Experimental Acanthamoeba infections in mice pretreated with methylprednisolone or tetracycline.

Human infections due to free-living amebas of the genus Acathamoeba have been reported sporadically, occasionally in individuals with underlying diseases. To determine if such infections may be considered opportunistic, groups of laboratory mice were pretreated with either methylprednisolone or tetracycline and inoculated intranasally with 1.075 times 10(4) Acanthamoeba castellanii isolated from a natural fresh water well. Results were compared with controls receiving either drug or amebas alone and with controls receiving saline injections with and without amebas. The mortality rate for those animals receiving methylprednisolone and amebas (50%) was found to be greater than the mortality in ameba controls (10%) (P equal 0.074). Similarly, the mortality rate for animals receiving tetracycline and amebas (60%) was higher than the mortality in the ameba controls (10%) (P equal 0.0286). Precise mechanisms for the increased mortality were unknown but were suspected to be due to the capacity of either corticosteroids or tetracycline to suppress host defenses, particularly those depending on neutrophils. The findings suggest a potentially pathogenic role for naturally occurring Acanthamoeba sp in humans with depressed host immunity.