Steve Roberts, DVM, MS, Dipl. ACVO, of the Animal Eye Center in Loveland, Colo., describes this phenomenon: "The exacerbation effect of topical corticosteroids relates to suppression of the normal healing response and the inflammatory response to microbes, and its enhancement of the breakdown of proteins within the collagen of the cornea. Within a few hours of a break in the corneal epithelium (outer layer), white blood cells (neutrophils or PMNs--polymorphonuclear leukocytes) move into the tear film and gain access to the epithelium and underlying corneal connective tissue (stroma). As the PMNs try to stop infection, they begin to remodel the wound area in preparation of healing by epithelial cell migration to form a new layer. If the initial influx of PMNs is reduced by the presence of steroids, then microbes may gain a foothold, making ultimate control difficult even with antibiotics or antifungal medications."