Uveitis in horses is a condition in which the uvea layer of the eye becomes inflamed. It is the most common cause of blindness in horses.

It is estimated that as many as 25% of horses worldwide are affected by some form of uveitis. Most veterinarians consider uveitis to be an immune-related disorder and, in some circumstances, it can be difficult to treat.

Uveitis can be recurring – referred to as Equine Recurring Uveitis (ERU) – or it can be acute with a single episode of limited duration.

Recurring uveitis often affects both eyes and eventually leads to blindness if left untreated. It is thought to be more common in humid regions, areas with high precipitation, as well as areas with clay and marsh-type soils. [1]

ERU is also commonly called Moon Blindness, a term coined in the 1600s when it was thought that recurring episodes of eye problems were related to the phases of the moon.

Equine Recurrent Uveitis

Uveitis involves inflammation of the uveal tract of the eye, which is the blood vessel-filled tissue beneath the sclera (white of the eye).

Uveitis can be acute, meaning it happens suddenly and lasts only a short period of time. More often, it is a recurring condition that gradually worsens over time.

Three types of Equine Recurring Uveitis have been described: classic ERU, insidious ERU and posterior uveitis.

Classic ERU

Classic recurring uveitis is the most common form of uveitis and is characterized by periods of inflammation within the eye, followed by periods of dormancy.

With each inflammatory phase, the disease progressively worsens and vision may eventually be lost. [2] Warmbloods and Icelandic horses appear to be at higher risk of developing Classic ERU.

Insidious ERU

In horses with insidious ERU, the inflammation never completely resolves. This form of uveitis leads to the gradual destruction of ocular tissues and degeneration of structures in the eye. [3] Insidious ERU is commonly seen in Appaloosa as well as some draft breed horses.

Horses with insidious ERU do not typically demonstrate eye discomfort. Owners may not recognize that their horse is experiencing uveitis until a cataract develops or the horse’s vision becomes impaired. [4]

Posterior Uveitis

Posterior uveitis is most commonly seen in Warmbloods and horses in Europe. It is characterized by changes and degeneration in the vitreous fluid and retina of the eye. [2]

The inflammation is mainly in the back or posterior part of the eye, hence the name.

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Causes of Uveitis

Many different microorganisms have been found to cause uveitis, but the most common is the Leptospira bacteria. In fact, this bacteria can be found in about 60% of horses with ERU. [5]

Though researchers are still working to understand how Leptospira triggers the disease, one theory is that it penetrates the eye where it is deposited in the ocular tissue, causing infection. [6]

The main carriers of Leptospira are small rodents, which excrete the bacteria in their urine. Swampy pastures present a higher risk for infections.

Oral mucous membranes, conjunctiva, nasal mucous membranes, and skin lesions may also be possible entry sites for pathogens. [1]

Other pathogens, viruses and illnesses that have been found to trigger uveitis include:

  • Borrelia spp.
  • Streptococcus spp.
  • Equine influenza
  • Equine viral arteritis
  • Equine herpesvirus

Parasitic infection from Onchocerca spp., Strongylus spp., and Toxoplasma spp. have also been associated with uveitis. [7]

Immune Response

ERU shows a strong resemblance to autoimmune uveitis in humans.

T cells are a type of white blood cell or lymphocyte that is involved in the immune system. These cells are thought to be the main cells that trigger episodes of uveitis.

When the T cells are activated, they overcome the blood retinal barrier to invade the eye. [17]

The T cells then cause severe inflammation within the eye, especially in the retina and iris. With each inflammatory episode, the retina further degrades until eyesight is eventually lost. [8]

This may also help to explain why some horses have episodes of inflammation following routine vaccinations or anthelmintic treatments when the immune system is stimulated. [4]

Other Causes

In addition to the above causes, various non-infectious factors may also trigger ERU. This includes trauma to the eye, abnormal growth of cells or tissue in the eye (neoplasia), and endotoxemia. [7]

Several additional possible causes of uveitis include: [1]

  • Congenital eye problems
  • Septicemia (severe general infection)
  • Severe keratitis (corneal ulcers)

A genetic component has been linked to ERU in German Warmbloods. A mutation on chromosome 20 may affect important components of the immune system to predispose these horses to ERU. In Appaloosas, another genetic mutation has been found affecting the major histocompatibility complex which is involved in defence against pathogens. [13]

The frequency of ERU in Appaloosas is eight times higher than in the general population.

Uveitis is also thought to be more severe and more likely to cause blindness in Appaloosas and both eyes are usually affected. [1][9]

Signs of Uveitis

Signs and symptoms of uveitis will vary depending on the form of this condition that your horse experiences. Early warning signs of uveitis and other eye problems in horses include: [3]

  • Eye redness
  • Increased tearing
  • Squinting
  • Swelling

During acute episodes, horse owners may notice the following symptoms:

  • Milky or hazy appearance to the eye
  • Greenish-yellow appearance to the iris
  • Blood or pus in eye
  • Sensitivity to light (photophobia)
  • Pain in the eye

Upon veterinary examination of the eye, the following clinical signs may be observed:

  • Lowered intraocular pressure
  • Iris adhesions
  • Corneal haze
  • Constricted pupil
  • Aqueous flare
  • Corneal scarring
  • Depigmentation
  • Lesions

Over time with recurring episodes of ERU, horses may experience secondary complications including: [1][15]

  • Cataracts
  • Retinal detachment
  • Atrophy of the globe often resulting in the “3rd corner of the eye”
  • Chronic keratitis (inflammation of the cornea)
  • Corneal opacity
  • Glaucoma
  • Blindness

Diagnosing Uveitis

The diagnosis of ERU is based on clinical signs and ophthalmic examination as well as a history of documented recurrent or persistent inflammation in one or both eyes. The more uveitis episodes that have occurred, the more changes that can be seen even during periods of dormancy. [1]

The attending veterinarian or specialist will examine both the front and back of the eye to identify signs of ERU and rule out other ocular conditions.

One diagnostic tool is a fluorescein stain, which can be used to assess the health of the cornea and rule out other eye diseases. A complete blood count and blood tests can also be performed, as well as serologic testing for Leptospira spp. [3]

A new software tool has been developed to aid in the early detection of uveitis as well as other ophthalmic diseases of the equine eye. This image analysis has the potential to help horse owners and veterinarians detect uveitis. [10]

LP Genetic Test

The Leopard Print (LP) genetic test can be used to identify Appaloosas with a higher risk for recurrent uveitis. This test determines how many copies of the LP mutation a horse has.

Research shows that horses with two copies of the mutation (homozygous LP/LP) have a higher risk of ERU compared to horses with no copies of this mutation (N/N). [3]

The Leopard Complex gene produces coat patterns that are characterized by leopard-like spots on a white coat. It is not yet known whether this gene is responsible for the higher rates of ERU in horses that are LP/LP or whether another gene is responsible.

However, it is recommended for horses with two copies of the LP mutation to have regular eye exams from a trained veterinary ophthalmologist to detect early signs of uveitis. [3]

Treating Uveitis

The two main objectives of uveitis therapy are to reduce pain and control inflammation. There is currently no cure for this disease, but several treatment options exist.

Treatments include nutritional therapy, topical drugs, immunosuppressive medication and various surgical procedures.

Common treatments for acute flare-ups of uveitis include: [1]

  • Mydriatics drugs to dilate the pupil and prevent cataract formation (atropine)
  • Topical corticosteroids to reduce inflammation (prednisolone, dexamethasone)
  • Systemic administration of non-steroidal anti-inflammatory drugs (flunixin meglumine, phenylbutazone)
  • NSAID eye drops to reduce inflammation (flurbiprofen, diclofenac, suprofen, bromfenac)
  • Systemic administration of antibiotics if bacterial infection is involved (rarely indicated)

Unfortunately, antibiotics do not appear to be effective at resolving ERU even when Leptopspria has been detected in blood work. However, they may be effective with some other types of bacterial infections. [2]

Systemic administration of corticosteroids is also not commonly used as it is associated with significant adverse effects such as laminitis. [11]

Diet & Management

For any horse experiencing an acute flare-up of uveitis it is generally recommended to implement stall rest, avoid bright light (use face masks if needed), and limit exercise.

Ensuring that your horse has a balanced diet with sufficient trace minerals as well as omega-3 fatty acids may also help to reduce inflammation. [16]

Omega-3 fatty acids are precursors for molecules called resolvins which are involved in dampening the immune response and supporting tissue repair. Therapeutics that mimic resolvins are currently under development for treating several immune-related conditions. [16]

Surgical Treatments for ERU

In some cases, surgery is the best option for relieving pain and inflammation from ERU. One such surgery that has been successful is vitrectomy, which is mainly performed in Germany.

In this procedure, vitreous fluid from the eye is exchanged in a minimally invasive way with a buffered salt solution. [5] Researchers believe vitrectomy is effective because of the removal of leptospiral infection in the vitreous cavity. [1]

Vitrectomy has proven particularly effective in Leptospira-associated ERU cases, while Leptospira-negative patients appear to be poorer candidates. [6]

However, this procedure is a specialized surgery that requires a high level of experience and skill on the part of the surgeon and surgical team. It also requires specific equipment that many veterinary facilities may not have. [1]

Other surgical procedures include injecting fibrinolytic drugs into the front part of the eye to break up blood clots or surgically removing blood clots and/or diseased tissues from the front of the eye.

Another surgical procedure involves injecting the antibiotic gentamicin into the vitreous cavity. [1]

Medications

Immunosuppressive drugs such as cyclosporine A have also proven helpful for some horses. Long-term studies following use of a slow-release implant which delivers this drug into the eye have shown a reduction in the number and severity of uveitis episodes with few complications. [6]

Another potential therapy for ERU which has not yet been well explored is mesenchymal stem cell (MSC) therapy.

This type of therapy has been effective in reducing immune cell activation in other animals and may help to do the same for ERU horses. However, more research is needed to verify the efficacy of this treatment. [12]

In the United States, a vaccine for the prevention of leptospiraemia is available and may help reduce the potential risk of ERU. [4]

Despite available treatments, a large number of ERU cases require enucleation (eye removal) to control pain and inflammation. This procedure can be performed under general anesthesia or standing sedation with a good prognosis for return to work if just one eye has been affected.

If both eyes are affected and do not respond to treatment, euthanasia may be necessary. [6]

Prognosis for ERU

Early diagnosis and treatment are associated with the best prognosis for horses diagnosed with ERU and are critical to preserving the affected eye.

The long-term prognosis is guarded, however. Many treatments can slow the progression of inflammation but they do not cure the problem.

More than 60% of horses with ERU are not able to return to previous levels of work and approximately 56% of horses will eventually become blind. Horses with glaucoma or cataracts are more likely to require enucleation. [3] Due to financial constraints and poorer quality of life, a significant portion of these horses (44.6%) are euthanized due to ERU. [14]

If you believe your horse is at risk for developing ERU or if symptoms are present, don’t hesitate to get your veterinarian involved right away.

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References

  1. Wallanke, B. et al. Infectious Uveitis in Horses and New Insights in Its Leptospiral Biofilm-Related Pathogenesis. Microorganisms. 2022.
  2. Ackerman, K. et al. In Vivo Biofilm Formation of Pathogenic Leptospira spp. in the Vitreous Humor of Horses with Recurrent Uveitis. Microorganisms. 2021.
  3. Young, Amy. Equine Recurrent Uveitis. UC Davis Veterinary Medicine. 2020.
  4. Gilger, B.C. and Michau, T.M. Equine recurrent uveitis: new methods of management. Vet Clin Equine. 2004.
  5. Fingerhut, L. et al. Neutrophil Extracellular Traps in the Pathogenesis of Equine Recurrent Uveitis (ERU). Cells. 2019.
  6. Malalana, F. What’s new in equine recurrent uveitis? In Practice. 2020.
  7. Witkowkski, L. et al. Multiple etiologies of equine recurrent uveitis – A natural model for human autoimmune uveitis: A brief review. Comp Immunol Microbiol Infect Dis. 2016.
  8. Degroote, R.L. and Deeg, C.A. Immunological Insights in Equine Recurrent Uveitis. Front Immunol. 2020.
  9. Sandmeyer, L.S. et al. Equine recurrent uveitis in western Canadian prairie provinces: A retrospective study (2002–2015). Can Vet J. 2017.
  10. May, A. et al. Artificial intelligence as a tool to aid in the differentiation of equine ophthalmic diseases with an emphasis on equine uveitis. Equine Vet J. 2021.
  11. Malalana, F. et al. Equine recurrent uveitis: Human and equine perspectives. The Vet J. 2015.
  12. Saldinger, L.K. et al. Horses with equine recurrent uveitis have an activated CD4+ T‐cell phenotype that can be modulated by mesenchymal stem cells in vitro. Vet Ophthalmol. 2020.
  13. Kulbrock, M. et al. A genome-wide association study identifies risk loci to equine recurrent uveitis in German warmblood horses. PLoS One. 2013.
  14. Gerding, J.C. and Gilger, B.C. Prognosis and impact of equine recurrent uveitis. Equine Vet J. 2015.
  15. Allbaugh, R.A. Equine recurrent uveitis: A review of clinical assessment and management. Equine Vet Educ. 2016.
  16. Lin, P. et al. The Future of Uveitis Treatment. Opthalmol. 2014.
  17. Deeg, C.A. Ocular immunology in equine recurrent uveitis. Vet Opthalmol. 2008.