Coccidiosis (sometimes called black scours) is caused by single-celled protozoan parasites of the genus Eimeria. The most important coccidians of cattle from a clinical perspective are E. zuernii and E. bovis.
Typically, young susceptible stock, 3 weeks to 6 months of age and congregated under moist, unhygienic conditions are particularly affected. Parasites are usually acquired in the first few months of life mostly from their herd mates as most young animals carry small numbers of coccidia but remain clinically normal.
Figure 1. The faeces of a cow with black scours may contain spherical eggs or oocysts of the coccidia parasite, Eimeria zuernii. Image courtesy of the National Centre for Veterinary Parasitology, ncvetp.org
The disease coccidiosis results from either overwhelming infection or the interaction of moderate levels of infection and stress such as weaning, poor nutrition, close confinement, transportation or the sudden onset of cold, wet weather.
Immunity develops by about six months of age, but it is specific to the infecting species of coccidia and can break down. Outbreaks of E. zuernii in 2-3 year old milking heifers have resulted from the peri-parturient suppression of immunity allowing reactivation of previous infections.
Beef calves and dairy calves in all environments can be affected. Coccidiosis has also been associated with post-weaning diarrhoea in beef calves in the northern dry tropics.
Coccidia inhabit the lower half of the small and large intestine of cattle.
Severe infection produces a sudden onset of profuse, dark, watery diarrhoea containing blood (black scours) and shreds of sloughed off intestinal lining. Tenesmus (straining to defecate) and soiling of the hind legs and the base of the tail in a faecal smudge pattern (in the shape of a half-moon between the pin bones from the animal switching its tail and smearing the area with blood-stained faeces) are characteristic.
Lethargy, weakness and severe dehydration and anaemia may also occur. Untreated cases show emaciation and can die within 5-7 days. Mild cases of coccidiosis may not be associated with clinical symptoms or mortality but may lead to poor appetite, poor condition and poor weight gain.
Chronic scours (diarrhoea) coccidiosis may also occur, due to irritation and haemorrhage, particularly of the large bowel, caused by large numbers of multiplying coccidian parasites.
Coccidia live in the small and large intestine of calves and excrete oocysts (type of egg) that are passed out with the dung. Oocysts become infective to cattle only after further development in the environment. In a moist environment this can take about 5-9 days at 20°C, and 2-5 days at 30°C. The resultant infective stages are retained inside the oocyst shell.
Cattle become infected when grazing pastures or drinking water contaminated with the infective oocysts. Once in the small intestine of calves, oocysts excyst (break open) releasing the infective stages which then invade the wall of the small intestine, these further develop into stages in the large intestinal wall. Clinical signs are associated with the release of oocysts from the wall of the large intestines.
The time between ingestion of infective oocysts and presence of the next generation of oocysts in the dung (the pre-patent period) takes about 16-21 days for E. bovis and 12-14 days for E. zuernii. Oocysts passed in faeces are tough and survive for long periods of time in the soil.
Bovine coccidiosis can be tentatively diagnosed on the basis of the clinical symptoms of bloody diarrhoea and tenesmus (straining) along with a history of stress.
Laboratory analysis of faecal samples from a number of affected calves for oocysts of E. zuernii and E. bovis is needed to confirm the diagnosis. However, oocyst output occurs over a short period and symptoms of disease do not always coincide with high oocyst counts. Maturing oocysts in the walls of the intestine can be seen at post mortem.
The sulphonamide compounds (available from veterinarians) and toltrazuril are active against coccidial infections. Treatment is best applied during the pre-patent period to ensure that animals do not suffer clinical disease. Damage to the intestinal wall occurs before oocysts are shed.
Prevention is based on sound animal husbandry practices to reduce stress and improve hygiene. Feed and watering points should not be contaminated with faeces, and concrete pens used for raising calves should be dry with washings discharged through well-constructed drainage systems to ponds. Additional weaning yards may be needed if yards currently in use are carrying considerable contamination.
Preventative drugs can be added to feed supplements or added to milk replacer to give protection against severe infection.