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Introduction Coccidiosis in cattle results from infection with one or more species of protozoan (single-celled) parasite of the genus Eimeria . These parasites affect the small and/or large intestine where they damage the intestinal wall. If numbers of parasites are high, this damage results in changes that lead to diarrhoea (scours), dysentery (bloody diarrhoea) and tenesmus (straining). Coccidia spp. are widespread in cattle farming, though they cause clinical disease mainly in calves from 3 weeks to 9 months of age. The parasites Coccidia are single-celled parasites that can infect a wide range of animals. These organisms are host specific i.e. coccidia from one animal cannot affect another species. This applies even to species as closely related as sheep and goats. Therefore birds do not transmit coccidiosis to cattle. There are over a dozen Eimeria species that can infect cattle, but only three species are considered to be pathogenic: Eimeria bovis, E. zuernii and E. alabamensis. The last species, E. alabamensis, has been shown in European studies to be an important pathogen of grazing calves, particularly within 10 days of turnout, but its status in Ireland is unknown at present. Life cycle Coccidial oocysts (similar to eggs) are excreted in the dung where, under suitable conditions of temperature, moisture and oxygen), they develop into sporulated oocysts, which are then capable of infecting cattle. Once ingested by a susceptible calf, the oocyst releases the sporozoites, which penetrate the cells lining the small or large intestine. Development continues in the gut wall through various stages until the coccidia mature and produce oocysts. A single ingested oocyst develops into thousands of new parasites, each of which destroys the epithelial cell in which it is growing, resulting in very rapid and severe damage to the gut. The pre-patent period (time between infection of the animal and the first appearance of oocysts in faeces) is typically about 21 days for E. bovis and E. zuernii , whilst it is shorter for E. alabamensis (8-12 days).
Figure 1 . Coccidia lifecycle.
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