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Musculoskeletal system: 1.5 The hindlimb 217

GENERALISED ORTHOPAEDIC DISEASES
VetBooks.ir IMMUNE-MEDIATED POLYSYNOVITIS LYME DISEASE (BORRELIOSIS)

Definition/overview Definition/overview
This condition is relatively uncommon and most Confirmed clinical cases are rare, but in certain
reported cases are in foals. parts of the world, such as the north east of the USA
and parts of the UK, cases are seen regularly. The
Aetiology/pathophysiology disease has a significant zoonotic risk in humans bit-
Immune-mediated polysynovitis is caused by depo- ten by infected ticks in endemic areas.
sition of immune complexes in the synovium and
complement activation as a sequela to a primary Aetiology/pathophysiology
inflammatory focus such as pneumonia, infected The causative agent of Lyme disease is a spirochaete,
umbilicus or a peripheral abscess. Rhodococcus equi Borrelia burgdorferi, which is transmitted to the horse
is the most commonly reported primary infec- through a bite of an infected tick of the Ixodes fam-
tive agent, but the condition has been reported ily. These ticks have a 2-year, three-stage life cycle
following equine herpesvirus-4 and streptococ- and they can become infected during any stage by
cal infections. Frequently, more than one joint is feeding on mammalian hosts, commonly the white-
involved. footed mouse in the USA and possibly deer in the
UK. The stage of the life cycle infecting the horse
Clinical presentation is unknown. Not all ticks are infected with the spi-
Affected animals are presented with an effusion rochaete and infection varies by tick species and
of one or more joints, gait stiffness and low-grade geographic region. The infective agent is capable
lameness. If the source of the primary infection is of non-specifically activating various cells of the
still active, the animal will have the relevant clinical immune system, leading to the production of pro-
signs. inflammatory mediators, which tend to localise in
joints and result in chronic arthritis.
Differential diagnosis
Infectious arthritis; idiopathic arthritis; Lyme Clinical presentation
disease. Lameness and stiffness in one or more limbs, as
well as low-grade fever, are the most common clini-
Diagnosis cal signs. The lameness is often caused by arthri-
Clinical presentation of multiple joint effusion but tis, which can involve more than one joint and
minimal lameness is suggestive of the condition. may become chronic. Other clinical signs include
Analysis of synovial fluid collected aseptically from chronic weight loss, swollen joints, muscle tender-
affected joints reveals a WBC count of <20 × 10 /l, ness and anterior uveitis. Neurological signs may be
9
with a mixture of healthy neutrophils and mononu- seen if the bacteria have penetrated the central ner-
clear cells. vous system.

Management Differential diagnosis
The primary cause should be identified and treated OA; OCD; polysaccharide storage myopathy;
accordingly. The condition is self-limiting, and chronic intermittent rhabdomyolysis; equine proto-
recovery is expected within a few weeks. The animal zoal myelitis; immune-mediated polysynovitis.
should be restricted to a box and treatment with sys-
temic chondroprotective drugs may be considered. Diagnosis
Corticosteroid therapy may be contraindicated in Diagnosis of the disease in horses is difficult, and
the face of a bacterial infection. presumptive diagnosis is commonly based on history,