Introduction to the Protozoa

Protozoa form the most primitive group in the animal kingdom. The bodies of all other animals are composed of many units, or cells, but those of the protozoa are a single cell. No matter how complex their bodies may be, and many of them are very much so, all the different structures are contained in a single cell. This complexity has made some investigators maintain that, instead of being considered single cells, protozoa should be thought of as non-cellular (see, for example, Boyden, 1957). This argument is essentially a verbal issue - a matter of how one wants to define "cell".

Protozoa are microscopic in size, only a few being visible to the naked eye. They differ from the Metazoa in being unicellular, but this difference is not as clearcut as might be supposed. Some protozoa have a syncytial stage in their life cycle in which there are no cell walls between the nuclei, and some species form colonies which swim as a unit and which contain somatic and reproductive organisms which look different. The difference between these and Metazoa is again partly a matter of definition, and gives a clue to how the Metazoa could have arisen.

The boundary between the Protozoa and certain one-celled plants, too, is not clearcut. For example, the whole group of slime molds are considered by protozoologists to be protozoa and assigned to the order Mycetozoorida in the class Sarcodasida, but botanists consider them fungi and assign them to the class Myxomycetes.

A still more confusing situation involves the plant-like protozoa which contain chlorophyll. Protozoologists assign them to the subclass Phytomastigasina, but botanists consider them green algae. The problem is that there are many species of colorless protozoa which differ from green ones only in that they lack chromatophores. Loss of chromatophores can be produced experimentally. It has been done in Euglena, for instance, by treatment with streptomycin or simply by growing the organisms at 34 to 35° C (Pringsheim and Pringsheim, 1952). This change of a plant into an animal would be just as astounding as the metamorphosis of Cinderella's pumpkin into a golden coach if the differences between the lower forms were as great as those between higher plants and animals. However, the principal difference is one of nutrition, and many species are quite plastic, their form of nutrition depending on circumstances. Indeed, many of the metabolic pathways of the phytoflagellate, Ochromonas malhamensis, aside from those in which its chlorophyll takes part, are so similar to those of men that Hutner has facetiously called it a humanoid!

In recognition of this situation, Ernst Haeckel proposed that the name Protista be applied to all single-celled organisms and that the group be considered intermediate between the animal and plant kingdoms. Relatively few modern taxonomists subscribe to this idea, perhaps less because of any defect in the idea itself than because they have been trained either as botanists or zoologists and not as biologists.

Since their discovery by Leeuwenhoek, some 30,000 species of protozoa have been described. They occur in practically all habitats where life can exist and are among the first links of the food chain on which all higher life depends. Floating in the plankton of tropic seas, they cause the luminous glow of waves and ship-wakes. Blooming off our coasts, they cause the red tide which deposits windrows of dead fish on shore. They abound in ponds and streams and in the soil. Their role in sewage purification is just beginning to be understood. Their skeletons cover the ocean floor and form the chalk we use in classrooms.

As parasites, protozoa play a double role. Malaria is still the world's most important disease. Trypanosomes have interdicted vast African grazing lands for livestock. Amoebae cause dysentery in man, and coccidia cause it in his domestic animals. But other protozoa, packing the termite's hind-gut almost solidly, digest the cellulose that it eats and feed it with their wastes and dead bodies. Fabulous numbers of protozoa swarm in the rumen and reticulum of cattle and sheep and in the cecum and colon of the horse, but their role is still not clear.

In this book we are concerned with the protozoan parasites of domestic animals. Our understanding of these forms can be enhanced by knowledge of the parasites of other animals and of free-living forms. For further information on the protozoa in general, reference is made to Doge'l (1951), Grasse (1952-53), Grell (1955), Hall (1953), Hyman (1940), Kudo (1954), Reichenow (1949-53) and Wenyon (1926).