Coccidia Life Cycle in Poultry

The life cycle of an Eimeria consists of two stages:

• an exogenous stage; sporogony in the external environment,
• an endogenous stage in the digestive tract of the chicken which consists of three essential phases: excystation or active departure of the sporozoites from the sporocysts, followed by several schizogonies or merogonies and gamogony.

1.  Exogenous stage

Infected birds excrete oocysts with their droppings in the external environment. The oocysts excreted in this way have to sporulate in order to become infectious.

Sporulation or sporogony is therefore an important stage in the parasitic cycle. It takes place outside the host in the external environment. About 48 hours at 25-28° C, or longer if the temperature is lower, are needed for the sporont inside the oocyst to transform itself into four sporocysts each containing two sporozoites (the infecting stages).

Litter offers conditions of temperature, moisture and oxygenation which permit this sporulation. If conditions are favourable, this sporulated oocyst may be resistant in the external environment for several months.

2 . Endogenous stage

2.1 Ingestion of sporulated oocysts and excystation

The chicken becomes infected by ingesting sporulated oocysts present in the environment: litter, feed and water contaminated by faeces of oocyst-excreting chickens. These oocysts are ground mechanically in the gizzard; their shells are destroyed, releasing the sporocysts. In the duodenum, the external wall of the sporocyst is dissolved under the action of host trypsin and bile salts and the sporozoites (infectious stages) actively come out of the sporocyst: this is called excystation.

2.2  Merogony or asexual multiplication

Depending on the species of parasite, the mobile sporozoites penetrate the intestinal or caecal epithelial cells. Cell invasion has not yet been completely elucidated, in particular the path adopted by the sporozoites of E. tenella in order to reach the caeca. Once the sporozoite has entered the cell, it becomes a trophozoite, the nucleus of which divides, resulting in the formation of schizonts or meronts. The meronts may contain several hundred merozoites. Once mature, the meronts cause the cells to burst and release the merozoites into the intestinal lumen. These merozoites in their turn penetrate neighbouring epithelial cells and repeat this process of asexual multiplication. Thus, depending on the species, two to four successive generations of merogonies may take place.

2.3 Gamogony or sexual reproduction

At the end of the asexual multiplications, the 2nd, 3rd or 4th generation merozoites invade the cells and develop into microgamonts (male gamonts) and macrogamonts (female gamonts) in accordance with an unknown determiner. The macrogamont effects its maturation without cell division and therefore engenders only one female gamete or macrogamete.

The microgamont produces a large number of mobile microgametes with two flagella which will fertilise the macrogametes. After fertilisation, the zygotes are surrounded by a thick wall (“wall-forming bodies”) and are transformed into oocysts. These oocysts are eliminated from the intestinal lumen and discharged with droppings into the external environment.

Thus, after ingestion of one oocyst, several days later thousands of oocysts are excreted into the environment. This period which has elapsed in the host between ingestion of an oocyst and excretion of the first oocysts in droppings is called the prepatent period. It is species-specific and varies from four to seven days according to the species of Eimeria in birds.