Why Ocular Microbiology Matters for the CPOA
As a CPOA working in an eye care setting, you are frequently the first person to interact with patients who have infectious eye conditions. Recognizing which organisms commonly cause ocular infections, understanding their clinical presentations, and knowing how to prevent transmission are essential skills for protecting both patients and clinic staff.
Bacterial Pathogens
Common External Disease Bacteria
- Staphylococcus aureus / Staphylococcus epidermidis: The most common eyelid and ocular surface organisms. Cause blepharitis, hordeolum (stye), bacterial conjunctivitis, and bacterial keratitis (corneal ulcer). Coagulase-negative staph (S. epidermidis) is the most common cause of post-cataract surgery endophthalmitis.
- Streptococcus pneumoniae: Causes aggressive bacterial conjunctivitis (often copious purulent discharge) and bacterial keratitis. Common in children.
- Haemophilus influenzae: Common cause of bacterial conjunctivitis in children, especially with concurrent upper respiratory infection. Often associated with otitis media.
- Pseudomonas aeruginosa: The most common bacterial cause of contact lens-related bacterial keratitis. Produces a rapidly progressive corneal ulcer with significant pain and hypopyon. Requires prompt, intensive antibiotic treatment.
- Neisseria gonorrhoeae: Hyperacute bacterial conjunctivitis with profuse purulent discharge. Highly contagious and can rapidly penetrate an intact cornea, leading to perforation. A sexually transmitted pathogen -- requires systemic antibiotic treatment.
💡 Clinical Tip: Patients with hyperacute conjunctivitis (profuse purulent discharge developing within 24-48 hours) should be seen urgently. Gonococcal conjunctivitis can perforate the cornea within days without treatment. Never treat hyperacute conjunctivitis empirically without the doctor evaluating the patient first.
Viral Pathogens
- Adenovirus: The most common cause of infectious conjunctivitis worldwide. Causes epidemic keratoconjunctivitis (EKC) -- highly contagious, follicular conjunctivitis with watery discharge and preauricular lymphadenopathy. EKC can cause significant corneal subepithelial infiltrates that reduce vision. No antiviral treatment available -- manage symptomatically. Extremely contagious by contact with secretions.
- Herpes Simplex Virus (HSV): Causes primary herpetic keratoconjunctivitis and recurrent herpetic keratitis. The hallmark is a dendritic (branching) epithelial ulcer that stains with rose bengal or fluorescein. Treatment: topical antiviral (ganciclovir gel) or oral acyclovir/valacyclovir. As noted: steroids are contraindicated in epithelial HSV keratitis.
- Herpes Zoster Ophthalmicus (HZV): Reactivation of varicella-zoster virus in the distribution of the first branch (V1) of the trigeminal nerve. Causes vesicular rash on the forehead and periocular skin plus conjunctivitis, keratitis, uveitis, and other severe ocular complications. Treated with systemic antiviral (acyclovir, valacyclovir).
- Molluscum contagiosum: A poxvirus causing umbilicated lid lesions that shed viral particles onto the conjunctival surface, causing chronic follicular conjunctivitis. Treated by removing or ablating the lid lesions.
Other Pathogens
- Chlamydia trachomatis: The leading infectious cause of preventable blindness worldwide (trachoma). In the U.S., causes inclusion conjunctivitis, a sexually transmitted form -- chronic follicular conjunctivitis often with superior pannus. Also causes neonatal inclusion conjunctivitis. Treated with systemic azithromycin or doxycycline.
- Acanthamoeba: A free-living protozoan in water and soil. Causes Acanthamoeba keratitis -- a severe, painful corneal infection almost exclusively in contact lens wearers who use tap water to rinse lenses or cases. Characteristic: severe pain disproportionate to clinical signs, ring infiltrate. Extremely difficult to treat and can be blinding.
- Candida and Aspergillus: Fungal pathogens causing keratitis, usually after corneal trauma with vegetable matter or in immunocompromised patients.
⚠️ Common Mistake: Treating all red eyes as bacterial conjunctivitis without involving the doctor. Adenoviral conjunctivitis, HSV keratitis, and Acanthamoeba keratitis all present with a red eye but require completely different management -- and antibiotics are useless or harmful for viral or protozoal infections. The CPOA documents symptoms and alerts the doctor; the doctor diagnoses.
Transmission Routes and Prevention
- Contact transmission: Most common route for bacterial and viral conjunctivitis. Prevent by handwashing, not sharing instruments, and disinfecting contact surfaces.
- Instrument contamination: Tonometer prisms, slit lamp chin rests, and trial contact lenses require proper disinfection between patients.
- Droplet transmission: Adenovirus and HSV can spread by respiratory droplets and contact with eye secretions. Staff should wear gloves when handling ocular secretions.
Key Takeaways
- Most bacterial conjunctivitis is caused by S. aureus, S. pneumoniae, or H. influenzae (children) and responds to topical fluoroquinolone antibiotics.
- Pseudomonas is the main pathogen in contact lens-related corneal ulcers -- aggressive treatment required.
- Adenovirus (EKC): most common viral cause; highly contagious; no antiviral treatment.
- HSV: dendritic keratitis; treat with antivirals; steroids are contraindicated in epithelial disease.
- Acanthamoeba: contact lens wearers using tap water; extremely severe, painful keratitis.
- Handwashing, instrument disinfection, and proper contact lens hygiene are the primary prevention measures.