Life Cycle

The environmentally resistant oocyst contains 4 sporocysts with 2 infective sporozoites per sporocyst, and is transmitted fecal-orally. Sporozoites will then exit the sporocyst to infect a host intestinal epithelial cell and undergo asexual replication for a pre-determined number of cycles. Then sexual replication occurs to produce an uninfective oocyst (1).  Sporulation occurs outside of the host because only when the oocyst is in the environment at the right temperature, oxygen level and relative humidity does it become infective (2).  Sporulation for most Eimeria species takes about 48 hours to occur (1).  During the patent period, you will see maximum oocyst shed around 6 to 8 days post inoculation and this will taper out around 10 days post inoculation (3). 

The number of Eimeria parasites ingested will determine the amount of damage to the host (4).  Depending on the Eimeria species, "a few" oocysts would cause an unapparent infection, "thousands" could cause clinical signs and "tens of thousands" could result in severe coccidiosis and perhaps mortalities in birds that have never been exposed to the disease before (4).

smallerlifecycle

Figure 1. The life cycle of a typical Eimeria species. Sporozoites (A) excyst from sporocysts in the intestinal tract of poultry. Sporozoites infect a host intestinal epithelial cell and initiate asexual replication (B-D). Asexual replication forms motile stages that exit and infect neighbouring host intestinal epithelial cells for a pre-determined number of cycles (C-D).  After asexual replication, sexual replication occurs (E-G) where an unsporulated oocyst is formed (H) and is then shed in the feces (I). Sporulation occurs in the environment to produce a sporulated, infective oocyst (I-L) (1).

References

1. Barta, J.R. Coccidiosis. In: eLS. 2001.

2. Fayer, R. Epidemiology of protozoan infections: the coccidia. Veterinary Parasitology 6:75-103. 1980.

3. Kheysin, Y.M. Life Cycles of Coccidia of Domestic Animals. University Park Press, Baltimore, MD. 1972.

4. Price, K.R. Use of live vaccines for coccidiosis control in replacement layer pullets. Journal of Applied Poultry Research  21:679-692. 2012.