This infectious disease affects sheep, calves, goats, and pigs. Chlamydial polyarthritis of sheep was first described in Wisconsin and has since been recognized in the western USA, Australia, and New Zealand. The disease was identified in calves from the USA, Australia, and Austria, and in pigs from Austria, Bulgaria, and the USA.

Etiology and Epidemiology

Strains of the causal agent, Chlamydophila psittaci, isolated from affected joints of sheep and calves are identical, but strain-specific antigens in their cell walls distinguish them from those that cause abortions in sheep and cattle (see Abortion in Large Animals).

The GI tract is of prime importance in the pathogenesis of chlamydial polyarthritis (see Intestinal Chlamydial Infections). The disease has been reproduced experimentally by oral inoculation. Because chlamydiae can be recovered from the feces of clinically normal calves and lambs, it is most likely the GI tract wherein the host and parasite stay frequently in balance. If there is a shift in favor of the chlamydiae, then a systemic infection and chlamydemia ensues; the ultimate site of replication is the synovial membrane. The GI tract also has been infected after experimental intra-articular inoculations. Chlamydiae are excreted in the feces and urine and transmitted via ingestion or, in some cases, inhalation.

Clinical Findings

Chlamydial polyarthritis is seen in lambs on range, on farms, and in feedlots. Morbidity may be 5–75%. Rectal temperatures are 102–107°F (39–41.5°C). Varying degrees of stiffness, lameness, anorexia, and a concurrent conjunctivitis (see Chlamydial Conjunctivitis) may be seen. Affected sheep are depressed, reluctant to move, and often hesitate to stand and bear weight on one or more limbs, but they may “warm out” of stiffness and lameness after forced exercise. Incidence of the disease in sheep on range is highest between late summer and early winter.

The disease affects cattle of all ages but calves 4–30 days old are affected more severely. Calves may have fever, are moderately alert, and usually nurse if carried to the dam and supported while sucking. They invariably also have diarrhea, which can be severe. Affected calves assume a hunched position while standing; their joints usually are swollen, and palpation causes pain. Navel involvement and nervous signs are not seen.

Chlamydial polyarthritis has been recognized in older pigs as well as in young piglets. The affected piglets become febrile and anorectic and may develop nasal catarrh, difficulties in breathing, and conjunctivitis. This condition has not been clearly differentiated from other infections that lead to polyserositis and arthritis in pigs.

Lesions

The most striking tissue changes are in the joints. In lambs, enlargement of the joints is not often noticed, but in chronic advanced cases, the stifle, hock, and elbow may be slightly enlarged. In calves, periarticular subcutaneous edema along tendon sheaths and fluid-filled, fluctuating synovial sacs contribute to enlargement of the joints. Most affected joints of lambs or calves contain excessive, grayish yellow, turbid synovial fluid. Fibrin flakes and plaques in the recesses of the affected joints may adhere firmly to the synovial membranes. Joint capsules are thickened. Articular cartilage is smooth, and erosions or evidence of marginal compensatory changes are not present. Tendon sheaths of severely affected lambs and calves may be distended and contain creamy, grayish yellow exudate. Surrounding muscles are hyperemic and edematous, with petechiae in their associated fascial planes.

Diagnosis

The history and careful examination of the pathologic changes in the joints and other organs can be of diagnostic value. Cytologic examination of synovial fluids or tissues may reveal chlamydial elementary bodies or cytoplasmic inclusions. Isolation and identification of the causative agent from affected joints confirms the diagnosis. Bacteriologic cultures of affected joints are usually negative, but Escherichia coli or streptococci occasionally may be isolated. If the joints of young calves are arthritic, and navel lesions are absent, chlamydial polyarthritis should be considered.

Clinical and pathologic features distinguish chlamydial polyarthritis from most other conditions that cause stiffness and lameness in lambs. Lambs with mineral deficiency or osteomalacia usually are not febrile. The abnormal osteogenesis in these 2 conditions and the distinct lesions of white muscle disease are virtually pathognomonic. In arthritis caused by Erysipelothrix rhusiopathiae, there are deposits on and pitting of articular surfaces, periarticular fibrosis, and osteophyte formation. Laminitis due to bluetongue virus infection can be differentiated clinically and etiologically. Detailed microbiologic investigations are required to differentiate chlamydial arthritis from mycoplasmal arthritis.

Treatment and Prevention

If begun early, therapy with long-acting penicillin, tetracyclines, or tylosin appears to be beneficial. More advanced lesions do not respond satisfactorily. Feeding chlortetracycline at 150–200 mg/day to affected lambs in feedlots reduces the incidence of chlamydial polyarthritis. No approved vaccines are available.

Last full review/revision March 2012 by C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS