Coccidia Life Cycle

All Eimeria species are monoxenous. The animal become infected through the ingestion of sporulated oocysts with contaminated water or feed. The life cycle of coccidia has two phases

• exogenous phase
• endogenous phase

Exogenous phase

The oocysts are excreted in faeces by the infected animal and to sporulate they require optimum temperature, humidity and oxygen tension. There is a direct relationship between the ambient temperature and time taken to sporulate. Cattle Eimeria sp take 10-14 days for sporulation at 10°C, however their sporulation is completed in 5-9 days and 2-5 days at 20°C and 30°C, respectively. At ambient temperatures below 10°C the sporulation will be slow and at temperatures above 60°C oocysts may die.

The unsporulated oocysts are more prone to extreme changes in climatic factors than the sporulated ones (Horton-Smith and Long, 1954) Oocysts can withstand freezing at -5°C to -8°C for several months ( Wilson and Morley, 1933; Marquardt et al., 1960; Schneider et al., 1972). Oocysts can overwinter in Norway on pastures and be infective to grazing animals in the next grazing season (Helle, 1970).

The endogenous phase of the life cycle starts when animals have ingested the sporulated oocyst.

Endogenous phase

Once a suitable host has picked up the sporulated oocysts with contaminated feed or water, the oocysts excyst in the small intestine under the influence of two stimuli, CO2 and trypsin-bile salts, and release eight sporozoites. These sporozoites enter the appropriate host cells, round up, and  sporozoite at this stage is called trophozoite which undergo nuclear division or schizogony and schizonts are formed. This is first generation of schizont or first generation of schizogony.

When the schizonts mature, the first generation of merozoites is released and they enter other appropriate host cells and continue the cycle of asexual development. In the new host cell, the merozoites round up to become trophozoites and undergo nuclear fission or asexual division as before, developing into second-generation schizonts and in turn into second-generation merozoites.

First-generation schizonts of Eimeria bovis are large and are found in endothelial cells of central lacteals in the small intestine, whereas large first-generation schizonts of Eimeria zuernii are in the connective tissue cells of lamina propria near the muscularis (Ernst and Benz, 1986).

In Eimeria alabamensis the sporozoite penetrates the nucleus of the intestinal epithelial cells.

Second-generation merozoites may further develop into subsequent generations of merozoites. This asexual reproduction does not continue indefinitely. Second-generation schizonts of Eimeria bovis and Eimeria zuernii  develop in epithelial cells of the caecum and upper colon. 

The number of merozoite generations (asexual multiplication generations) varies between two and five from species to species. After the fixed number of schizogony (merozoite generations), last-generation merozoites start the sexual phase (gametogony).

The last-generation merozoites change into trophozoites which, instead of repeating schizogony, differentiate into macrogametocytes (macrogamont) and microgametocytes (microgamont). These grow to full size and each macrogametocyte in turn gives rise to one macrogamete. Each microgametocyte gives rise to a large number of biflagellate microgametes.

The macrogamont and microgamonts of Eimeria bovis and Eimeria zuernii are also found in the epithelial cells of the caecum and upper colon. The young microgametes contain small granules in the vicinity of the nucleus, which later enlarge and scatter over the cytoplasm. The large granules are found on the periphery of the cell and are called “wall forming granules”, which form the wall of the oocyst after the fertilization of the macrogamete. Fertilization of the macrogamete by the microgamete results in formation of a zygote. The zygote lays a wall around itself to form an oocyst. The oocyst breaks out of host cell into the intestinal lumen and is excreted with faeces.

The prepatent period (the time required from day of infection to the start of oocyst excretion) is 17-22 days for Eimeria bovis, while for Eimeria zuernii it is 16-19 days. The prepatent period for Eimeria alabamensis is 6- 8 days.

The number of oocysts produced depends on various factors such as dose of infection, immunity and number of merozoites ultimately produced.