Causes of Conjunctivitis
Conjunctivitis is caused by two broad categories: infectious agents (viruses and bacteria) and noninfectious etiologies (allergic, mechanical/irritative/toxic, immune-mediated, and neoplastic), with viral conjunctivitis being the most common overall cause. 1
Primary Classification Framework
The 2024 American Academy of Ophthalmology guidelines provide a clear algorithmic approach to understanding conjunctivitis causes 1:
Infectious Causes
Viral (Most Common Overall)
- Adenovirus - accounts for approximately 80% of viral conjunctivitis cases 2
- Herpes simplex virus (HSV) - can cause primary or recurrent disease with distinctive vesicular lesions
- Varicella zoster virus (VZV) - presents with dermatomal rash and severe pain
- Enterovirus 70 and Coxsackievirus A24 - major causes of acute hemorrhagic conjunctivitis 1
- Epstein-Barr virus (EBV) - typically presents as follicular conjunctivitis with systemic symptoms
- Measles, mumps, rubella - seen in unvaccinated individuals 1
- Molluscum contagiosum - causes chronic follicular conjunctivitis with characteristic umbilicated eyelid lesions
- Zika virus - transmitted via Aedes mosquito
- SARS-CoV-2 (COVID-19) - presents with mild follicular reaction 1
Bacterial (Second Most Common Infectious Cause)
- Gonococcal - requires urgent recognition due to risk of corneal perforation; consider sexual abuse in children 1
- Chlamydia trachomatis - leading infectious cause of global blindness; can cause cicatricial disease 1
- Other bacterial pathogens causing acute purulent conjunctivitis
Noninfectious Causes
Allergic (Affects up to 40-50% of population)
- Seasonal/perennial allergic conjunctivitis - triggered by environmental allergens (grasses, pollens), outdoor air pollution, climate factors 1
- Vernal conjunctivitis
- Atopic conjunctivitis
- Giant papillary conjunctivitis (GPC) - often contact lens-related
Mechanical/Irritative/Toxic
- Keratoconjunctivitis sicca (dry eye) - one of the most frequent causes of conjunctival inflammation 1
- Blepharitis - another most frequent cause requiring treatment of underlying problem 1
- Contact lens-related keratoconjunctivitis
- Medication-induced/preservative-induced - from long-term eye drop use 3
- Superior limbic keratoconjunctivitis (SLK) - may indicate thyroid disorder 1
- Rosacea conjunctivitis
- Floppy eyelid syndrome - should prompt sleep apnea evaluation 1
- Giant fornix syndrome
Immune-Mediated and Other
- Parinaud oculoglandular syndrome - most commonly from cat scratch disease (Bartonella henselae) or tularemia 1
- Ligneous conjunctivitis - genetic plasminogen deficiency causing pseudomembrane formation 1
- Mucus fishing syndrome - cyclical mechanical trauma 1
- Neoplastic causes
Critical Clinical Distinctions
Key differentiating features by etiology 1, 4, 5:
- Viral: Watery discharge, follicular reaction, preauricular lymphadenopathy, more common in adults
- Bacterial: Mucopurulent discharge with matted eyelids on waking, more common in children
- Allergic: Itching (most consistent sign), bilateral presentation, chemosis, mucoid discharge
Secondary Conjunctival Inflammation
Always consider underlying systemic or ocular diseases 1:
- Dry eye and blepharitis are the most frequent causes requiring treatment of the primary condition
- Systemic atopy
- Acute systemic infections (gonorrhea requiring systemic therapy)
- Thyroid disorders (with SLK)
- Sleep apnea (with floppy eyelid syndrome)
Important Clinical Pitfalls
The guidelines emphasize that conjunctival inflammation is often secondary rather than primary disease 1. Treatment must address the underlying problem—for example, treating dry eye or blepharitis rather than just the conjunctival inflammation itself. Missing sexually transmitted infections (gonorrhea, chlamydia) is particularly dangerous as these require systemic antibiotics in addition to topical therapy 1, 5.