The Disease

Coccidiosis is most common in young lambs aged 2 - 8 weeks. It is becoming increasingly important in sheep production, particularly in intensive production systems (Barragry, 1992).

Coccidiosis is caused by single celled parasites (protozoa) called Eimeria, which undergo a simple life cycle in the gut. Oocysts, containing 8 Eimeria parasites, are shed in the faeces of infected animals. They can survive on the ground for up to a year. If a lamb swallows some oocysts, they break open in the gut and release the 8 parasites, which invade the gut wall. Each parasite grows and multiplies by repeated asexual division to produce a hundred or more daughter parasites. Eventually, the daughter cells break out and invade fresh areas of the gut wall and repeat this process. Within 10 - 14 days the parasites will have multiplied by up to a millionfold. By this stage of infection, parts of the gut wall are packed with parasites which develop into male and female sex cells. The female sex cells are fertilised and secrete an oocyst wall around them, then drop off the gut wall to be excreted in the faeces and complete the cycle (Blewitt and Angus, 1991).

The two pathogenic species in sheep, E. crandalis and E. ovinoidalis, are found in the ileum, and infection with E. ovinoidalis also occurs in the caecum and colon. Both impair the absorptive capacity of these parts of the gut, thereby provoking scour. E. ovinoidalis also damages the gut's capacity for regeneration, thus precipitating a more severe and prolonged disease.

Virtually all sheep flocks are infected with coccidia, but only some lambs develop clinical disease (Catchpole et al., 1975). Clinical coccidiosis occurs when damage to the gut is sufficiently severe to cause dysfunction. This normally occurs at the beginning of the parasite's sexual multiplication stage, when parasite numbers reach their peak. The lambs may suffer from dullness, inappetance, diarrhoea, dehydration, weight loss, tenesmus, rectal prolapse and anaemia (Barragry, 1992).

The source of the initial outbreak of clinical coccidiosis is likely to be either residual contamination in the environment or low levels of oocyst shedding by the ewes (Barragry, 1992). Since almost all lambs become infected with coccidia, it is not clear why some lambs become clinically ill while others do not. The level of the environmental infection, particularly with pathogenic species, is probably the most important factor, but other factors, such as nutritional and climatic stress, may be involved. It is difficult to predict the occurrence of clinical coccidiosis, as some flocks are troubled by it every year while others, with similar management systems, are not (Blewitt and Angus, 1991).