of the horse’s skeleton until a problem develops. Often the first assessment of skeletal strength occurs when injuries arise and skeletal weaknesses become evident. Although many things including genetics, hormones, physical exercise and diet influence bone density, the importance of calcium cannot be overemphasized for protecting correct growth and development in young horses, and soundness in mature horses.

Dr Jennifer Stewart BVSc BSc PhD is an equine veterinarian, a member of the Australian Veterinary Association and Equine Veterinarians Australia, CEO of Jenquine and a consultant nutritionist in Equine Clinical Nutrition.

All content provided in this article is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your equine veterinarian.