BIOLOGICAL RACES IN PARASITIC PROTOZOA

Summary Biological races are defined as such subdivisions of a morphological species as are distinguishable by differences in biological characters only. Though biological races are well represented in diverse groups of parasitic Protozoa, their true nature is in most cases concealed under separate specific names. In parasitic Protozoa biological races differ from each other in various aspects of their host‐parasite relations. These may be manifested in host‐restriction (or host‐specificity), as in trypanosomes of the brucei group, containing races confined to man and to other mammals; in intestinal amoebae and flagellates comprising human and batrachian races; and in human and simian malaria parasites ( Plasmodium ). Likewise, races parasitic in the same vertebrate host may be restricted to distinct vectors, as in piroplasms ( Babesia ) of cattle and dogs. There are races which cause different diseases in the same host, usually with a distinct localization in its tissues, e.g. Leishmania in man and dogs, and trypanosomes of the evansi group, causing dourine and surra. Others again vary in the degree of virulence to the host, e.g. piroplasms of the genera Theileria and Babesia. A variation in pathogenicity to one or more hosts characterizes some geographical races of the same parasite, e.g. local strains of human Plasmodium , and of Trypanosoma evansi. Finally, in certain races various differential characters may be combined. Variations in the host‐parasite relationship, as manifested by different races of Protozoa, are determined by the degree of their adaptation to the physical and chemical conditions in the host. These conditions include, among others, body temperature, quality of the digestive juices, nutrition (diet), and properties of the serum (parasiticidal action). The mutual adaptation of parasite and host involves reciprocal reactions, among which the serological reactions of the host are especially interesting, for they throw light on the intimate constitution of parasitic microorganisms. There is reason to believe that biological races of some parasitic Protozoa‐like the intraepecific ‘types’ of bacteria‐differ from each other in antigenic composition, while retaining common antigens characteristic of the species as a whole. Antigenic constitution is, therefore, correlated with the biological peculiarities of these races (at least in blood parasites), and since antigens consist of qualitatively different chemical substances, the distinction between biological races resolves itself into differences in the chemical structure of the antigenic components of the organisms in question. Biological races in parasitic Protozoa appear to be stable and hereditarily fixed. The biological criteria for their differentiation and the constancy of their diagnostic characters are advanced as arguments in support of an independent taxonomic status for biological races.