Etiology

Bacterial causes:  The most common bacterial pathogens are Staphylococcus aureus, coagulase-negative staphylococcus, and Pseudomonas aeruginosa. The other causative bacteria are Staphylococcus pyogenes and Staphylococcus pneumoniae.

Viral causes: The most common causative agent is Herpes simplex virus (HSV) . Varicella-zoster virus (VZV) and cytomegalovirus can also cause viral keratitis.

Fungal: Fungal etiologies account for only 5% to 10% of all corneal infections. They are most often precipitated by trauma to the cornea with subsequent exposure to plant or vegetable material. The most commonly implicated species organisms include Aspergillus, Fusarium, Scedosporium apiospermum, phaeohyphomycetes, Candida albicans, and other Candida species.

Protozoan: Acanthamoeba is a free-living protozoan found in freshwater and soil that can cause keratitis and corneal ulcers, primarily in contact lens wearers.