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Gregarines & CoccidiaThe gregarines and coccidia have been placed in the phylum Apicomplexa and represent a group of protozoans that lack cilia, reproduce sexually, use micropores for feeding, move by body flexion or gliding, and produce oocysts containing sporozoites as the infective form (Tanada and Kaya, 1993). The gregarines are divided into two groups, the eugregarines and neogregarines. The major difference between these groups is that eugregarines do not reproduce vegetatively in the host while the neogregarines do. The neogregarines are thought to be more primitive and are much more virulent since they can build up in huge numbers in the host tissues. EugregarinesAbout 1,400 species of eugregarines are known in two major groups- aseptate species that have one "body" compartment, and septate species which have two. Both groups primarily consist of pathogens of annelids and arthropods (a few are commensuals living in the gut lumen). Typically, oocysts are ingested by the host and the spore wall dissolves in the host's gut juices, releasing sporozoites (usually eight). If the eugregarine species is pathogenic, these forms enter the midgut epithelia and grow. At a certain stage, they exit the cell, killing it. Having reentered the gut lumen, the trophozoites mature into gamonts which pair in "syzygy" around which a cyst forms. One gamont produces microgametes and one produces macrogametes which fuse to form zygotes, the only diploid stage. Zygotes undergo division to form the eight new sporozoites per oocyst (Tanada and Kaya, 1993). Eugregarines are often rather benign to the host because the infection level depends entirely on the number of oocysts ingested. Those species that inhabit the midgut are rarely seriously pathogenic; however, they have been reported to block the gut of the host (Harry, 1967). Some species inhabit the gastric caeca and these cause more serious pathologies than the midgut-inhabiting species (Tanada and Kaya, 1993). Ascogregarina (aseptate, in mosquitoes) and Gregarina (septate, in roaches) are the best known genera. NeogregarinesBecause the neogregarines undergo multiple divisions (called schizogony or merogony) after entering the host cells, their numbers are not necessarily related to the number of oocysts ingested by the host. The resulting "merozoites" spread the infection to other tissues in the host and undergo yet another division before undergoing sexual reproduction (Canning, 1964). Neogregarines are transmitted via contaminated food, or by cannibalism of infected hosts. Effects of neogregarine infection vary from a juvenile hormone effect on the host to host mortality. Some of the better known genera are Mattesia in beetles and moths, Ophryocystis in tenebrionid beetles, Farinocystis in Tribolium. Biological Control: It is generally agreed that eugregarines have little potential as biological control agents. They are typically commensuals and the lack of a merogony in the host is associated with lack of virulence and inability to efficiently produce sufficient quantities of infectious forms for use in biological control programs. However, some neogregarines, notably Mattesia sp. have been evaluated as biological control agents (Brooks, 1988). One species on dermestid pests of stored grain showed good potential in simulated warehouse conditions and another species may have some utility for control of sawtooth beetles (Lord, 2003 ). CoccidiaOf the Coccidia, 1% of species are restricted to insects, most are pathogens of vertebrates. They differ from the gregarines in that the mature gamonts are intracellular in host tissues. Like the gregarines, however, the life cycle is primarily haploid, with diploidy only occurring before meiosis. Typically, the coccidia enter host midgut cells, migrate to the hemocoel, and then to the fat body tissues. The host is killed primarily due to multiple replication cycles in fat body and, in a few species, other tissues. Of the more common genera, Adelina is the best known. Adelina species infect Coleoptera, Lepidoptera, Orthoptera, Embioptera, Diptera, Collembola, and other invertebrates. Biological Control: Because coccidia are typically found in animals other than insects (including vertebrates), no field testing has been conducted on the few species for which there appears to be some potential for controlling host populations (Brooks, 1988). Recent studies on one species infecting crickets have reported physiological effects on the host (Dolgikh, 1998), but no recent studies have been conducted to utilize species in this pathogen group as biological control agents. |
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