Isoenzyme polymorphism in entamoeba histolytica : an epidemiological survey in a rural South African population.

Abstract

Isoenzyme characterisation of Entamoeba histolytica into pathogenic and non-pathogenic zymodemes substantiated previously held views that this parasite con5titutes two distinct strains or even sub-species that are morphologically identical but vary in their pathogenicity. A reappraisal of the epidemiology of amoebiasis and investigation of the patho-physiological relationships between these pathogenic and non-pathogenic zymodemes and their host was therefore indicated. Only pathogenic zymodemes were isolated from hospitalised patients with amoebic liver abscess (ALA) and amoebic dysentery (AD). In the amoebiasis endemic peri-urban population of Durban, I. histolytica occurred at an overall prevalence of 10%. Carriers of non-pathogenic zymodemes constituted 9% of the population. A key observation was that asymptomatic infections with pathogenic zymodemes occurred at a prevalence of 1%. Higher prevalence of E. histolytica occurred in association with poor sanitary conditions. Furthermore., both pathogenic and non-pathogenic zymodemes tended to cluster into family units suggesting person-to-person transmission of the parasite by the faecal-oral route. Although invasive amoebiasis occurs far more frequently in males than females (8:1) both pathogenic and non-pathogenic zymodemes are equally distributed in male and female E. histolytica cyst passers. Ninety percent of carriers of pathogenic zymodemes spontaneously cleared their infections and remained asymptomatic throughout the study period of 2 years while 10% developed AD which required treatment with metronidazole. No spontaneous changes in zymodemes from the non-pathogenicto the pathogenic type was observed in a longitudinal study. The serological response of asymptomatic carriers of pathogenic zymodemes (100% seropositive) was identical to that of patients with ALA or AD with a high proportion (94-100%) of them being strongly seropositive. The prevalence of seropositivity amongst subjects who were not infected by E. histolytica (13% seropositive) was not statistically different (p>0,5) from that of the random population of this endemic area (19% seropositive) and carriers of non-pathogenic zymodemes (21% positive); the prevalence of strongly seropositive reactions among this group was only between 2-4%. It is concluded that a positive serological response is directly due to past or present contact with pathogenic zymodemes. This is further substantiated by the observation that the proportion of seropositive subjects was found to increase dramatically in a population near Cape Town where an outbreak of invasive amoebiasis (ALA and AD) occurred indicating a high prevalence of pathogenic zymodemes in this community. Another community in northern Transvaal (Gazankulu) where ALA and AD does not occur was, as expected, uniformly seronegative. Axenic growth of pathogenic zymodemes was possible but could not be accomplished with the non-pathogenic zymodemes. Even though monaxenic growth together with Trypanosoma cruzi was possible with both strains, the pathogenic zymodemes tended to grow more prolificly. No zymodeme changes from non-pathogenic to pathogenic and vice versa were observed with such changes in culture conditions. Cyst production by the pathogenic zymodemes in vivo was confirmed experimentally, thereby demonstrating the ability of pathogenic E. histolytica to independently complete their life-cycle thus giving it the ability to propagate itself successfully as a species.