Experimental amebiasis. I. Pathogenicity of axenically cultured Entamoeba histolytica in the brain of the newborn mouse.

Axenically cultured Entamoeba histolytica inoculated intracerebrally into newborn Swiss mice invaded the cerebrum and produced multiple abscesses containing viable trophozoites. As few as 20 amebae of a virulent strain (HM-1:IMSS) occasionally produced fatal disease, 200 killed about 75% of animals and higher doses regularly killed all animals. In contrast, avirulent strains (HK-9 and HB-301:NIH) failed to produce acute brain disease in comparable time periods even when mice were inoculated with as many as 20,000 amebae. Two other strains (1295 and H-458:CDC) were of intermediate virulence. High doses of avirulent amebae often produced hydrocephalus as a late manifestation. In newborn, 3 week-old, and 6-week-old mice resistance to infection increased with age, and older animals often responded late to virulent strains by developing hydrocephalus.