THE BIONOMICS OF COPROPHILIC PROTOZOA

Abstract

SummaryThe coprophilic Protozoa are forms capable of living in faeces. They form a loose ecological group of considerable practical and economic importance.The development of the conception of coprophilic Protozoa is traced in the article, and it is shown that at first they were frequently confused with or taken for parasitic species, many of them being considered as pathogenic. Further research showed that they were in reality free‐living species which had become adapted to life in dung.The faecal environment is peculiar in being characterized by abundance of decaying organic matter, vast numbers of bacteria with eonsequent restriction of free oxygen, and restriction of fluid to small pockets and channels between the solid particles. Hence the only Protozoa able to live in undiluted faeces are polysaprobic species whose methods of locomotion permit them to move in restricted volumes of fluid or in a viscous medium. Some coprophilic Protozoa are holozoic, while others are saprozoic. Most of them flourish best at low temperatures, but they are tolerant of wide variation in the hydrogen‐ion concentration of the medium and can withstand temporary deficiency of oxygen and abnormally high concentrations of carbon dioxide. They are all well adapted to withstand desiccation by anabiosis or cyst formation. Although they can live and multiply in normal faeces so long as they remain sufficiently moist, many of them are not able to survive in pathological faeces from cases of diarrhoea and dysentery. The factors inhibiting growth in this medium are the presence of drugs, or of unaltered bile salts, an unusually high osmotic pressure, or the admixture of urine with the fluid stool. The presence of mucus in the faeces, on the other hand, is favourable to the growth of coprophilic Protozoa. Although urine is lethal to the active forms, it probably does not act as an important limiting factor in nature, since with normal faeces it merely bathes the surface and does not become admixed to any extent.A classified list is given of all the Protozoa reported from human and animal faeces.While the drying up of the faeces leads to the disappearance of active Protozoa, the dilution of the faeces artificially or by rain leads to the development of a succession of forms analogous to that which takes place in a hay infusion or in the waters of a polluted river during self‐purification.Coprophilic Protozoa may be classified, according to their degree of adaptation, into three groups, namely: (a) Oligocoprophilic forms, able to live in faeces only after dilution; (b) Mesocoprophilic forms, able to live in some undiluted faeces, especially if of loose consistency, but flourishing only after dilution; (c) Polycoprophilic forms, able to flourish in undiluted normal faeces.The medical and veterinary importance of the coprophilic Protozoa lies in the possibility of confusing them with or taking them for true parasitic forms. Some of the classical cases of pseudo‐parasitism, due to this cause, are described.Coprophilic Protozoa gain access to the faeces either by passing through the alimentary canal in the encysted condition, or by contamination of the stool after defaecation by air‐, water‐, or soil‐borne cysts. The former are referred to as endogenous forms, the latter as exogenous.Coprophilic Protozoa may be distinguished from parasitic species by the fact that they may be readily cultivated at room temperature but die out at 37o C.A close ecological relationship exists between the Protozoa of sewage and the coprophilic Protozoa. The latter occur characteristically in sedimentation tanks and Imhoff tanks, but are also represented in sprinkling filters and activated sludge. They play a part in the. purification of sewage.A similar close ecological relationship exists between coprophilic and soil Protozoa, almost all the coprophilic species occurring also in the soil from which they were originally derived and to which they are continually being returned.Coprophilic Protozoa also occur in natural manure, and it has been suggested that the loss of nitrogen from stored dung owing to the activities of bacteria and moulds might be averted by ‘seeding’ with suitable coprophilic forms.Many species of coprophilic Protozoa are useful indicator species, and their presence in drinking‐water supplies is suggestive of contamination with faecal matter or sewage.Coprophilic Protozoa are of evolutionary significance, since they recapitulate the various steps by which free‐living species may have become adapted to the parasitic habit, namely: (a) Close relationship to established parasitic forms. (b) Occasional excystment and temporary existence in the lower bowel, (c) Facultative parasitism in cold‐blooded hosts, (d) Established parasitism, but with the ability to lead a coprozoic existence temporarily.The factors involved in this change are discussed and the conclusion reached that coprophilic forms may evolve into true parasites.