Bovine coccidiosis may be on the increase in the United Kingdom. The disease almost invariably affects cattle less than one year old, and often follows turnout. However,older cattle may develop the disease. The condition has been reported by some organic farms to be a problem.
Coccidiosis is caused by single celled parasites (protozoa) called Eimeria, which undergo a simple life cycle in the gut.
Eimeria are species specific and 13 species have been isolated from cattle.
Of these 13 species, Eimeria zuernii, E. bovis and recently E. alabamensis are the most common and most pathogenic. Eimeria zuernii and E. bovis are classically associated with disease in housed calves, whereas, E. alabamensis has been reported in both housed and more especially grazed cattle
Oocysts, containing eight Eimeria parasites, are shed in the faeces of infected animals.
They can survive on the ground for up to a year.
If a calf swallows some oocysts, they break open in the gut and release the eight 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, 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, completing the cycle.
The life cycle from ingestion to patency takes 15-17 days for E. zuernii and 15-20 days for E. bovis. Oocyst production during infection with a single species lasts for 5-12 days, but may be prolonged in multiple species infections.
The presence of large enough accumulations of oocysts to cause disease results from farming practices that allow groups of cattle, especially if they have been stressed by transport and by mixing with others, to ingest food or water contaminated with faeces. Infections can occur both indoors, on damp and faeces-contaminated bedding, or outdoors, around drinking and feeding troughs.
The disease is manifested by enteritis, diarrhoea and, in severe cases, dysentery.
The diarrhoea or dysentery is sometimes accompanied by severe straining that may lead to rectal prolapse. The larger the infective dose, the more severe the clinical signs).
Light infections are self-limiting, but severe infections can be fatal if untreated. Diagnosis is made on the basis of clinical signs plus the finding of large numbers (>2,000) oocysts per gram of faeces
However, oocysts counts can sometimes be unreliable as healthy animals can pass more than 10 million oocysts per gram of faeces, animals can die of coccidosis before any oocysts are shed and oocyst output may be transient - an animal that is dying of coccidiosis may show very few oocysts .
The oocysts are very resistant to external conditions and will survive for up to two years in suitable environments. They can resist moderate frosts, but are susceptible to drought, high temperatures and the chemical action of ammonia.
