Arthritis is inflammation of one or more joints and is often associated with bacterial or viral infection, although trauma may also be involved.
A wide range of bacteria may infect joints. Most are normal residents of skin, faeces or soil. Erysipelothrix and Chlamydophila are most commonly involved in Australian sheep. Erysipelothrix, a common dip inhabitant, enters the body via shearing cuts, marking and mulesing wounds, and grass seed penetrations. Chlamydophila are believed to be acquired by inhalation, ingestion or via the conjunctiva.
The arthritis of goats known as big knee, is caused by caprine arthritis encephalitis virus, and is common in dairy goats but very rare in meat goats of the type selected for live export.
The infecting organisms may be disseminated through the blood stream and pass into many or all joint cavities. In some joints the infection may spontaneously resolve without permanent damage, while in other joints infection may persist and result in lameness and other changes.
There is joint pain on movement and restricted movement. Pain and swelling about one or more joints in the early stages may subside in a few days, but usually persists in the knee and hock.
In early stages the joint fluid is increased, and less viscid than normal. It then becomes cloudy and clots form in recesses of the joint cavity. The lining of the joint is velvety and reddened. Persistent inflammation will lead to erosion of the normally-smooth joint surface, especially about the margins. Thickening of the joint capsule enlarges the joint. Irregular small masses of bone and cartilage are attached at the joint margin.
With chronic lameness there will be atrophy of the muscle of the affected limb.
Specimens required for laboratory determination of pathogens include aseptically collected joint fluid; chilled and unopened joints with the joint capsule intact; and acute and convalescent sera from affected sheep submitted chilled for chlamydial serology. It is often difficult to demonstrate the presence of organisms in chronic lesions.
Chronic cases are likely to have irreversible joint changes and treatment is unlikely to be of benefit. Oxytetracycline antibiotics are recommended when treating earlier cases, although procaine penicillin or erythromycin may be more effective in some circumstances.
Routine prevention is based on minimising risks of wounds and traumatic injury, and ensuring optimal general hygiene. A vaccine is available to prevent erysipelas arthritis in lambs.