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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Conjunctivitis is inflammation of the mucous membrane that covers the posterior aspects of the eyelids and nictitating membrane (palpebral conjunctiva) and the superficial surface of the sclera (bulbar conjunctiva). It may be infectious or noninfectious.

Pathophysiology!!navigator!!

  • Conjunctivitis is a nonspecific finding
  • Infectious and noninfectious diseases of the lids, cornea, sclera, anterior uvea, nasolacrimal system, and orbit as well as systemic diseases can result in conjunctivitis
  • Conjunctiva is a mucous membrane so it can also reflect systemic dysfunction through changes in color and in vascular appearance, as in anemia and jaundice
  • Environmental allergies or irritants may cause conjunctivitis
  • Habronemiasis is a parasitic conjunctivitis caused by aberrant migration of Habronema larvae. Habronemiasis may occur concurrently with SCC, which makes histologic examination of affected tissues crucial
  • Onchocerciasis can cause conjunctivitis, keratitis, and keratouveitis. The causative agent is Onchocerca cervicalis and the insect vector is the female Culicoides. Migrating larvae may invade the conjunctiva, cornea, and anterior uvea resulting in inflammation
  • The development of SCC has been associated with cell damage caused by the UV component of solar radiation. Animals with higher levels of exposure to sunlight or that live in high altitudes are more prone

Systems Affected!!navigator!!

Ophthalmic

Genetics!!navigator!!

No proven genetic basis for conjunctivitis. However, breed predilection for ocular SCC suggests genetic influence.

Incidence/Prevalence!!navigator!!

Common

Geographic Distribution!!navigator!!

None identified for conjunctivitis specifically. However, animals with higher levels of exposure to UV light are more prone to development of SCC.

Signalment!!navigator!!

  • Neonates—conjunctivitis may be associated with neonatal maladjustment syndrome, septicemia, uveitis immune-mediated hemolytic anemia, and environmental irritants; subconjunctival or episcleral hemorrhages may occur secondary to birth trauma or neonatal maladjustment syndrome. Conjunctivitis secondary to pneumonia is seen most commonly in 1–6 month old foals
  • Adults—ocular SCC prevalence increases with age

Signs!!navigator!!

  • Conjunctivitis—conjunctival hyperemia, chemosis, and ocular discharge vary with type of disease
  • Onchocerciasis—limbal conjunctival thickening and depigmentation, corneal edema, vascularization, stromal cellular infiltrate
  • SCC has 2 characteristic appearances—proliferative mass which may or may not be ulcerated, or diffuse thickening and ulceration of tissue. May resemble granulation tissue or just an area of increased redness in the conjunctiva
  • Habronemiasis—appearance ranges from granulomas, nodules, to small raised caseated plaques on the conjunctiva
  • Dermoid—pigmented mass involving the limbus and varying degrees of the cornea

Causes of Conjunctivitis!!navigator!!

  • Infectious
    • Parasitic—Habronema megastoma, H. muscae, Draschia megastoma, Onchocerca cervicalis, Thelazia lacrimalis, Trypanosoma spp.
    • Viral—adenovirus, equine herpesvirus types 1, 2, 4, and 5, equine infectious anemia, equine viral arteritis, influenza
    • Bacterial—Moraxella equi, Streptococcus equi ssp. equi, Rhodococcus equi, Actinobacillus spp., leptospirosis
    • Mycotic—Aspergillus spp., Fusarium
    • Protozoal—equine protozoal myeloencephalitis
  • Neoplastic—SCC, lymphoma, papilloma, hemangioma, hemangiosarcoma, mastocytoma, melanoma, multiple myeloma, etc.
  • Secondary to other ocular/adnexal disease—ulcerative keratitis, corneal stromal abscess, anterior uveitis, equine recurrent uveitis, obstructed nasolacrimal duct
  • Secondary to trauma—orbital fractures, scleral perforation
  • Secondary to environmental causes—foreign bodies and debris, allergic reactions to dust, environmental pollutants
  • Secondary to systemic disease—polyneuritis equi, vestibular disease syndrome, African horse sickness, epizootic lymphangitis, neonatal maladjustment syndrome

Risk Factors!!navigator!!

  • Recumbent foals are at risk for conjunctivitis secondary to environmental irritants
  • White, chestnut, and palomino coat color predispose to ocular SCC
  • Lightly pigmented animals and those residing in areas with high UV indices are at the greatest risk
  • Warm weather and climates with heavy fly populations are risk factors for habronemiasis and other parasitic infections

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Conjunctivitis is a nonspecific sign, reflecting the eye's limited mechanisms of response to injury. It is critical to differentiate primary conjunctivitis from conjunctivitis associated with ocular or systemic disease
  • Nodular/mass lesions of conjunctiva—habronemiasis, SCC, mastocytoma, hemangioma, hemangiosarcoma, papilloma, and other neoplastic infiltrates, fungal granulomas, nodular necrobiosis, pseudotumors, dermoids, foreign body reaction

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Other Laboratory Tests!!navigator!!

  • Cytology to identify mycotic, bacterial causes of conjunctivitis
  • Culture and sensitivity of mucopurulent discharge may be considered if a primary bacterial cause is suspected (uncommon in horses)
  • Habronemiasis—conjunctival scraping reveals eosinophils, mast cells, neutrophils, plasma cells, rarely larvae
  • Biopsy and histopathology should be performed for mass lesions

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

Complete ophthalmic examination is indicated to identify adnexal and ocular causes of conjunctivitis, including a thorough adnexal examination, fluorescein staining, and examination for signs of anterior uveitis.

Conjunctival Biopsy

  • Onchocerciasis—microfilariae, eosinophils, lymphocytes
  • Habronemiasis—eosinophils, mast cells, neutrophils, plasma cells, rarely larvae
  • SCC—epithelial cells with neoplastic characteristics
  • Lymphoma—large population monomorphic lymphocytes with neoplastic characteristics

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

  • Most horses with simple conjunctivitis associated with parasitic, bacterial, viral, and environmental causes can be treated on an outpatient basis
  • Treatment of some systemic and ocular diseases associated with complicated conjunctivitis may require hospitalization

Nursing Care!!navigator!!

Ensure topical medications can be administered appropriately.

Activity!!navigator!!

  • Restriction of activity may be required in cases where conjunctival disease is associated with systemic illness
  • If environmental irritation is suspected, then exposure to the inciting substance should be restricted or eliminated
  • Animals with ocular involvement/disease should not be ridden if visual status is compromised

Diet!!navigator!!

No change in diet is necessary. Hay should be fed at ground level rather than elevated hay racks or bags to avoid further irritation of the conjunctiva by dust and debris.

Client Education!!navigator!!

If there is evidence of self-trauma, a protective hood covering the affected eye should be placed on the horse. The client should be instructed to contact the veterinarian if the condition worsens in any way or shows little to no signs of improvement.

Surgical Considerations!!navigator!!

  • Treatment of conjunctival neoplasia may involve local resection, with adjunctive beta-irradiation, cryotherapy, radiofrequency hyperthermia, or topical chemotherapy
  • Small lacerations of the conjunctiva will heal without primary closure. Large lacerations should be sutured with fine absorbable suture
  • Conjunctival foreign bodies and debris can usually be removed with topical anesthesia and liberal flushing of conjunctival fornices

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Parasitic conjunctivitis—habronemiasis: topical 0.03% echothiophate iodide (ecothiopate iodide; phospholine iodide) and neomycin/polymyxin B with dexamethasone every 12 h. Multifocal lesions will require oral ivermectin. Intralesional triamcinolone may reduce size of granulomas, but this long-acting steroid must be used with extreme caution and not at all if the cornea is compromised in any way. Onchocerciasis—systemic ivermectin with topical anti-inflammatories. Thelazia—topical phospholine iodide, flush conjunctival fornix
  • Conjunctival lacerations—treat with prophylactic broad-spectrum antibiotic topically
  • Allergic conjunctivitis—topical corticosteroid, reduce/eliminate exposure to inciting cause if possible, lubricating ophthalmic ointment before turnout
  • Other systemic medication as indicated by concurrent systemic disease

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

The patient should be rechecked soon after beginning therapy (3–4 days), with specific time frame determined by disease and severity. Subsequent rechecks are dictated by the specific diagnosis, the severity, and response to treatment.

Prevention/Avoidance!!navigator!!

  • Fly control in barns and pastures, fly hoods, and frequent periocular administration of insect repellent can help prevent the development of habronemiasis
  • A preventative health program including regular deworming with avermectins will help prevent habronemiasis and onchocerciasis
  • The incidence of allergic/environmental conjunctivitis can be reduced/prevented by avoidance of the inciting agent
  • Treat any underlying ocular or systemic disease that may promote the conjunctival disease
  • Limit solar exposure in lightly pigmented animals to decrease the incidence of SCC

Possible Complications!!navigator!!

Possible complications of treatment include depigmentation in region of treatment, recurrence, and metastatic spread (if neoplastic).

Expected Course and Prognosis!!navigator!!

  • Infectious conjunctivitis usually responds to appropriate treatment
  • Failure to respond or recurrence suggests an unidentified underlying cause (i.e. recurrent bacterial conjunctivitis associated with an unrecognized foreign body)
  • Course and prognosis of conjunctival neoplasia depend on the specific type of neoplasia and the extent of invasion of surrounding tissues
  • Viral conjunctivitis may be recurrent
  • Allergic conjunctivitis is often difficult to eliminate completely owing to the nature of the horse's environment
  • Many systemic diseases that have conjunctivitis as a clinical sign can have serious and life-threatening consequences

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Systemic absorption of topically applied medication is possible. Benefits of treatment should be considered against any risks posed to the fetus.

Synonyms!!navigator!!

N/A

Abbreviations!!navigator!!

SCC = squamous cell carcinoma

Suggested Reading ⬆ ⬇

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

Gilger BC. Equine ophthalmology. In: Gelatt KN, ed. Veterinary Ophthalmology, 5e. Ames, IA: Wiley, 2013:1560–1609.

Gilger BC. Equine Ophthalmology, 3e. Philadelphia, PA: WB Saunders, 2017.

Author(s) ⬆

Author: Caroline Monk

Consulting Editor: Caryn E. Plummer

Acknowledgment: The author acknowledges the prior contribution of Caryn E. Plummer.