Bones are an important structural component of the body. Healthy bones allow your pet to move freely, and express normal behaviours. Unhealthy bones on the other hand, can be a cause of pain, and they may impact your cat’s quality of life. One form of bone disease is that of brittle bones, a term used to describe bones that are structurally weak, and prone to breaking. This condition usually arises due to a deficiency or imbalance of calcium.

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Role of Calcium in cats 

Calcium is an essential mineral, meaning that it is essential for the normal function of the body. Many people know about the need for calcium to grow strong bones, but calcium also plays a role in the contraction of muscle as well as the transmission of impulses through the nervous system. It enters heart cells and plays an important role in the mechanism of the heart pumping too. Calcium is also required to form blood clots, an essential defence mechanism against cuts and wounds. 

Calcium Balance within the Feline Body

Despite the important role of calcium in the body, problems can occur with too much, as well as too little, so the body regulates how much calcium is available at any one time. The levels of calcium in the blood are related to another compound, phosphate. Phosphate binds to calcium making it less available or unbinds from it to make it more available. Excess calcium can be stored in bones and then removed from bones if needed.

If calcium levels get too high, calcitonin hormone is produced which prevents breakdown of bone calcium stores and allows calcium to be lost through the kidneys. If calcium levels get too low, parathyroid hormone (PTH) is produced. PTH triggers the loss of calcium from bone and reduces the loss of calcium through the kidneys. This hormone is produced by the parathyroid gland, a very small paired gland in the neck region.

Both calcium and phosphate are obtained from the diet. How much is absorbed depends on the type of food, the health of the intestinal tract, and the levels of vitamin D in the body. Vitamin D must also be obtained from the diet. Unlike us, cats cannot make vitamin D from exposure to the sun’s UV light. PTH increases the activity of vitamin D to improve calcium absorption when calcium levels are low.

Osteodystrophy in cats

When bones develop in an abnormal way, it is known as osteodystrophy. Diets deficient in calcium, phosphate and vitamin D can lead to this disease as can an imbalance in the calcium to phosphate ratio. This is known as nutritional hyperparathyroidism. Abnormal production of PTH can also cause osteodystrophy.

Juvenile rickets in young and growing animals affects the parts of the bones where growth occurs. It usually occurs with diets deficient in phosphate and vitamin D. An imbalance in the calcium to phosphate ratio is the most likely cause of the condition with those animals being fed an all-meat diet (ironically!) most likely to be affected.

During the crucial stage of bone development as a cat grows, nutritional osteodystrophy results in bones becoming soft. The normal forces of movement through affected bones can result in these soft bones becoming deformed. Symptoms worsen in the first few months of a kitten’s life, with affected cats becoming less willing to move. When they do move, they may appear lame, or unable to control their muscles. Normal activity may cause a bone to break (pathological fracture) or may cause a sudden deformity in the leg as the soft bone folds over instead of breaking (folding fracture).

Following an examination, your vet may recommend blood tests to assess calcium and phosphate levels, as well as x-rays to assess the density of the bone and the damage from any fractures. If caught early, before the bones have become damaged or distorted, it is possible to correct the condition by fixing the dietary imbalance.

Osteomalacia in cats

The softening of bone structure in a fully developed bone is known as osteomalacia. When caused by nutritional hyperparathyroidism, adult-onset osteomalacia has similarities to juvenile osteodystrophy. However, as the bones were fully grown, osteomalacia can appear less severe than osteodystrophy. Feeding an all-meat diet is again the typical cause.

Deformities may occur in the spine, creating a kink or overarched back. Eventually the long bones of the limbs may become brittle enough to fracture through normal movement. Affected cats may also have a poor coat condition or may seek out unusual objects to eat (known as pika). As bones within the body complete growth at different times, it is possible for an individual to have both osteodystrophy and osteomalacia present at the same time.

As with the juvenile disease, recovery with a corrected diet can be complete if detected before any fractures or deformities occur. To limit the risk of fractures, affected kittens and cats should have their activities limited for several weeks, especially jumping. Your vet may recommend a commercially-available balanced diet or referral to a veterinary nutritionist to ensure any new diet corrects the problem.

Renal Secondary Hyperparathyroidism

Although not typical, brittle bones can uncommonly occur with progressive kidney failure. As kidney function declines, phosphate levels can increase resulting in low calcium. As decreasing levels of calcium cause increased PTH to be produced, calcium stores are increasingly removed from bone. The kidneys also play a role in the activation of vitamin D. Low levels of the active form of this vitamin also trigger production of PTH.

More often, your cat would develop symptoms of kidney disease first, including increased drinking, altered appetite, weight loss and vomiting. The loss of calcium from bone with this condition is not usually severe enough to cause symptoms, but tooth loss, lameness, and pathological fractures could occur with advanced disease. Diets formulated for kidney health have adjusted levels of calcium and phosphate to limit these effects. Where suitable for your cat, your vet may discuss a kidney-focused diet if your pet is affected by kidney disease.

Other Disease Processes

A hereditary form of brittle bones can occur, known as osteogenesis imperfecta. An exceptionally rare genetic condition, it affects the long bones of the limbs, causing them to be more slender in shape, with a thin outer wall. Due to an abnormal gene involved in collagen production, it is not related to a calcium imbalance, but can present itself in a similar way, with pathological fractures.

Several health conditions can trigger sudden or temporary low calcium levels in the blood, including eclampsia in milk-producing queens, and after removal of the thyroid gland to treat hyperthyroidism. With any cause, there may be no symptoms if it is mild. If it is more severe, affected animals may demonstrate weakness, tremors and seizures before there is any concern about bones becoming brittle. Your vet can monitor calcium levels in the blood if they are concerned that this could be a cause of your pet’s symptoms.

Prevention of brittle bone disease in cats

Although not all cases of brittle bones can be prevented, the most common cause is an incorrect diet. Not all available diets on the market offer complete and balanced nutrition. So it is important to read the labels on foods to determine if they provide everything your pet needs. Well-known brands will usually offer complete nutrition with the correct calcium to phosphate ratio. If in doubt, look for more information on the brand’s website, or discuss with your vet.

If you would prefer to feed your cat a home-prepared diet, it is important to get this balanced from the start. Your vet can refer you to a veterinary nutritionist that can create recipes for you to prepare that ensure both balanced nutrition and the correct calcium and phosphate levels for your pet’s life stage. As your cat’s needs change throughout its life, you may need to get this diet reviewed periodically.

Conclusion

Brittle bones can lead to some painful complications that impact your cat’s life and welfare. Feeding a nutritionally appropriate diet can prevent most causes of the condition. Your vet can provide advice on how best to go about this for your circumstances, as well as investigate any concerns with your cat through blood tests and x-rays where necessary.

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