Date of Award
Master of Science (MS)
Professor A. D. Dingle
Primary amoebic meningo-encephalitis (PAME) is being recognized with increasing frequency throughout the world since its initial discovery by Dr. M. Fowler in 1961. The disease, which has been fatal in all but two reported cases, is mediated by the amoeba-flagellate, Naegleria, which enters the host through a nasopharyngeal route. Naegleria was considered a free-living saprophyte until the discovery of PAME.
As of 1973, no PAME cases had been reported in Canada and it was not known whether this was due to incorrect diagnosis of meningitis cases, hostile climatic conditions that precluded pathogenic Naegleria growth to infective levels or complete lack of pathogenic strains in Canada.
Naegleria is easily isolatable from soil and fresh water sources and, consequently, a search for a pathogenic strain was undertaken in southern Ontario. Only water accessible to the public was sampled. A grossly polluted canal yielded a pathogenic strain capable of producing PAME when intranasally introduced into test animals.
A comparative analysis was carried out using the pathogenic strain and three other isolates selected for diversity of natural environment and/or non-pathogenicity. NB-1, a non-pathogenic laboratory Naegleria long used as a tool for the study of cell differentiation, was used as a standard.
The cultural needs, optimal temperatures for growth, the doubling rate, temperature tolerance, rate of transformation form amoeba to flagellate, effect of sub-lethal heat shock during this transformation process, cyst survival ability, resistance of amoebae and cysts to desiccation with alcohol and soap solutions, as well as various morphological measurements and taxonomic observations of the amoeba, flagellate and cyst phases were determined for the various strains.
The most distinctive and consistent feature found within the five strains was the thermophilic nature and temperature tolerance of the pathogens. Non-pathogenic strains failed to thrive at temperatures above normal body temperature. In addition, the cyst walls of the most virulent pathogen did not have distinctive pores, a feature normally found in Naegleria.
Experimental pathologic testing of the pathogenic strain showed a positive relationship between inoculum size and fatality rate. Tests showed that the rate of intracranial infestation with Naegleria exceeded the number of the individuals that succumbed to the disease. Also, some mice that appeared to be near-death from PAME were capable of recovering.
The virulence of the pathogen was not affected by the media used for sub-culturing and the age of the test animals had little effect on the fatality rate.
The pathogenic strain demonstrate a virulence and survival time (i.e., period from time of inoculation until death from PAME) that correlated well with studies conducted by other investigators using pathogenic Naegleria isolated directly from PAME victims.
Davies, Joan Florence Christine, "Primary Amoebic Meningo-Encephalitis and Isolation of the Causative Naegleria Amoebae From Ontario Waters." (1974). Open Access Dissertations and Theses. Paper 540.