You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

As animals get older they often lose muscle mass - and eventually muscle wastage is obvious particularly along the spine (the processes become more prominent) and on the hindlegs, which appear very thin. But why does this happen ?

Muscle wastage is a loss of muscle volume and it usually causes weakness, and sometimes an inability to exercise properly. Muscle wasting can be caused by a number of factors :

• If an animal does not eating sufficient food it breaks down body tissues to release the energy that it's body cells need, so it loses body weight - which initially is mainly stored body fat, but also involves a loss of muscle mass . Thin animals do not have much fat to lose and so they breakdown their body muscle quite soon in the process, and cats and other obligate carnivores have an enzyme system which is geared up to breakdown proteins to release energy. Unfortunately, they can not down-regulate these enzymes when food is in short supply, so they breakdown their body proteins, including muscle.
• If an animal can not digest and/or absorb food properly  due to a gastrointestinal disorder, pancreatic disease or liver disease, it has the same effect as not being able to eat sufficient food.
• If an animal has a metabolic disorder  which greatly increases the rate of energy use (called catabolism)  it may not be able to maintain body weight and condition, and may breakdown  muscle tissue to help meet it's high energy needs. The most striking disease of this type is malignant cancer in which loss of body weight (including muscle mass) can be extremely rapid. Another example is hyperthyroidism (or thyrotoxicosis) in old cats.
• Muscle wasting  will occur if muscles are not used - a process called atrophy. This type of wasting is often seen in older, large and giant breed dogs that have osteoarthritis (eg associated with hip dysplasia) because the discomfort, or a limited range of movement in the limbs, does not allow them to use their muscles properly.
• With advancing age there is a natural increase in body fat as a proportion of body weight, and a decrease in lean body mass - which includes a decrease in muscle volume as well as an increase in total body fat. 
• Various muscle diseases , such as inflammation (myositis) can lead to muscle wastage.
• Various neurological diseases can lead to muscle wastage.

So, if you notice that your pet is developing muscle wasting  get the advice of your veterinarian. Some of these disorders - such as hyperthyroidism in cats, can be treated successfully. Conditions which are causing discomfort or pain can often be improved with medication.  

Finally, it is a good idea to maintain an exercise routine for your pet as it gets older because this will help to maintain muscle function and prevent disuse atrophy.

Updated October 2013