Chicken Eimeria Pathogenicity

The presence of coccidia does not necessarily signify coccidiosis. The disease is more complex than a simple association of the coccidia with its host. Pathogenicity depends on the species of parasite and on the quantity of oocysts ingested, the sensitivity of the host, and the environmental situation.

The different phases of parasitic development are accompanied by the rupture of the intestinal cells and are not without effects on the physiology of the host.

Pathogenicity varies in accordance with the species of Eimeria in chickens. It depends on the site of development, on the size of the different parasitic stages and on their location (intestinal villi or more deeply in the crypts or cells of the lamina propria).

Generally, the small stages confined to the epithelial cells on the surface of the villi above the nuclei of the host cells (E. acervulina, schizonts of E. maxima) cause little damage since the cells at the peak of the villi are ageing (close to natural dispatch).

On the other hand, the meronts of E. tenella, E. necatrix and E. brunetti develop more deeply in the lamina propria (deep in the intestinal wall). They become enormous and cause the rupture of blood vessels, resulting in bleeding into the intestinal lumen and death of the chickens with severe infections. These three species are highly pathogenic.

The gamonts of E. maxima, which are very large in hypertrophied cells, often take a sub-epithelial position and are responsible for malabsorption of nutrients.

E. acervulina and E. mitis are somewhat less pathogenic but often cause subclinical coccidiosis accompanied by less nutrient absorption, while E. praecox does not cause lesions.

The pathogenic effect and the number of oocysts excreted increase in accordance with the dose of oocysts ingested.

A dose of 5 x 106 oocysts of E. acervulina invariably produces intestinal lesions accompanied by anorexia and weight loss; however, even during very severe infections, deaths are rarely observed. On the other hand, a dose of 5 x 104 oocysts of E. tenella or E. necatrix is sufficient to kill most susceptible chickens.

Under natural conditions, chickens are infected by several species, and there may be competition between species colonising the same intestinal site.