Myositis is an inflammation or infection of a muscle and tendinitis is an inflammation or rupture of tendons. The most frequent cause of myositis is wounds penetrating into a muscle. It can also occur from extension of an infection from nearby tissues, such as a bone infection. Occasionally it is caused by an intramuscular injection. Myositis causes pain, stiffness and swelling. If a limb is involved, there is usually lameness. The infection may break through the skin and discharge pus.
A tendon is a fibrous cord by which a muscle is attached to a bone. Tendons can rupture or become inflamed causing tendinitis. Trauma or injury is the most common cause of tendinitis and rupture of tendon. If the tendon is ruptured, the muscle cannot function properly.
The diagnosis of myositis and tendinitis is made by the symptoms and a physical examination. Treatment of inflammation involves the use of antibiotics. Some cases will require surgical opening and draining of the infected muscle. Tendinitis usually responds to rest, pain relievers, and restricted exercise of the affected area. Sometimes, corticosteroids are used. If the injury becomes chronic or the tendon is ruptured, surgical repair will be necessary.