Factors Affecting the Virulence of Naegleria fowleri for Mice

Incubation temperature, growth phase, and strain of parasite were examined for their effect upon the virulence of Naegleria fowleri for mice. Agitated cultures of amebae incubated at 30 and 37°C were more virulent than amebae cultured at 23 and 44°C. Mortality was greater for mice inoculated with amebae harvested at late logarithmic and early stationary growth phases than it was with amebae harvested at early logarithmic and late stationary growth phases. Virulence varied greatly among the 10 strains of N. fowleri tested. Also, it appeared that virulence of amebae decreased with increased time in axenic culture. These results suggest that factors affecting the virulence of N. fowleri for mice may also influence the outcome of natural exposure and infection in man. Naegleria fowleri is a free-living ameboflagellate which produces a fatal meningoencephalitis in man. It is also able to produce a similar progressive and fatal disease in mice (see Griffin, 1978 for a recent review). There are numerous publications concerning the virulence of N. fowleri for laboratory animals. Most of these accounts describe the invasiveness of amebae with respect to route and size of inoculum. In contrast, little information is available on the effect of culture conditions upon the virulence of N. fowleri amebae. Wong et al. (1975) have suggested that pathogenicity of N. fowleri may be influenced by the strain virulence, growth phase, and cultural conditions of amebae. Recently (Wong et al., 1977), they demonstrated that prolonged maintenance of N. fowleri amebae in axenic medium reduced the organism's virulence for mice. Griffin (1972) stated that the ability of N. fowleri to grow at high temperatures seemed directly related to virulence. In this study, we demonstrate that incubation temperature, growth phase, and ameba strain affect the virulence of N. fowleri for mice. Materials and Methods