Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Protozoa
Apicomplexa
Class Coccidiomorpha - Order Coccidia

Coccidia are intracellular parasites inhabiting the epithelial and other Cells of the intestine, Liver, Kidneys, and certain other Organs in both vertebrate and invertebrate animals. Like all Sporozoans, they are characterized by a strict alternation of sexual and asexual reproduction. Asexual reproduction occurs through multiple fission, or schizogony. The majority of coccidia have a single host, with sporogony occurring entirely (or at least partially) in the external environment.

Let us examine The life cycle of a monoxenous (single-host) cocidian belonging to the genus Eimeria, which parasitizes various vertebrate hosts, including rabbits (Figs. 6, 7), cattle, and poultry.

The MAIN STAGES OF the Life Cycle of Eimeria, which parasitizes the epithelial cells of the rabbit's Small Intestine, are shown in Figures 6 and 7. Infection occurs when oocysts containing sporozoites enter the host's intestine. After the membranes are digested, the sporozoites emerge and invade the intestinal epithelial cells. They begin to grow and multiply via schizogony, forming successively an immature, a growing, and finally a mature schizont.

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Fig. 6 Cross-section of a rabbit intestinal villus showing various Developmental Stages of Eimeria magna:

1 — epithelial cells; 2 — macrogametes; 3 — microgametocyte with microgametes;

4 — early-stage schizont; 5 — schizont breaking down into merozoites; 6 — late-stage schizont with numerous nuclei.

Fig. 7

Developmental cycle of the cocidian (Eimeria magna) parasitizing rabbits:

A, B, C — 1st, 2nd, and 3rd generations of schizogony; D — gametogony; E — sporogony. 1 — sporozoites; 2 — young schizont; 3 — growing multinucleate schizont; 4 — schizont disintegrating into merozoites; 5 — merozoites; 6 — development of macrogamete; 6a — development of microgametes; 7 — microgametes; 8 — oocyst; 9 — oocyst initiating sporogony; 10 — sporocyst with four sporoblasts and residual body; 11 — development of sporoblasts; 12 — mature oocysts containing four spores, each with 2 sporozoites.

Inside the mature schizont, multiple fission produces merozoites—small, mononuclear cells arranged relative to one another like the segments of a mandarin orange.

Merozoites emerge from the schizont into the lumen of the intestine or other organ, invade neighboring cells, and transform back into schizonts, undergoing further schizogony. This process repeats several times, leading to a exponential increase in the parasite population. However, the number of schizont generations is limited, not exceeding 4–5 in species of the genus Eimeria. After several asexual generations, the sexual phase begins. Merozoites that invade host cells give rise to gamonts—the stages that develop into female Gametes (macrogametes, laden with reserved nutrients) and male gametes (spermatozoa, formed following repeated nuclear division of the microgametocyte).

The spermatozoa are equipped with two flagella and actively penetrate the macrogametes. Fertilization yields a zygote, which secretes a robust two-layered wall and transforms into an oocyst. At this stage, the oocysts are typically excreted in the feces. Their further development (sporogony) takes place outside the host's body in the external environment.

Under favorable conditions (moist soil, temperatures of 20–25°C, and adequate oxygen supply), The Nucleus within the oocyst divides. Cytoplasm aggregates around the nuclei, forming four sporoblasts. These secrete their own walls and develop into spores, or sporocysts (four spores in Eimeria). Following further division, each Spore Forms 2 sporozoites.

Upon reaching this stage, the oocyst becomes infective. If ingested by a host, the sporozoites emerge from the spores and initiate a new developmental cycle.

Certain coccidia are causative agents of severe diseases in domestic animals, collectively known as coccidiosis.

Coccidia of the genus Eimeria are of particular importance: E. magna, E. intestinalis, and several other species cause rabbit coccidiosis and frequently trigger mass mortality in livestock. Coccidiosis in chicks (E. tenella) and cattle (E. zurnii/sinithi, etc.) is also extremely dangerous.

Control measures against coccidiosis are primarily preventive, aimed at preventing the ingestion of sporocysts contaminated in feed and Water. Additionally, A number of specific therapeutic drugs have been developed to combat coccidiosis.

QUESTIONS FOR SELF-check and review:

1. What structural and life-cycle modifications have evolved in sporozoans in connection with their parasitic mode of life?

2. How does the life cycle of the malaria parasite (Plasmodium) proceed?

3. What makes this disease hazardous to humans?

4. What are the key Features of the cocidian life cycle?