Many of the parasites which have the greatest effect on our collective health and economy, and which have influenced the fates of nations, are protists. Most significantly, they include the agents of sleeping sickness and malaria. Sleeping sickness is caused by a kinetoplastid flagellate. The direct impact of sleeping sickness on humans is now much reduced, but in the 19th century, it wreaked destruction in Africa, facilitated by ill-informed moralistically motivated colonialists (Ford 1971). Malaria, caused by Plasmodium - an apicomplexan alveolate - remains the number one infectious disease affecting people.
Protists embrace many species which are found within invertebrate and vertebrate animals, within plants, or even within other protists. Some of these 'endobionts' are clearly pathogenic, but the nature of the relationship between others seem to be more benign. At one end of the spectrum Phytophthora, a stramenopile, levels large tracts of native woodlands of Australia by invading the plants through their roots. On the other hands, opalines live happily in the backsides of frogs, while hypermastigid flagellates and entodiniomorph ciliates probably supply their hosts with the capacity to digest their food.
Parasites are drawn from many groups of protists. The adaptive form varies from amoeboid (Entamoeba), amoebo-flagellate (Naegleria), flagellated (trypanosomes), ciliates (Balantidium), to sporozoa (Plasmodium). Some groups have diversified as parasites (Microsporidia, Apicomplexa). The parasitic life style often leads to organizational regression. Organisms which may have a clear body form or have organelles such as flagella, may lose those features while they are located within other cells. In some cases, this regression is extreme. Microsporidia, which lack flagella, dictyosomes, mitochondria, are now believed to be a highly derived form of eukaryotes, and Myxozoa are a highly derived form of metazoan.
Groups of Protistan Parasites
|Name of group||Organization||Composition|
|Alveolates||sporozoan, flagellates, ciliates||Apicomplexa is a major group comprised almost entirely of parasitic protozoa. It includes the agents of malaria (Plasmodium). A small number of the dinoflagellates and ciliates may also parasitize invertebrates.|
|Entamoebae||amoebae||a few genera of amitochondriate amoebae, e.g. Entamoeba|
|Excavates||flagellates, amoebae||diplomonads (e.g. Giardia) and retortamonads are parasitic; (others are free living)|
|Opisthokonts||fungal, animal||a small proportion of fungi and animals are parasitic, though the important ones may be important to human health and economy; microsporidia (e.g. Nosema) and myxospora occur here.|
|Parabasalids||flagellates||e.g. Trichomonas, and the hypermastigids which live in the intestines of termites and some other insects|
|Pelobionts||flagellates||A few species, e.g. in Mastigina, are parasites|
|Pseudospora||amoeb-flagellates, one genus||Pseudospora|
|Ramicristates||amoebae||some genera of amoebae are facultative parasites or parasites most usually of the digestive tracts of invertebrates and vertebrates. e.g. Acanthamoeba,|
|Rosette agent||un-named parasite|
|Stramenopiles||fungal, flagellated, amoeboid||A few taxa are parasites, such as oomycete fungi, Blastocystis, and the opalines|
Genera of parasitic protists which have not been studied by electron microscopy and for which no clear identity has emerged (after Patterson, 1999):