Life cycle of coccidia 

Coccidia are strongly host specific (can infect only one species of host or additional intermediate hosts). So if Your chicken get coccidia - Your health will not be disturbed !
BUT - there is always a 'BUT' - Veterinary medicine uses the word 'coccidia' as a pool of very different species - as they treat them mostly with the same medicine (antibiotics-coccidiostatics). So -sometimes- your veterinarian tells you the diagnose 'coccidia' he/she is not able to tell you the species he/she found i.e. in your puppies guts. Try to find out more about coccidia!

The infection of animals by coccidia is the result of an ingestion of water or food contaminated by the infectious stage - the sporulated oocysts . Most of the time it is a matter of hygiene. The oocysts wall is mechanically (or/and enzymatically) broken in the stomach/gut.

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Sporocysts (A) are set free and are exposed to enzymes (trypsin and bile). As the result sporozoites (the invaders) are released (B). They are characterized by typical organells (see sporozoite). Sporozoites move actively and enter epithelial cells for their further development (1).

What about Invasion!

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Once intracellular (in its parasitophorous vacuole), the sporozoite rounds up and develops to a so called schizont of the first generation (schizont syn. meront) (2)(3). Merozoite formation takes place within the schizonts. Depending on the species hundreds or thousands merozoites are formed.

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By destroying the host cell the released merozoites can invade new epithelial cells (4) and develop to schizonts II (second generation) (5). Merozoites of generations differ in size and quantity.

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Released merozoites II may give rise to an additional third generation of schizonts (6)(some Eimeria species even developed a fourth merogony step) or switch to sexual developmental stages (s.c. gamonts). We still do not know the trigger for 'male' or 'female' development. Even if you infect a chicken with a single sporozoite - the so called female macrogamet and male microgamets are formed. By the way all sporozoites and merozoites are 'halpoid' - like sperms.

(7 - male called microgamont)

(8 - female called macrogamont).
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The process of fertilization (9+10 - microgametes enter actively macrogametes) leads to the formation of an intrazellular zygote (with highly impermeable oocyst wall). The zygote becomes a young oocyst thereby destroying the host cell (11). Oocysts (unsporulated type see 11) are discharged in the faeces.

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Sporulation (12) takes place in warm and wet soil (litter). Probes derived directly from faeces of infected animals will mostly contain unsporulated oocysts. Probes from the litter will show sporulated oocysts with 4 sporocysts containing 2 sporozoites each (Eimeria type).

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