Mycobacterium fortuitum keratitis. Clinicopathologic correlates and corticosteroid effects in an animal model

Mycobacterium fortuitum keratitis is an indolent infection of traumatized corneas in humans. To study this disorder in an animal model, 10(4) M fortuitum organisms (10 microliters) were inoculated into the stroma of both corneas of 16 New Zealand albino rabbits. Eight of the rabbits were also given bilateral subconjunctival injections of methylprednisolone acetate (20 mg in 0.5 ml) at the time of inoculation. Two corticosteroid-treated and two untreated rabbits were selected each week after inoculation for histopathological examination and quantitative cultures. Corneal lesions in corticosteroid-treated eyes were characterized clinically by indolent ulcerations and satellite lesions that slowly enlarged; on histopathologic examination at each week, acute inflammation and microorganisms were consistently present. Corneal lesions in untreated eyes were characterized clinically by small infiltrates that progressed little over time; at weeks 1 and 2, light microscopic examination showed intrastromal granulomatous and/or mixed acute and chronic inflammation with focal intrastromal necrosis, but at weeks 3 and 4 there was no evidence of active disease. Organisms could not be identified microscopically in corneas of any untreated rabbits. Mean values for quantitative cultures of corneas were higher in corticosteroid-treated rabbits after week 1, although standard deviations were large. These results suggest that M fortuitum keratitis in rabbits is made worse by corticosteroid use. Clinical and histopathologic changes were compared with human disease and found to be similar in corticosteroid-treated rabbits.