anterior uveitis - equine

Introduction to Immune Privilege in the Eye

The eye is recognized as an immune-privileged organ, functioning under unique immunological constraints to protect its specific structures and maintain vision. Immune privilege is crucial for preventing detrimental inflammatory responses to foreign antigens, which could otherwise impair critical functions and lead to vision loss. Structural barriers such as the blood-retinal barrier, along with active homeostasis mechanisms like immune suppression and tolerance, help maintain this privilege.

Immune-mediated eye diseases are common in various species, with equine recurrent uveitis (ERU) being one of the most significant conditions affecting horses, leading to chronic pain and potential blindness.

Overview of Equine Recurrent Uveitis (ERU)

ERU is globally acknowledged as the leading cause of blindness in horses, particularly affecting appaloosas due to their genetic predisposition. This debilitating condition is immune-mediated, but the exact underlying cause remains elusive. Current research indicates that mechanisms of action likely vary from case to case; the presence of autoantigens—specific proteins that can trigger an immune response against ocular tissue, particularly retinal antigens—has been commonly observed in affected horses. Furthermore, epitope spreading, a process where immune responses broaden to target additional antigens, plays a significant role in the disease's induction and progression, complicating treatment strategies.

Both genetic predispositions and environmental exposures contribute to the development of ERU, with research revealing that no single triggering antigen has been definitively identified. This underscores the complexity of ERU's pathogenesis and the need for comprehensive management approaches.

Leptospira as a Trigger

Leptospira bacteria, particularly L. interrogans, are commonly associated with ERU and are considered a significant potential trigger for the condition. Infection typically occurs through the consumption of contaminated water, with clinical signs manifesting 12-24 months post-exposure. The diagnosis of Leptospira-induced ERU requires vitreocentesis—a procedure to extract vitreous fluid—and subsequent antibody testing, as serum testing alone has proven ineffective. Epidemiological studies suggest a strong clinical association with ERU predominantly in the USA and specific regions in Europe, while the correlation appears weaker in the UK.

Clinical Presentation of ERU

ERU presents in three distinct forms:

• Classic ERU: Characterized by clearly defined episodes of inflammation, resulting in sudden onset of pain and discomfort.

• Insidious ERU: Involves low-grade inflammation without overt signs of discomfort, making it difficult to diagnose; this variant is particularly common in Appaloosa and draft horse breeds.

• Posterior ERU: Affects primarily the vitreous and retina, often presenting with minimal discomfort, and is also more frequently seen in draft breeds. Diagnosis relies on identifying clinical signs of ocular pain and inflammation alongside assessing indicators of chronicity.

Diagnosing ERU: Clinical Signs

Typical acute uveitis signs include:

• Pain and blepharospasm: Horses may squint or keep their eyes closed.

• Epiphora and miosis: Excessive tearing and constricted pupils are common.

• Aqueous changes: These include flare (flare refers to the presence of protein in the aqueous humor) and hypopyon (pus in the anterior chamber).

• Conjunctival and episcleral hyperaemia: Increased blood flow leading to redness of the conjunctiva.

• Corneal edema: This results in a hazy appearance of the cornea and the vitreous.

Signs of chronicity may include corneal changes, iris rests (anomalies of the iris), synechiae (adhesions between the iris and other structures), pigment changes on the cornea and iris, cataracts, and potential signs of glaucoma.

Treatment of Acute Uveitis

The treatment of acute uveitis requires aggressive anti-inflammatory and analgesic therapy to alleviate pain and prevent lasting damage. Key treatment points include:

• Administration of mydriatics, such as atropine, to soothe muscle spasms and mitigate potential cataract formation.

• Use of systemic NSAIDs like Flunixin meglumine, which are preferred over topical corticosteroids due to the risk of exacerbating conditions involving ulcerative keratitis.

• An initial intensive treatment period of 1-2 weeks, with a gradual tapering process over 2-4 weeks to avoid rebound flare-ups.

Long-Term Management of ERU

Prognosis and Options

The prognosis for vision in cases of ERU is generally poor, especially for Appaloosas, widely believed to have a genetic susceptibility to such autoimmune diseases. Long-term management options include:

• Long-term corticosteroid therapy: This option necessitates careful monitoring to detect and manage potential adverse effects on the horse's health.

• Long-term systemic NSAIDs: These pose similar risks to corticosteroids; thus, strict adherence to recommended dosages is essential to avoid complications.

• Ciclosporin A (CsA) implants: These have shown effectiveness in reducing the frequency of recurrence in affected individuals.

• Pars plana vitrectomy (PPV): An advanced surgical option that may be suitable for some cases, particularly those associated with Leptospira, despite the inherent surgical risks.

• Emerging treatments: Intravitreal gentamicin injections and autologous mesenchymal stem cells have been proposed as innovative therapies, demonstrating potential benefits in managing ERU.

• Adjunctive management: This includes responsible adjustments in care and environmental factors to minimize triggers for ERU, such as dietary management and avoiding exposure to potential pathogens.

Conclusion and Key Points

ERU is a multifaceted autoimmune disease with the capacity to severely impair vision. Seamless communication and collaboration between veterinarians and horse owners are imperative for early detection and proper management tailored to the individual case, aiming for better outcomes. The prevalence and impact of ERU reflect a complex interplay of environmental and genetic factors, warranting continued research for better understanding and management strategies.

References

Baake et al., 2019; Gilger and Hollingsworth, 2017; Deeg et al., 2006; Gerding and Gilger, 2016; Fischer et al., 2019; Voelter et al., 2020; and others.