Feline Conjunctivitis Study Guide

Overview and Clinical Importance

Conjunctivitis is the most common ocular disorder in cats, characterized by inflammation of the conjunctiva–the thin mucous membrane lining the inner eyelids and covering the sclera. Unlike dogs where conjunctivitis is typically secondary to other conditions, feline conjunctivitis is predominantly caused by primary infectious agents, most notably feline herpesvirus-1 (FHV-1). This distinction is crucial for the NAVLE, as it fundamentally changes the diagnostic and therapeutic approach.

The condition affects cats of all ages, breeds, and genders, though young cats in multi-cat environments are most commonly affected. Understanding the etiology, clinical presentation, and treatment of feline conjunctivitis is essential for board examination success and clinical practice.

Anatomical Considerations

The conjunctiva is a mucous membrane consisting of palpebral conjunctiva (lining the eyelids), bulbar conjunctiva (covering the sclera), and the conjunctiva of the nictitating membrane (third eyelid). Key functions include:

• Tear film distribution: Facilitates spreading of tears across the ocular surface
• Immune defense: Contains conjunctiva-associated lymphoid tissue (CALT) with antibody-producing cells
• Goblet cells: Produce mucin component of tear film

Etiology of Feline Conjunctivitis

Feline conjunctivitis differs fundamentally from canine conjunctivitis in that cats have primary conjunctival pathogens that do not require predisposing factors.

Summary of Etiologies

Feline Herpesvirus-1 (FHV-1) Conjunctivitis

FHV-1 is the most common cause of feline conjunctivitis and represents a major category on the NAVLE. It is a DNA alphaherpesvirus that establishes lifelong latency in the trigeminal ganglia following primary infection.

Pathophysiology

• Primary infection: Occurs 2-6 days post-exposure; epitheliotropic virus causes necrosis of conjunctival and corneal epithelium
• Latency: Approximately 80% of cats become latent carriers; virus resides in trigeminal ganglia
• Recrudescence: Stress, illness, or corticosteroid use triggers reactivation; approximately 45% of carriers develop recurrent disease

Clinical Signs

Primary Infection (Kittens)

• Bilateral conjunctivitis with serous to mucopurulent discharge
• Upper respiratory tract disease (sneezing, nasal discharge, fever)
• Blepharospasm and photophobia
• Duration: 10-20 days; typically self-limiting

Recurrent Infection (Adults)

• Often unilateral (same eye repeatedly affected)
• Milder signs than primary infection
• May or may not have respiratory signs
• Triggered by stress (moving, boarding, new pets, surgery)

Corneal Manifestations

Chlamydia felis Conjunctivitis

Chlamydia felis (formerly Chlamydophila felis) is a Gram-negative obligate intracellular bacterium and the second most common cause of feline conjunctivitis. It primarily affects young cats under 9 months of age.

Key Clinical Features

• Unilateral onset → bilateral progression: Typically starts in one eye, becomes bilateral within 1-3 weeks
• Severe chemosis: Marked swelling of conjunctiva is characteristic
• Hyperemia of nictitating membrane: Third eyelid involvement prominent
• Serous → mucopurulent discharge: Progresses with chronicity
• Follicular conjunctivitis: Lymphoid follicle formation with chronic infection
• NO corneal involvement: Unlike FHV-1, Chlamydia does not cause corneal disease unless co-infected

Diagnosis

• PCR (preferred): Conjunctival swab; most sensitive and specific
• Cytology: Giemsa stain may reveal intracytoplasmic inclusion bodies in epithelial cells (basophilic inclusions)
• Serology: Only useful in unvaccinated cats

Treatment

Exam Focus: Doxycycline for at least 4 weeks is the treatment of choice for C. felis. Shorter courses (7-14 days) do NOT eliminate infection and lead to recurrence. Always follow doxycycline with water or a small amount of food to prevent esophageal stricture in cats!

Mycoplasma Conjunctivitis

Mycoplasma felis and Mycoplasma gateae are small, cell wall-deficient bacteria. Their role as primary pathogens is controversial, as experimental infection often requires prior immunosuppression.

Clinical Features

• Conjunctival swelling with serous to mucoid discharge
• Pseudomembrane formation - characteristic finding
• May be associated with upper respiratory signs
• Often occurs as co-infection with FHV-1 or C. felis

Treatment

• Doxycycline: 10 mg/kg PO q24h for 3-4 weeks
• Topical oxytetracycline: q6-8h as adjunct

Feline Eosinophilic Keratoconjunctivitis

Feline eosinophilic keratoconjunctivitis (FEK) is an immune-mediated condition characterized by infiltration of eosinophils into the cornea and conjunctiva. It may be associated with FHV-1 infection in up to 75% of cases.

Clinical Features

• White to pink raised plaques: Typically on cornea, starting from limbus
• Granular or cobblestone appearance: Classic description
• Corneal vascularization
• Conjunctival thickening, depigmentation
• Often unilateral initially; may become bilateral

Diagnosis

Cytology is diagnostic: The presence of even ONE eosinophil on conjunctival or corneal cytology is considered pathognomonic. Mast cells are also typically present.

Treatment

Diagnostic Approach

Complete Ophthalmic Examination

Laboratory Testing

PCR testing is the gold standard for diagnosing infectious causes. Important considerations:

• FHV-1 PCR: High sensitivity but also detects latent carriers; interpret with clinical signs
• C. felis PCR: Sensitive and specific; preferred over cytology
• Feline respiratory panel: Tests for multiple pathogens simultaneously
• FeLV/FIV testing: Consider in cats with recurrent or severe infections

Exam Focus: A positive FHV-1 PCR does not confirm active disease–many normal cats are latent carriers. Diagnosis is typically clinical: compatible history, clinical signs, and response to treatment. Reserve PCR for atypical cases or when ruling out co-infections.

Treatment of FHV-1 Conjunctivitis

Antiviral Therapy

Supportive Care

• Topical antibiotics: Oxytetracycline, erythromycin, or tobramycin q6-8h to prevent secondary bacterial infection
• Artificial tears: Hyaluronate preparations help FHV-1-induced tear film deficiency
• Stress reduction: Environmental modification; Feliway pheromones
• Nutritional support: Ensure adequate hydration and nutrition during acute illness

Memory Aids for Board Exams

"HERPES" Mnemonic for FHV-1 Features

• H - Hides in trigeminal ganglia (latency)
• E - Epitheliotropic (attacks conjunctival and corneal epithelium)
• R - Recurrent disease (stress-triggered reactivation)
• P - Pathognomonic dendritic ulcers
• E - Eighty percent become carriers
• S - Sequestra (corneal) in chronic cases

"CHEM" Mnemonic for Chlamydia felis

• C - Chemosis (marked swelling)
• H - Hyperemia of nictitating membrane
• E - Exclusively conjunctival (no corneal disease)
• M - Minimum 4 weeks doxycycline

Differentiating Features Quick Reference