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Coccidiosis is a parasitic disease caused by the development and multiplication of coccidian (Eimeria spp.) in the intestinal tract. Coccidia are ubiquitous; they exist wherever chickens are reared and their survival is assured by a highly resistant form of transmission - the oocyst - which may survive for several months in the environment. Despite hygiene measures, drug prophylaxis and vaccination, coccidian remain a major problem in poultry rearing. It is estimated that the global cost of prevention of coccidiosis in chicken is $200 to $300 million per year. Seven species of Eimeria are currently recognised in chickens: 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 beyond the junction of the caeca during severe infections E. necatrix performs its schizogony in the jejunum-ileum and its gamogony in the caeca. E. paraecox 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 the 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 Eimeria species are specific for chickens and cannot infect other types of fowl, birds or mammals.