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- A -

Alae: Flattened, wing-like expansions of the cuticle of nematodes (cervical, caudal or lateral).

Apical complex: A complex set of organelles found at the anterior end of the protozoans of the Phylum Apicomplexa.  The apical complex has a role in the penetration of host cells.

Arrested development: See hypobiosis.

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- B -

Bradyzoite:  A stage in the life cycle of protozoa of the family Sarcocystidae (tissue-cyst-forming coccidia).  In particular this is the term used to describe the merozoite which forms within the tissue cyst in the intermediate host (and rarely within the definitive host).  This infectious stage rarely infects new cells within the intermediate host, rather it is the infectious stage for the definitive host.

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Cercaria: In the trematode life cycle, the cercaria is a larval trematode (which may or may not have a tail, depending on the species).  The cercaria develops from the germinal cells of the sporocyst or  redia and emerges from the snail (intermediate host) to later become (in most species) the metacercaria.

Coenurus:  Larva of Taenia multiceps which is a fluid filled cyst containing many invaginated scolecies.

Control:  Reduction of disease incidence, prevalence, morbidity or mortality to a locally acceptable level as a result of deliberate efforts. Continued intervention measures are required to maintain the reduction.

Cysticercoid:  A single, evaginated scolex that is embedded in a small solid cyst that are typically found in small intermediate hosts such as arthropods (Cyclophyllidian tapeworms).

Cysticercus:  Larval stage in the cestode life cycle which is a fluid filled cyst containing an attached single invaginated scolex typically found in mammal intermediate hosts (cyclophyllidian tapeworms)

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Definitive Host: Found in all parasitic life cycles. The definitive host is the host in which the sexual stages of the parasite ("adults") mature.  If there are no sexual stages in the life cycle, then the definitive host is the host in the life cycle considered most important to humans.

Direct Parasite Life Cycle: A life cycle of a parasite in which the pre-parasitic larvae develop in the environment  as free-living stages in which the eggs will EITHER hatch and the pre-parasitic larvae are entirely free-living (e.g. Ostertagia ostertgia) and the infective stage is usually an L3 and infection occurs via ingestion of infective stage or skin penetration OR the eggs do NOT hatch and pre-parasitic larvae develop inside their eggs (e.g. Ascaris suum)  and infective stage is usually the egg containing the infective larva and infection of the definitive host occurs via ingestion of the infective stage.

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Embryonated egg:  (Also called a "Larvated egg"):  A nematode egg with a developed larva inside it.  Most nematode eggs leave the host in the morula stage and develop in the environment to the embryonated stage (the stage just before hatching).  A few nematode eggs (ex. Strongyloides spp., Dictyocaulus arnfieldi) are embryonated at the time they leave the host.

Ensheathed:  A larval nematode that is within the retained cuticle of the previous stage.

Eradication: Permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts. Intervention measures are no longer needed.

Exsheathment: The final step in molting of the infective stage when the old cuticle (sheath) is lost and the new nematode stage emerges inside the definitive host.  It is a requirement for the establishment for infection in the host.

Extinction: The specific organism no longer exists in nature or the laboratory.

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- F -

Feces: Waste matter eliminated from the bowels; excrement. (The word "feces" is plural but can be used with either a singular or plural verb.)

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Gametocyte: The stage in the life cycle of some protozoa (Apicomplexa) which is destined to become a gamete (macro- or micro-).

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- H -

Helminths: A general term referring to the nematodes (roundworms), trematodes (flat worms, flukes), cestodes (tapeworms) and acanthocephalans (thorny-headed worms).

Hydatid Cyst:  Larva of Echinococcus spp. which is a large fluid filled cyst containing many invaginated scolecies and daughter cysts that also contain many invaginated scolecies.  There are two types of hydatid cysts:  E. granulosus has uniloculular (one-chambered) cyst and E. multilocularis has a multi-locular (many chambered) cyst.

Hypobiosis: In parasitology: A temporary halt in nematode development within the host at an early stage in the parasitic phase of the life cycle in nematodes with direct life cycles only.  Nematodes arrest as immature forms in the definitive hosts at a time when conditions in the external environment pose a hazard to survival of free living pre-parasitic stages (usually in winter or dry seasons).  Once conditions are more favorable, the arrested development resume their development to adults the life cycle continues.

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Intermediate host: A stage in the life cycle that is essential to parasite development (with some exceptions), and the mode of transmission of the parasite to the definitive host is usually by predation. Intermediate hosts are not required for all parasite life cycles.

Indirect Life Cycle: A life cycle of a parasite in which the pre-parasitic larvae develop inside an appropriate intermediate host.

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- L -

Life cycle: The stages an organism goes through over time containing a possible pre-parasitic and a parasitic phase.  Usually represented as a circle beginning with the formation of a individual and ending with the production of the next generation.  In parasitology the life cycle of the parasite gives the investigator predictive value for understanding pathogenesis and clinical signs, and for understanding the epidemiology and  control of the parasite.  Life cycles can either be direct or indirect.

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- M -

Meront: See schizont.

Merozoite: The invasive stage that is the end result of schizogony.

Metacercaria: Stage of trematode life cycle that the cercaria after invading the second intermediate host or attaching itself  to vegetation, develop into.  When ingested by the definitive host, the metacercaria will develop to the adult stage.

Metacestode:  Mature tapeworm larvae.

Miracidium: The pyriform, ciliated larva of a trematode that developed in and hatched from the egg.  The miracidium  will penetrate the snail and undergo development to the next stage in the lifecycle.

Myiasis:  Infestation of organs and tissues of vertebrates by larval dipterans (true flies).

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- N -

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- O -

Occult infection: An infection with adult worms but no corresponding diagnostic stage.   For example in infections with Dirofilaria immitis (heartworm) some hosts may not have circulating microfilaria (this may be a result of immune-mediated clearance of the microfilaria, a single-sex infection, chemosterilization of the adult worms by drug treatment, or senescent adults).  This would be known as an "occult heartworm infection".

Oncosphere:  In a cestode life cycle, the oncosphere is the hexacanth (6-hooked) embryo found in the egg.

Oviparous:  Laying eggs in which the embryos have developed little or not at all.  Seen, for example, in the nematodes Toxocara canis and Ancylostoma caninum.

Ovoviviparous:  Condition in which the organism develops in an egg and hatches within the adult female.  Thus a live motile organism emerges from the adult female.  Seen, for example, in the nematode Dirofilaria immitis and in some sharks and snakes.

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- P -

Paedogenesis: A process in the molluscan intermediate host which  involves the production of new individuals by a single larval form in which one trematode egg may eventually develop into hundreds of adults.

Parasite: Organisms that live temporarily or permanently on or within  other living organisms (plant or animal hosts) for the purpose of obtaining food.

Parasitic Phase of Life Cycle: The development and maturation to adult males and females occurring in the definitive host of a parasite life cycle.

Parasitism: A two species association in which one species, the parasite, lives on or in a second species, the host, for a significant period of its life and obtains nourishment from it.  The parasite may or may not cause disease in the host.

Parasitology: The study of host-parasite relationships.  Traditionally this area of study has focused on parasites belonging to the protozoa, helminths and arthropods.

Paratenic host (transport host): A type of intermediate host in which immature helminths may survive for indefinite periods but do not undergo development. Further parasitic development depends upon infection of the definitive host, which is usually by predation on the paratenic host. Completion of a life cycle may happen with or without a paratenic host, but the presence of a paratenic host may serve to make infection of the definitive host by the parasite more efficient.

Parthenogenesis: A form of reproduction in which an unfertilized egg develops in to a new individual.

Periparturient Rise in Fecal Egg Counts (PPR): An increase in the number of parasite eggs in the feces of animals around parturition. This can be pronounced in ewes, sows, and goats.

Phagolysosome: A phagosome that has fused with a lysosome within the phagocyte.

Phagosome: A vacuole within a cell that was formed during phagocytosis.

Pre-parasitic Phase of Life Cycle: The development to the infective stage occurring outside the definitive host in the environment or in a second (intermediate) host of the parasite life cycle.

Predilection site:  The site within a host where the parasite is normally found.

Prepatent Period (PPP): The period of time from infection until mature adult parasites are producing eggs or larvae.

Procercoid: The larvae of Pseudophyllidian tapeworms in which after a crustacean ingests a ciliated coracidium, the procercoid develops in the body cavity of the crustacean.

Pleurocercoid: Second larval type in Pseudophyllidian tapeworms in which after the crustacean is eaten by freshwater fish and the procercoid is liberated, the pleurocercoid is develops in the muscles of the new host and possesses the characteristic scolex and is the only stage which is infective to final host.

Procercoid: The larvae of Pseudophyllidian tapeworms in which after a crustacean ingests a ciliated coracidium, the procercoid develops in the body cavity of the crustacean.

Proglottid: One segment of the body (strobila) of a tapeworm.  Proglottids may be immature, mature (male and/or      female sex organs present) or gravid (full of eggs).

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- R -

Redia: In the trematode life cycle, the redia is the larval form (possessing an oral sucker) that develops from the sporocyst and the redia will give rise to the cercariae in the snail (intermediate host).

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- S -

Schizont (meront): An asexual reproducing stage of some protozoa. It reproduces by schizogony.

Schizogony: In schizogony the nucleus of the original invading protozoan undergoes repeated divisions to form many nuclei within the cell.  Then each nucleus becomes surrounded by cytoplasm and a cell membrane forms around each new organism.  These new stages within the schizont are known as merozoites.

Scolex:  The anterior end ("head") of the adult cestode, it is the hold-fast organ that anchors the worm in the GI tract.

Somatic Migration: Migration in the host by a parasite through the lungs into the systemic circulation distributed throughout the body and encyst in the tissues.

Spicules: Chitinous structures found in male nematodes, usually paired.  They are inserted in the female genital opening during copulation.

Sporocyst: In the trematode life cycle, the miracidium will invade a snail,  lose the cilia and develop into an elongated sac, called the sporocyst, from which the  redia  will develop (in some species the sporocyst gives rise to a daughter sporocyst).
Sporozoite: The initial invasive stage of most of the Apicompexan protozoa.

Strobila:  The string of proglottides which make up the "body" of  a tapeworm.

Strobilocercus:  A type of larva of cyclophyllidian tapeworms (i.e. the larva of Taenia taeniaeformis).

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- T -

Tachyzoite: Extra-intestinal stage of Toxoplasma that are found in in vacuoles muscle, liver, brain, and lung found in intermediate and definitive hosts.  Sporulated oocysts are ingested and liberated sporozoites rapidly penetrate the intestinal wall and through the blood.  It is the invasive and proliferative stage of Toxoplasma.

Tetrathyridium:  Worm like larva with an invaginated scolex found only in Mesocestoididae.

Tracheal Migration: Migration in the host by a parasite into alveoli up the respiratory tree and then swallowed and then mature in the intestine.

Transport host: See paratenic host.

Trophozoite: The stage of the protozoa in the host which feeds and grows until division commences.

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- U -

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Vector: An intermediate host in a parasitic life cycle, that may or may not  (paratenic host) be essential for the development of the parasite.  A vector will seek out the definitive host in the life cycle to prey on it, whereas other  intermediate hosts will not.

Visceral Larva Migrans: Nematode larvae migration in hosts that are suitable for long survival but are unsuitable for development to the adult stage.  The larva worms wander for a time in the hosts tissues.

Viviparous:  Condition in which the embryo develops within the female and is nourished by the mother (no egg shell exists at any time).  For example:  mammals.

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- Y -

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