Phyulm : Apicomplexa

Class: Sporozoa

Sub-class: Coccidia

Order: Eucoccidia

Sub-order: Eimeriina

Family : Eimerridae

General features of Eimerridae

• These organisms are commonly called coccidia and typical intercellular parasites of epithelial cells of intestine of vertebrates.
• All forms have single host in which they undergo asexual and sexual cycles of reproduction.
• Gametes formed through fusion of microgamete with macrogamete develops independently inside the epithelial cells of intestine of host and leaves host after developing protective cystic wall in form of oocyst.
• Sporocyst number varies according to species, in which there is one or more sporozoites.
• This family contains 16 genera and some 1340 species of which Eimeria and Isospora are most important causing infection often called as coccidiosis.
• Lifecycle is usually homoxenous.

Generic identification of coccidian parasite

Note: Scabrous: Rough and covered with scabs

Morphology of Eimerridae

• Oocysts are usually oval, egg-shaped, ellipsoidal, sub-spherical or spherical.
• Cyst wall is generally bi-layered, clear and transparent, outer walls may be light colored and smooth scabrous or spiny. Inner wall is usually thinner.
• There may be small thinning at the narrower pole called micropyle. It may be covered by cap-like structure called micropylar cap or polar cap.
• Inside the cyst, cytoplasm of uninucleate zygote is usually coarsely granular which usually fills the entire space in the beginning and later it contracts into ball (Sporont).
• Dark granules may be seen near micropyle called polar granules.
• Sporogony starts with sporulation of oocyst which begins with devision of sporont either in two or four small sporocyst. In some species, a part of protoplasm, left after spore formation, called oocytic residuum. Stieda body is present at the more pointed end of each sporocyst. Each sporocyst then divides to form two or four banana shaped elongate bodies called sporozoites.
• Left out protoplasmic granular mass is called sporocystic residuum.
• Sporozoites are usually sausage or comma shaped or banana shaped with central nucleus.
• Merozoites are usually banana-shaped with centrally placed round nucleus.

Fig: Typical sporulated oocyst of Eimeria

Transmission

• Transmission usually occurs through ingestion of contaminated food.

Lifecycle

• Generally, life-cycle is divided into three phases.

1. Infection schizogony
2. Gametogony and oocyst development

• Sporulation
• Host gets infection when they ingest sporulated oocyst through feed or water.
• After ingestion, oocyst reaches to intestine of host cells (epithelial cells, crypts, villi) parasite may also reach endothelial cell
• Sporozoites are released from sporocyst through action of bile or trypsin.
• Sporozoites then invades epithelial cells of intestine, rounds up and become trophozoite.
• Within few days, nucleus of trophozoites divides through repeated binary fission, resulting in production of elongate uniform bodies, called merozoites.
• These merozoites then invade new host cells and starts cycle again. Generation of these schizonts varies in different species. Number and size of merozoites also varies in different species. Each mature schizont is enclosed in thin distinct wall containing 8 or 16 to hundreds or thousands of merozoites with centrally placed nucleus.
• After subsequent number of generations, merozoites released then enter new cells of host and undergo gametogony. Most of merozoites turn into female gametocyte called macrogametocytes (macrogamonts) and few merozoites become male gametocyte called microgametocyte or microgamonts.
• Microgamonts nucleus undergoes multiple division with production of large number of comma shaped microgametes. Macrogamonts also matures to microgametes.
• Microgamete then fertilize macrogametes resulting zygote formation. Zygote develops to oocyst with formation of oocyst wall. Oocyst breakout host cells, enter intestinal lumen or pass out in feces in unsporulated form.

Sporulation

• Sporulation of oocyst takes place outside host cell.
• Unsporulated oocyst contains nucleated mass of protoplasm enclosed by resistant wall.
• Under suitable conditions, (oxygenation, high humidity and optimal temperature of around 27⁰C), nucleus divide twice and protoplasmic mass forms four conical bodies radiating from central mass.
• Each of these nucleated cones becomes rounded to form sporoblast. In some species, remaining protoplasm forms oocyst residual body.
• Each sporoblast secretes wall of refractile material and becomes sporocyst. Protoplasm within oocyst divides into banana shaped sporozoites.
• Oocyst now consist outer wall enclosing sporocyst, each containing sporozoites. This is known as sporulated oocyst and is infective stage.
• This whole sporulation process takes 2-4 days under optimal condition.
• Protoplasm remaining within sporocyst, forms sporocyst residual body and this gives sporocyst knob-like structure at one end, which is known as steida body.

Prepatent period

• Time between ingestion of first infective oocyst to the time when first oocysts appear in the faeces of host.
• Varies from few days to few weeks in different species.

Patent period

• Time from the first discharge of oocyst to time of last discharge of oocyst in faeces of host after single infection.

Genus Eimeria

• Oocyst contain four sporocyst. Each sporocyst contain four sporozoites.
• Identification is generally based on oocyst size, morphology and knowledge of host animal.

Coccidiosis in cattle

Species of Eimeria occurring in cattle are:

• bovis
• zuernii
• alabamensis
• auburnensis
• brasiliensis
• bulidnonensis
• canadensis
• cylindrica
• ellipsoidals
• pellito
• subspherical
• wyomingensis

Most commonly occurring and pathogenic species in cattle are E. zuernii and E. bovis.