Pathophysiology and Clinical Manifestations of Coccidiosis

In the field situation, single species infections are rare. Mixed infections are seen as a rule. As only some of the cattle coccidia species are considered pathogenic, the intensity of pathology inflicted would vary with the pathogenic coccidia species involved and the number of pathogenic coccidia oocysts ingested.

The severity of damage to the intestine is related to the number of oocysts of pathogenic species ingested. When the number of oocysts is lower, non-immune healthy animals may tolerate the infection without any visible signs of clinical disease. Though the intestinal cells are destroyed, their rapid replacement results in minimal damage. However, if the exposure of non-immune healthy animals is to a large number of oocysts, the damage to the intestine is widespread and severe. It includes mucosal and submucosal oedema of both the caecum and colon, followed by “Morocco” leather appearance of the caecal and colonic mucosae (in Eimeria bovis infection). Fibrin strands are seen on the mucosa. Later the lesions become more marked, congested and ulcerated; haemorrhages extend to the submucosa and muscular tissue.

In brief the disease is associated with a loss of blood, fluid, albumin and electrolytes in the gut. Sloughed intestinal mucosa, fibrin casts, may be seen in faeces and may also be tinged with blood. Sometimes blood loss is considerable.

In mild coccidiosis episodes, animals have little or no diarrhoea, faeces may contain blood and affected animals may be anorexic. In severe cases due to severe damage to the intestinal epithelium, the faeces are liquid and bloody may contain mucus and fibrin. Animals are dehydrated, weak and listless. Affected animals typically strain to defecate which may result in rectal prolapse. The tail and hindquarters of the animals is soiled with faeces, which results in loss of hair in these areas. The animals will have rough hairs, drooping ears and sunken eyes. Some of the severely affected animals die due to the disease.

The pathological changes induced by Eimeria alabamensis are characterized by catarrhal inflammation of the small intestine. It results in profuse watery diarrhoea, poor appetite, depression, abdominal pain and reduced growth. The disease normally strikes 3 to 7 days after release to the pasture.

Coccidiosis also predisposes animals to secondary bacterial infections. Recovery from a severe episode may require several weeks. Neurologic signs have been reported during coccidiosis outbreaks in calves and weaned beef cattle (Fanelli, 1983).

Under the circumstances, most animals become infected, but only some suffer and show clinical symptoms (10 to 15% of animals) described above. Subclinical coccidiosis can result in reduced growth. Nielson et. al. (2003) observed in a Danish study that there was a significant reduction in daily weight gain amongst calves excreting more than 5,000 OPG at a time in the course of the season as compared to those that excreted less than 5,000 OPG in whole season. Daugschies et. al. (1986) reported that the animals infected experimentally with 50 to 100,000 oocysts of Eimeria bovis suffered from weight loss and reduced feed intake. Weight loss in animals suffering from coccidiosis has been attributed to reduced feed intake and endogenous loss of organic compounds via parasite – induced lesions in the intestinal mucosa rather than changes in intestinal absorption (Daugschies et. al.,1998).