The main concern from coccidiosis is that disease is not related to a single Eimeria spp. Infections result from a mixture of Eimeria species that invade different parts of the intestine, as shown in Figure 1. The intracellular parasite invades and destroys the epithelial cells of the host, causing severe damage to the intestinal wall. Nine different Eimeria species are known in poultry, only five to seven of which are associated with diseases in commercial flocks. Whereas an E. praecox infection is generally considered to produce little pathology, an infection with E. acervulina and E. mitis can result in mild enteritis followed by fluid loss and the malabsorption of nutrients.
In more severe cases, inflammation of the intestinal wall with local bleeding (haemorrhages) and sloughing of epithelia (E. brunetti, E. maxima) or complete villar destruction resulting in extensive haemorrhage and death (E. necatrix, E. tenella) is seen in infected chickens.
Most of the highly pathogenic species invade the lower parts of the digestive tract. The life cycle of Eimeria is relatively short, from four to six days, and consists of two developmental stages; exogenous (in faecal matter) and endogenous (in host digestive tract). The exogenous stage starts after the release of unsporulated (non-infective) oocysts in the faeces. Sporulation of the oocyst occurs in the faeces and is encouraged by the right temperature, humidity and access to oxygen in the litter.
The endogenous stage starts after ingestion of the infective oocysts by the chicken. (Step 3, figure 2) In the micro-environment of the gizzard, sporozoites are released from the oocyst. Further down the digestive tract, the sporozoites invade and destroy epithelial cells and start the highly efficient reproduction cycle. This involves several rounds of asexual reproduction, followed by sexual differentiation, fertilisation and the shedding of unsporulated oocysts.