Canine Conjunctivitis Study Guide

Overview and Clinical Importance

Conjunctivitis is inflammation of the conjunctiva, the thin mucous membrane lining the inner eyelids and covering the sclera. It represents one of the most common ophthalmic conditions encountered in canine practice. The conjunctiva serves as a protective barrier against infections and foreign bodies while contributing to tear film production and ocular surface health.

Unlike feline conjunctivitis, which is predominantly infectious (primarily caused by feline herpesvirus-1), canine conjunctivitis is most commonly secondary to underlying conditions such as keratoconjunctivitis sicca (KCS), allergies, eyelid abnormalities, or systemic diseases. Primary bacterial conjunctivitis is exceedingly rare in dogs.

Anatomy of the Conjunctiva

The conjunctiva is composed of three anatomical regions:

Palpebral conjunctiva: Lines the inner surface of the eyelids

Bulbar conjunctiva: Covers the anterior sclera (white of the eye)

Fornix: Transition zone between palpebral and bulbar conjunctiva (forms the conjunctival sac)

In dogs, the conjunctiva also covers the third eyelid (nictitating membrane), which provides additional protection. The third eyelid contains the gland of the nictitating membrane, contributing approximately one-third of tear production.

Etiology and Classification

Classification by Type

Common Causes of Canine Conjunctivitis

Keratoconjunctivitis Sicca (KCS/Dry Eye)

KCS is the most common cause of chronic or recurrent conjunctivitis in dogs. The condition results from inadequate aqueous tear production by the lacrimal gland and gland of the third eyelid.

Etiology: Immune-mediated destruction of lacrimal glands (most common, approximately 80% of cases), canine distemper virus, sulfonamide toxicity, neurogenic (facial nerve damage), congenital, iatrogenic (third eyelid gland removal)

Breed Predisposition: American Cocker Spaniel, English Bulldog, West Highland White Terrier, Cavalier King Charles Spaniel, Lhasa Apso, Shih Tzu, Pug, Yorkshire Terrier, Boston Terrier

Clinical Signs: Thick mucoid to mucopurulent discharge, conjunctival hyperemia, corneal vascularization, pigmentation, dull lusterless cornea, blepharospasm, corneal ulceration (secondary)

Schirmer Tear Test (STT) Interpretation

KCS Treatment Options

Allergic Conjunctivitis

Allergic conjunctivitis is common in dogs with atopic dermatitis and may be seasonal or year-round depending on the allergen. Clinical signs include bilateral conjunctival hyperemia, chemosis (conjunctival edema), clear serous discharge, and pruritus manifesting as face rubbing or pawing at eyes.

Common allergens: Pollen, dust mites, mold spores, grass, weeds

Treatment: Topical corticosteroids (dexamethasone, prednisolone acetate) after ruling out corneal ulceration via fluorescein stain; oral antihistamines; allergen avoidance when possible

CRITICAL: NEVER use topical corticosteroids until fluorescein stain confirms absence of corneal ulceration. Steroids inhibit epithelial healing and can cause corneal perforation if an ulcer is present.

Follicular Conjunctivitis

Follicular conjunctivitis is an immune-mediated condition of young dogs (typically less than 2 years) characterized by hyperplasia of lymphoid follicles on the bulbar surface of the third eyelid. It represents an exaggerated immune response to chronic antigenic stimulation.

Breed Predisposition: Large breeds with droopy eyelids, prominent eyes, or large palpebral fissures

Clinical Signs: Enlarged follicles on inner third eyelid (cobblestone appearance), mild ocular discharge, redness

Treatment: Mild cases may not require treatment; topical corticosteroids or NSAIDs for symptomatic relief; many dogs outgrow the condition

Exam Focus: Follicular conjunctivitis has an excellent prognosis. Most young dogs outgrow the condition, though recurrence may require repeated treatment courses.

Plasma Cell Conjunctivitis (Plasmoma/Atypical Pannus)

Plasma cell conjunctivitis is an immune-mediated condition strongly associated with German Shepherd Dogs. It affects the third eyelid (nictitating membrane) and is closely related to chronic superficial keratitis (pannus), often occurring concurrently in approximately 70% of pannus cases.

Clinical Signs: Thickened, hyperemic third eyelid margin with multifocal depigmentation; cobblestone or irregular conjunctival surface; bilateral

Histopathology: Plasma cell and lymphocyte infiltration

Treatment: Topical cyclosporine (0.2% Optimmune or compounded 1-2%) or tacrolimus; topical corticosteroids; lifelong therapy required

Chronic Superficial Keratitis (Pannus/Uberreiter's Syndrome)

Pannus is an immune-mediated, progressive corneal disease that begins at the temporal or ventrotemporal limbus with vascularization and pigmentation advancing centrally. UV light exposure exacerbates the condition, with higher prevalence at increased altitudes.

Breed Predisposition: German Shepherd Dog (strong), Belgian Tervuren, Greyhound, Border Collie, Australian Shepherd, Siberian Husky

Clinical Signs: Bilateral; progressive corneal vascularization, pigmentation, and granulation tissue; typically NOT painful (no blepharospasm); normal STT values

Treatment: Topical cyclosporine or tacrolimus; topical corticosteroids; UV-blocking dog goggles (Doggles); lifelong management; cannot be cured but can be controlled

Infectious Causes

Canine Distemper Virus (CDV)

CDV can cause bilateral mucopurulent conjunctivitis as part of systemic infection. May lead to permanent KCS due to viral destruction of lacrimal glands. Look for concurrent respiratory signs, GI signs, and neurological abnormalities.

Canine Herpesvirus-1 (CHV-1)

CHV-1 can cause conjunctivitis and keratitis in adult dogs, though it is less common and typically self-limiting compared to feline herpesvirus. Clinical signs include unilateral or bilateral conjunctivitis with serous to mucopurulent discharge.

Bacterial Conjunctivitis

Primary bacterial conjunctivitis is RARE in dogs. Most bacterial involvement is secondary to underlying conditions (KCS, eyelid abnormalities, foreign bodies). Common organisms include Staphylococcus spp. and Streptococcus spp.

Exam Focus: If you see a question about 'primary' bacterial conjunctivitis in dogs - be skeptical! Always look for an underlying cause. Treatment with antibiotics alone without addressing the primary etiology leads to treatment failure.

Secondary Causes: Eyelid Abnormalities

Diagnostic Approach

A systematic ophthalmic examination is essential for identifying the underlying cause of conjunctivitis.

Complete Ophthalmic Examination Sequence

• Schirmer Tear Test (STT) - FIRST, before applying any drops; assesses tear production
• Gross Examination - Symmetry, discharge character, periocular changes
• Eyelid/Eyelash Examination - Entropion, ectropion, distichiasis, ectopic cilia
• Third Eyelid Examination - Follicles, plasmoma, foreign bodies (examine bulbar surface)
• Fluorescein Stain - Rule out corneal ulceration before steroids
• Tonometry - Intraocular pressure if glaucoma or uveitis suspected
• Additional Testing - Cytology, culture, PCR as indicated

Fluorescein Staining

Fluorescein is a water-soluble dye that stains exposed corneal stroma (hydrophilic) but not intact epithelium or Descemet's membrane (hydrophobic). Positive staining indicates epithelial defect (corneal ulcer).

Technique: Apply minimal fluorescein; flush excess with saline; examine under cobalt blue light

Positive Stain: Green uptake indicates epithelial loss; avoid corticosteroids

Descemetocele: Deep ulcer to Descemet's membrane shows 'doughnut' pattern - stain uptake around edges with clear center (Descemet's membrane does not stain)

Treatment Principles

The key to successful treatment is identifying and addressing the underlying cause. Symptomatic treatment alone leads to recurrence.

Topical Antibiotics

Topical Anti-Inflammatory Agents

Corticosteroids: Prednisolone acetate 1%, dexamethasone 0.1% - Potent anti-inflammatory; CONTRAINDICATED with corneal ulceration

NSAIDs: Diclofenac, flurbiprofen, ketorolac - Milder anti-inflammatory effect; may delay epithelial healing

Prognosis

• Allergic conjunctivitis: Good to excellent with allergen management; may recur seasonally
• Follicular conjunctivitis: Excellent; most dogs outgrow
• KCS: Guarded to good with lifelong therapy; better prognosis if STT greater than 2 mm at diagnosis
• Pannus/Plasmoma: Cannot be cured; controlled with lifelong therapy; younger dogs have more aggressive disease
• Secondary to eyelid abnormalities: Excellent following surgical correction

Memory Aids for NAVLE

KCS = 'Keep Crying Stopped'

K - Keratoconjunctivitis

C - Cyclosporine treatment

S - Schirmer test less than 10 mm/min

German Shepherd Eye Disease = 'GPS'

G - German Shepherds predisposed

P - Pannus and Plasmoma

S - Sun (UV) exposure worsens

Exam Order = 'STT-FLO'

S - Schirmer Tear Test FIRST

T - Third eyelid examination

T - Tonometry

F - Fluorescein stain

L - Lids examination

O - Other testing as needed