[Protective immunity against Naegleria meningoencephalitis in mice]

This study is to verify the protective ability against experimental Naegleria meningoencephalitis by immunization with Naegleria fowleri in mice. Naegleria fowleri, strain 0359, and Naegleria gruberi, strain EGB, were used in this study, and cultured in CGVS medium axenically. Inbred BALB/c mice, weighing about 20g, were immunized by three intraperitoneal injection of 1 x 10(6) N. fowleri trophozoites at the interval of one week. This N. fowleri trophozoites antigen was fixed with 5 percent formaldehyde. N. fowleri trophozoites from culture were homogenized with sonicator at 4 degrees C as monitored by phase contrast microscopy, and their membrane and cell content preparations were made for the immunization of mice. Their inoculation dose in volume was equivalent to the 1 x 10(6) trophozoites in each injection for immunization. And N. gruberi trophozoites, which was fixed with 5 percent formaldehyde, were also used for immunization. Mice were inoculated intranasally with 5 x 10(4) N. fowleri trophozoites in a 5 microliter suspension under anesthesia by as intraperitoneal injection of about l mg secobarbiturate. Nervousness, rotation or sluggish behaviour were observed in the mice which were infected with N. fowleri. Necrotic lesion was demonstrated in the anterior portion of brain, especially in the olfactory lobe. The inflammatory cell infiltration with numerous N. fowleri trophozoites was noticed. This pathological changes were more extensive in the control than in the experimental groups. Mice were dead due to experimental primary amoebic meningoencephalitis that developed between 8 days and 23 days after inoculation. Mortality rate of the mice was low in the immunized experimental group. Mean survival time, which is the survival duration of mice from the infection to death, was prolonged significantly in the immunized mice except in the mice immunized with N. fowleri membrane. Even in the mice immunized with N. gruberi, survival time was delayed. In summary, the effectiveness of immunization is demonstrated in terms of protective immunity against Naegleria meningoencephalitis in mice.