E. tenella

E. tenella develops in the caeca but can colonise the terminal ileum and the rectum in severe infections.

The parasitic cycle takes six to seven days, taking place deep in the epithelial cells of the caecal glands or crypts of Lieberkühn, explaining the severity of the tissue lesions and the high degree of pathogenicity of this species. This is reflected in considerable mortality due to haemorrhages which occur 5 to 6 days p.i. when the meronts II burst.

The sporozoites invade the epithelium of the tip of the villi and cross the epithelial cells. They cross the basal membrane and, taken over by macrophages, they advance into the lamina propria to the crypts of Lieberkühn. The sporozoites then penetrate the epithelial cells of these glands from the base and develop into first-generation meronts under the cell nucleus. The merozoites I, released into the lumen of the gland, infect neighbouring epithelial cells and develop rapidly into large (50 mm) second-generation meronts which emigrate into the submucousal layer. When the meronts II burst (5 days p.i.), they cause severe tissue lesions. Up to 3 days p.i., no lesion is observed, except for some rare petechiae. On days 5 and 6 p.i., the caeca are filled with partially coagulated blood. Observation of the contents of the caeca under the microscope reveals the presence of numerous meronts and merozoites II.

From day 6, oocysts appear in the caecal contents, which become inflexible. Caseous material appears and pseudomembranes are detached from the caecal mucous membrane. On day 7, a large reddish caseous plug is formed, shaped like a sausage containing blood and cell debris. On day 8 this plug obstructs the caecum completely. The patent period in which oocysts are excreted varies from 10 to 12 days.

E. tenella is the most pathogenic species for chickens. Thirty thousand oocysts can be enough to kill them. The haemorrhages due to the bursting of the meronts II 5 to 6 days p.i. are responsible for the death of fowl. This may occur before the symptoms such as loss of appetite or weight loss appear.

The signs observed during E. tenella infection are: bloody faeces, anaemia, loss of appetite and weight, and a high mortality rate. In view of its location in the caecum, E. tenella infection has little effect on nutrient absorption; however, it plays a very important role in the development of certain bacteria, including pathogenic bacteria such as salmonella.

Score 0: No macroscopic lesion. The wall is thin and presents characteristic longitudinal grooves and mucousal folds. The contents are homogeneous and creamy and do not contain any particles. The colour is chestnut.

Score 1: Some petechiae are scattered on the caecal wall. The caecal contents are normal. The petechiae are more visible on the serous membrane than on the mucousal side.

Score 2: The lesions are more numerous and there is blood in the caecal contents. The caecal wall is slightly thickened. Normal caecal contents are present.

Score 3: A large quantity of blood or caecal plugs is present. The caecal walls are greatly thickened. The faecal contents have practically disappeared.

Score 4: The caecal wall is very distended with blood or large caseous plugs. Faecal debris is no longer visible; it is enclosed in the caseous plugs. Score 4 is assigned to dead fowl.