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ProtozoaProtozoa are the most taxonomically diverse group of insect pathogens and, to confuse matters further, major changes currently are occurring in the understanding of their taxonomic relationships. All Protozoa are single-celled eukaryotes and they range in their interactions with insects from commensualists and mutualists, to plant and animal pathogens vectored by insects, to acute insect pathogens (Tanada and Kaya, 1993). Of some 14,000 described species of Protozoa, about 500 are pathogens of insects. Many are chronic pathogens that may debilitate a host without producing obvious disease symptoms but some species are extremely virulent, causing stunted growth, slow development, and early death (Tanada and Kaya, 1993). Entry into the host is typically by ingestion, but some can invade through the cuticle. Some species may be transovarially transmitted from infected females to their offspring. Species that invade the cells of the host are usually found in the cell cytoplasm and are typically more pathogenic than extracellular species. Some protozoans exhibit tissue tropism, infecting only certain tissues or organs, others are systemic. The neogregarines often exhibit tissue tropism, the tissue or tissues infected being a species specific characteristic. No toxins have been found to be associated with protozoa in insects. Death or debilitation of infected hosts may be, for example, the result of competition for metabolites, disruption of normal cell and tissue function, or blockage of the gut or other organs by extracellular species. Unlike the viruses and some bacteria, protozoans do not typically cause the rupture of the host's integument. Some insects such as mosquitoes become chlorotic or whitish in color when infected, and some lepidopteran or coleopteran larvae may appear puffy. Many species of protozoans, however, do not typically cause outward signs of disease (Tanada and Kaya, 1993). The insect-pathogenic protozoa are currently recorded from four major groups of the Protozoa: |
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