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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • ERU is a common cause of blindness and chronic discomfort in a wide variety of horses. ERU is used, confusingly, to refer to a group of recurrent or persistent immune-mediated diseases of multiple origins, as well as a specific diagnosis. Recurrent attacks of uveitis are the hallmark of ERU. Additionally, some horses will present with a chronic and persistent course of disease
  • ERU is characterized by recurrent episodes of intraocular inflammation of variable degrees of intensity that are interrupted by periods of quiescence

Pathophysiology!!navigator!!

ERU appears to have characteristics of an infection-mediated autoimmune disease. The triggers for ERU are not completely understood. ERU can occur as a late sequela to systemic infection with ocular signs developing months after exposure. Hypersensitivity to infectious agents such as Leptospira interrogansis possible.

Systems Affected!!navigator!!

Ophthalmic

Genetics!!navigator!!

Unknown but some genetic predisposition has been shown in German Warmblood and Appaloosa horses. There is evidence of a genetic link between ERU and IL-17A and IL-17F in German Warmblood horses.

Incidence/Prevalence!!navigator!!

1–2% of American horses are affected.

Signalment!!navigator!!

Breed Predilection

L. interrogans-seropositive Appaloosas were 8.3 times as likely to develop uveitis as other breeds, and 3.8 times more likely as other breeds to lose vision following development of uveitis.

Mean Age and Range

While all ages can be affected, a large proportion of horses present before the age of 12 years.

Predominant sex

None

Classification!!navigator!!

ERU is currently classified as follows:

  • Classic ERU is characterized by active bouts of intraocular inflammation followed by periods of quiescence or minimally detectable inflammation. The iris, ciliary body, and choroid are primarily affected
  • Insidious ERU has the hallmark feature of low-grade intraocular inflammation that does not appear to be outwardly painful but that results in a gradual and steady level of destruction, which culminates in the degeneration of multiple intraocular structures. Breed predisposition: Appaloosa, draft, and Knabstrupper breeds
  • Posterior ERU is characterized by predominately posterior segment inflammation (e.g. vitreous, retina, and choroid), but mild anterior inflammation is commonly present

Signs!!navigator!!

  • Classic ERU—lacrimation, blepharospasm, photophobia, and miosis, as well as variable degrees of corneal edema, conjunctival hyperemia, ciliary injection, aqueous flare, hyphema, intraocular fibrin, and hypopyon. Miosis may result in secondary posterior synechiae and/or dyscoria. Acute cases generally have low IOP, while chronic and/or insidious ERU may be associated with intermittent elevations in IOP. Recurrent bouts of inflammation may cause cataracts, intraocular adhesions, phthisis bulbi, eventually resulting in vision loss
  • Insidious ERU—conjunctival and episcleral vascular hyperemia, mild to moderate blepharitis, focal or diffuse corneal edema, corneal vascularization, aqueous flare, iris discoloration (hyperpigmentation and/or depigmentation), and corpora nigra degeneration, as well as iris atrophy and miosis, corneal fibrosis, posterior synechia, and pigment of the anterior lens capsule. Focal and/or diffuse cataracts and lens sub/luxations are possible. Secondary glaucoma is common
  • Posterior ERU—vitritis (cloudy or hazy vitreous), chorioretinal scarring, retinal detachments (peripapillary linear traction band detachments are most common), and retinal degeneration
  • In chronic cases, regardless of which type of ERU, corneal vascularization, endothelial degeneration resulting in persistent corneal edema, band keratopathy, synechiae, cataract formation, and alterations in iris color may occur. Secondary glaucoma and phthisis bulbi can occur and irreversible blindness is a common sequela in many cases of ERU

Causes!!navigator!!

  • While the pathogenesis is clearly immune mediated, the true underlying cause often remains unknown
  • Hypersensitivity to infectious agents such as L. interrogans serovar pomona is commonly implicated as a possible cause
  • Toxoplasmosis, salmonellosis, Streptococcus, Escherichia coli, Rhodococcus equi, borreliosis, strongyles, onchocerciasis, parasites, and viral infections have also been implicated

Risk Factors!!navigator!!

  • Appaloosa or Warmblood breed
  • Leptospira infections increase the risk for ERU

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Ulcerative keratitis
  • Corneal stromal abscess
  • Glaucoma
  • Endothelial immune-mediated keratitis
  • Heterochromic iridocyclitis with keratitis

CBC/Biochemistry/Urinalysis!!navigator!!

None

Other Laboratory Tests!!navigator!!

  • Antibody titer evaluation from both aqueous humor and serum can be useful to determine the degree of intraocular antibody production or if a horse has systemic leptospirosis
  • The ratio of aqueous humor to serum antibody titers (i.e. Goldmann–Witmer coefficient) to determine if intraocular antibody production is taking place as opposed to leakage of serum antibodies into the eye. A c-value >1 suggests local antibody production and >4 provides greater confidence of active leptospiral uveitis. Detection of Leptospira organisms by culture or PCR in aqueous or vitreous humor samples may be considered.
  • Leptospiral titers for L. pomona, L. bratislava, and L. autumnalis should be requested in the USA. Additionally, L. grippotyphosa titers should be evaluated in Europe. Positive titers for serovars of 1:400 or greater are of importance

Imaging!!navigator!!

Ocular ultrasonography to assess the condition of the posterior segment or to evaluate for cataract and luxation or subluxation of the lens and to look for retinal degeneration and detachments.

Pathologic Findings!!navigator!!

  • In acute stages, lymphocytic infiltration with neutrophils can be found in the uveal tract, resulting in edema and plasmoid vitreous and aqueous
  • The chronic stages manifest by corneal scarring, cataract formation, and chorioretinitis and retinal degeneration

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

  • The major goals of treatment of ERU are to suppress inflammation, prevent or minimize the frequency and intensity of recurrent bouts of uveitis, decrease or eliminate pain and discomfort, and preserve vision
  • Specific prevention and targeted therapy are often not possible as the etiology remains elusive in many cases
  • Therapy will last for weeks, and often requires months of slow and deliberate tapering-off of medications
  • Initially, stall rest with minimal turn out and avoidance of bright sunlight is recommended

Activity!!navigator!!

Activity should be reduced pending resolution of clinical signs.

Client Education!!navigator!!

  • A complete cure is not possible in most affected horses
  • Medical treatment of the disease can be both time-consuming and expensive. The owner should be educated about the potential for recurrence and the debilitating and blinding nature of this disease

Surgical Considerations!!navigator!!

  • Pars plana vitrectomy in horses with ERU has been used successfully to remove vitreal debris and infectious organisms
  • A sustained release cyclosporine A (ciclosporin) drug-releasing device surgically inserted into the suprachoroidal space can effectively suppress the intensity of the uveitis in many eyes with ERU. The drug should theoretically be released for a period of 3–5 years
  • Low-dose intravitreal gentamicin (4 mg) injections have been utilized over the past few years with increasing success

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Topical prednisolone acetate (1%) or dexamethasone (0.1%) should be applied a minimum of 4–6 times per day initially
  • Systemic corticosteroids may be beneficial in severe, refractory cases of ERU but should be used with caution
  • Topical NSAIDs (flurbiprofen, indomethacin (indometacin), diclofenac, suprofen, bromfenac every 24 h to every 8 h) are effective at reducing the intraocular inflammation when a corneal ulcer is present
  • Flunixin meglumine (0.25–1.1 mg/kg PO every 12 h) and phenylbutazone (1 g IV or PO every 12 h) are frequently used systemically. Aspirin (15 mg/kg/day) is still commonly used to manage chronic uveitis; however, its efficacy is questionable
  • Mydriatic and cycloplegic medications (atropine 1%) minimize synechiae formation by inducing mydriasis, and alleviate some of the pain of ERU by relieving spasm of ciliary body muscles (cycloplegia)
  • Antibiotic treatment for horses with positive titers for Leptospira remains speculative but streptomycin (11 mg/kg IM BID) or enrofloxacin (7.5 mg/kg SID) may be a good choice for horses at acute and chronic stages of the disease. Penicillin G sodium (10 000 U/kg IV or IM QID) and oxytetracycline (5–10 mg/kg IV BID) may be beneficial. Oral doxycycline (10 mg/kg BID) does not enter the aqueous or vitreous of normal horse eyes at therapeutic levels, but might reach higher levels in inflamed eyes
  • Preservative-free, low-dose (4 mg) gentamicin has been demonstrated to suppress recurrent episodes of uveitic attacks.

Precautions!!navigator!!

Horses receiving topical atropine should be monitored for colic.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Regularly scheduled follow-up examinations should be carried out at least weekly during the first 2–3 weeks once a positive response to treatment has been established
  • Once tapering off of topical and systemic medical therapy is initiated, the horse should be carefully monitored for signs of recurrent inflammation

Prevention/Avoidance!!navigator!!

  • In horses where the disease tends to flare up after routine vaccination or deworming, prophylactic treatment of the eye may be beneficial
  • While a Leptospira vaccine is presently available, no proof exists that it will prevent the development of Leptospira-induced ERU

Possible Complications!!navigator!!

ERU can potentially result in blindness.

Expected Course and Prognosis!!navigator!!

The prognosis for ERU is usually poor, but the disease may be controlled with medical (conventional or suprachoroidal cyclosporine-releasing devices) or surgical (i.e., low-dose intravitreal gentamicin [4 mg] injection) intervention.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Systemic infection by the ERU-causing organism.

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

Infectious agents such as Leptospira can be a health risk for people.

Pregnancy/Fertility/Breeding!!navigator!!

Leptospira infection may lead to abortion.

Synonyms!!navigator!!

  • Periodic ophthalmia
  • Moon blindness
  • Iridocyclitis

Abbreviations!!navigator!!

  • ERU = equine recurrent uveitis
  • IL = interleukin
  • IOP = intraocular pressure
  • NSAID = nonsteroidal anti-inflammatory drug
  • PCR = polymerase chain reaction

Suggested Reading ⬆ ⬇

Brooks DE.Ophthalmology for the Equine Practitioner, 2e. Jackson, WY: Teton NewMedia, 2008.

Dwyer AE, Crockett RS, Kalsow CM. Association of leptospiral seroreactivity and breed with uveitis and blindness in horses: 372 cases (1986–1993). J Am Vet Med Assoc 1995;207:1327–1331.

Fischer BM, McMullen Jr RJ, Reese S, Brehm W. Intravitreal injection of low-dose gentamicin for the treatment of recurrent or persistent uveitis in horses: Preliminary results. BMC Veterinary Research, 2019; doi.org/10.1186/s12917-018-1722-7

Gilger BC, Hollingsworth SR. Diseases of the uvea, uveitis, and recurrent uveitis. In: Gilger BC, ed. Equine Ophthalmology, 3e. Ames, IA: Wiley Blackwell, 2017:369–415.

McMullen Jr RJ, Fischer BM. Medical and surgical management of equine recurrent uveitis. Vet Clin Equine 2017;33:465–481.

Author(s) ⬆

Author: Richard J. McMullen Jr.

Consulting Editor: Caryn E. Plummer

Acknowledgment: The author and editor acknowledge the prior contribution of Andras M. Komaromy and Dennis E. Brooks.