Why is Pseudomonas keratitis considered an emergency?

Pseudomonas aeruginosa produces proteases and exotoxins that can rapidly destroy corneal tissue, potentially perforating the cornea within 24-48 hours if untreated. It is the most virulent common corneal pathogen and is strongly associated with extended wear soft contact lenses. Any contact lens wearer with a painful red eye and a corneal infiltrate needs same-day emergency evaluation.

What is the pathognomonic lesion of herpes simplex keratitis?

The dendritic ulcer is the pathognomonic lesion of HSV keratitis. It appears as a branching, tree-like (dendritic) epithelial defect that stains with fluorescein. The terminal end-bulbs (swollen endpoints on the branches) are particularly characteristic. This pattern distinguishes it from other corneal ulcers and guides treatment with topical antivirals.

How is adenoviral conjunctivitis distinguished from bacterial conjunctivitis?

Adenoviral conjunctivitis produces watery (not thick purulent) discharge, prominent follicles on the tarsal conjunctiva, and often a preauricular lymph node (swollen lymph node in front of the ear). Bacterial conjunctivitis produces thick mucopurulent discharge that causes lid crusting, particularly in the morning. Viral conjunctivitis is self-limiting; bacterial conjunctivitis responds to antibiotics.

What makes contact lens wearers uniquely at risk for Acanthamoeba keratitis?

Acanthamoeba is ubiquitous in water and soil. Contact lenses create a microenvironment at the corneal surface where Acanthamoeba can attach and colonize, particularly when exposed to water containing the organism. Risk factors include swimming, showering, or bathing with contact lenses in, using tap water to rinse contact lens cases, and using homemade saline solutions. Proper lens care (only using sterile solutions, keeping lenses away from water) dramatically reduces risk.

Common Ocular Pathogens: Bacteria, Viruses, and Parasites

Why Ocular Pathogen Knowledge Matters

Recognizing the likely cause of an eye infection guides triage, infection control precautions, and appropriate urgency. A CPO who understands that Pseudomonas keratitis in a contact lens wearer requires emergency referral, while a typical viral conjunctivitis is self-limiting, provides better patient care and protects other patients and staff from contagion.

Ocular pathogens are broadly classified into bacteria, viruses, fungi, and parasites. For the CPO exam, the focus is on the most clinically significant organisms in each category.

Bacterial Pathogens

Staphylococcus aureus and Staphylococcus epidermidis

Staphylococci are normal inhabitants of the eyelid skin and margins. When they overgrow or the ocular defenses are compromised, they cause:

Bacterial blepharitis: Chronic eyelid margin inflammation with scaling, crusting, and dilated blood vessels.
Bacterial conjunctivitis: Mucopurulent discharge, red eye, morning lid crusting.
Staphylococcal marginal keratitis: Peripheral corneal infiltrates caused by immune reaction to staph exotoxins, not direct infection.

Streptococcus pneumoniae

Streptococcus pneumoniae (pneumococcus) is a common cause of bacterial conjunctivitis and, more seriously, bacterial keratitis. It tends to produce a more acute, copious purulent discharge than staph species. It was historically the most common cause of bacterial corneal ulcers before fluoroquinolone prophylaxis became standard.

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative bacterium that has a particular affinity for contact lens-associated corneal infections. It is the most feared bacterial corneal pathogen because it can destroy corneal tissue with alarming speed, sometimes perforating the cornea within 24 to 48 hours of infection.

Risk factors for Pseudomonas keratitis:

Extended wear soft contact lens use
Overnight contact lens wear
Contaminated contact lens solutions
Swimming with contact lenses

Presentation: severe pain, dense white corneal infiltrate, significant anterior chamber reaction (hypopyon), and copious mucopurulent discharge. This is an ocular emergency requiring immediate referral.

A contact lens wearer who calls your office with a red, painful eye should never be told to simply remove their lens and wait. Pseudomonas keratitis is an emergency. Triage these patients for same-day evaluation.

Viral Pathogens

Adenovirus

Adenovirus is the most common cause of viral conjunctivitis (pink eye). It presents with:

Watery discharge (not purulent like bacterial)
Follicular conjunctival reaction (bumps on the conjunctiva under the eyelid)
Preauricular lymphadenopathy (enlarged lymph node in front of the ear)
Highly contagious, spreading rapidly through contact

Epidemic keratoconjunctivitis (EKC) is a severe form caused by specific adenovirus serotypes that involves both the conjunctiva and cornea. Subepithelial infiltrates (SEIs) can develop weeks after the acute infection, causing photophobia and visual blurring.

There is no specific antiviral treatment for adenoviral conjunctivitis. Management is symptomatic (cool compresses, lubricants). Meticulous infection control is essential to prevent spread in the office.

Adenoviral conjunctivitis is highly contagious and spread by direct contact. In a clinic, this means disinfecting all surfaces and instruments the patient touched, changing gloves between patients, and informing the patient to avoid sharing towels or shaking hands until the infection resolves (usually 10-14 days).

Herpes Simplex Virus (HSV)

Herpes simplex virus type 1 (HSV-1) is responsible for most cases of herpes keratitis. After primary infection (often subclinical or presenting as a cold sore), the virus establishes latency in the trigeminal ganglion. Reactivation causes:

Dendritic ulcer: The pathognomonic (disease-defining) lesion of HSV keratitis. A branching, tree-like epithelial defect visible with fluorescein staining. Terminal end-bulbs are characteristic.
Geographic ulcer: A more advanced, amoeba-shaped lesion from a coalescent dendritic pattern.
Stromal keratitis: Immune-mediated stromal inflammation that can lead to corneal scarring and permanent vision loss with recurrences.

Treatment is with topical antivirals (ganciclovir gel or trifluridine) and sometimes oral acyclovir or valacyclovir. Steroids are added very carefully only when there is significant stromal or endothelial involvement and always with concurrent antiviral coverage.

Chlamydia trachomatis

Chlamydia trachomatis causes two distinct ocular syndromes:

Inclusion conjunctivitis (adult or neonatal): A chronic follicular conjunctivitis from a sexually transmitted chlamydial strain. Adults present with a unilateral or bilateral mucopurulent conjunctivitis with prominent follicles, often with concurrent genital infection. Neonates acquire it during delivery.
Trachoma: A different chlamydial serotype that causes repeated infections leading to conjunctival scarring (trichiasis), corneal vascularization, and ultimately blindness. Trachoma is the leading infectious cause of preventable blindness worldwide.

Acanthamoeba

Acanthamoeba is a protozoan parasite that lives in water and soil. Acanthamoeba keratitis is rare but devastating, and almost exclusively occurs in contact lens wearers, particularly those who expose their lenses to water (showering, swimming, using tap water to rinse lenses or cases).

Clinical features:

Severe, out-of-proportion pain relative to the amount of observed inflammation
Ring-shaped corneal infiltrate (pathognomonic when present)
Very slow to respond to treatment; may require months of intensive antimicrobial therapy
Can mimic herpetic keratitis, leading to delayed diagnosis

Acanthamoeba keratitis is strongly associated with contact lens water exposure. Educating patients never to wear contact lenses in water (swimming, showering, hot tubs) is a key preventive measure.

Key Takeaways

Staphylococcus and Streptococcus are common causes of bacterial conjunctivitis and blepharitis.
Pseudomonas keratitis is an emergency associated with contact lens wear; it can perforate the cornea within 24-48 hours.
Adenovirus causes highly contagious viral conjunctivitis with watery discharge and follicular reaction.
HSV keratitis presents with a dendritic ulcer on fluorescein staining; antivirals are required.
Acanthamoeba keratitis is almost exclusively a contact lens wearer's disease linked to water exposure.