Development of septic polysynovitis and uveitis in foals experimentally infected with Rhodococcus equi.
Authors: Huber Laura, Giguère Steeve, Berghaus Londa J, Hanafi Amanda, Vitosh-Sillman Sarah, Czerwinski Sarah L
Journal: PloS one
Summary
Rhodococcus equi typically causes pyogranulomatous pneumonia in foals, but extrapulmonary manifestations such as uveitis and polysynovitis occur and have been presumed immune-mediated; however, their pathogenesis remains unclear. Researchers experimentally infected 28 foals aged 3–4 weeks via intratracheal inoculation with either high (10⁸ CFU; n=16) or low (10⁷ CFU; n=12) bacterial loads, then monitored clinical signs and performed necropsy 14 days post-infection, culturing R. equi from aqueous humour and synovial fluid samples. High-inoculum foals developed significantly more extensive lung involvement (31.8% versus 14.4% of lung affected), and both polysynovitis (20 of 28 foals) and uveitis (14 of 25 foals) occurred with greater frequency and severity in this group; critically, R. equi was cultured directly from aqueous humour (11 foals) and synovial fluid (14 foals), indicating these are active septic infections rather than purely immune-mediated sequelae. These findings reframe polysynovitis and uveitis as direct infectious complications correlating with pneumonic burden rather than secondary inflammatory phenomena, suggesting that systemic or haematogenous dissemination occurs alongside pulmonary disease severity in naturally infected foals. For practitioners managing R. equi-affected foals presenting with joint or ocular signs, this evidence supports aggressive antimicrobial therapy targeting systemic infection and warrants careful monitoring of respiratory status as a predictor of extrapulmonary disease development.
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Practical Takeaways
- •Uveitis and joint swelling in foals with R. equi pneumonia indicate septic infection requiring aggressive systemic antimicrobial therapy; these are not simply secondary immune reactions
- •Disease severity in the lungs directly predicts risk of systemic dissemination to joints and eyes, so early detection and treatment of respiratory signs is critical to prevent complications
- •Foals presenting with concurrent polysynovitis and uveitis should be assumed to have active septic infection in these tissues until proven otherwise, guiding diagnostic and therapeutic decisions
Key Findings
- •High-dose R. equi inoculum (1×10⁸ CFU) produced significantly greater lung involvement (31.8% vs 14.4%) compared to low-dose (1×10⁷ CFU)
- •Polysynovitis developed in 20 of 28 foals (71%) and uveitis in 14 of 25 foals (56%), with significantly higher risk in high-inoculum group
- •R. equi was cultured from aqueous humor in 11 foals and synovial fluid in 14 foals, indicating direct septic infection rather than purely immune-mediated disease
- •Aqueous humor protein concentration was significantly elevated in high-inoculum group, correlating with disease severity