Coccidiosis in Chickens

Coccidia are obligatory parasitic protozoa (sporozoa) belonging to the phylum of the Apicomplexa characterised by the presence of an apical complex in the free stages of the cycle (sporozoites and merozoites) which invade the epithelial cells.

Eimeria have a direct life cycle (only one host), are very specific to hosts, to sites of development (intestines) and to cell types (epithelial cells of the intestinal villi or cells of the crypts).

Nine species of Eimeria have been described in chickens; seven are currently recognised as valid and colonise very clearly demarcated parts of the intestine.

In chickens, these locations and the lesions produced are so characteristic that this helps to diagnose the species present during a post-mortem examination.

• E. acervulina completes its entire cycle in the duodenum, but may spread to the ileum during severe infection.
• E. tenella is located almost exclusively in the caeca. It may extend to both sides of the junction of the caeca during severe infections.
• E. necatrix performs its schizogony in the jejunum-ileum and its gamogony in the caeca.
• E. praecox colonises the duodenum and the jejunum.
• E. mitis colonises mainly the ileum.
• E. maxima infects the middle intestine (jejunum and beginning of the ileum, on either side of Meckel's diverticulum) but most often ascends into the duodenum.
• E. brunetti descends the length of the intestine during infection and is located preferentially in the rectum and cloaca. When infections are severe, it may ascend to the distal part of the ileum and the proximal part of the caeca.

These seven species are specific to chickens and cannot infect other types of fowl or birds or mammals.

Although each species has specific development sites in the intestine, sometimes it is difficult to identify them during multiple infections.  

They are distinguished by:

- the morphology of their oocyst, the form of resistance and of dissemination of the parasite in the external environment;

- their intestinal location for endogenous development;

- their pathogenicity: characteristic intestinal lesions, type of diarrhoea (e.g., with or without blood).

Depending on the dose and the pathogenicity of the infecting species, coccidiosis causes clinical disease with considerable associated economic damage, i.e., mortality, reduced weight gain and increased feed conversion. Coccidiosis infections also enhance multiplication of concomitant infections with salmonellas (e.g., S. enteritidis, S. typhimurium) or Clostridium perfringens.