Etiology

• Clinical disease is associated with the presence of pathogenic coccidian (E. zuernii Eimeria zuernii or E. bovis), infections, which occur in the lower small intestine, cecum and colon Eimeria species.
• E. alabamensis has been reported to cause enteritis in first-season grazing calves in the first week following turnout and also later in the grazing season.

Predisposing factors

General

• The disease is usually associated with a previous stressful situation such as weaning Calf weaning and feeding, transport Transporting cattle, overcrowding Housing, changes in feed or severe weather. Coccidiosis has been reported (1985) as a contributory factor associated with bovine parvovirus infection in North Queensland, but more recent studies to support this theory have not been identified.

Pathophysiology

• Following ingestion, sporulated oocysts excyst releasing sporozoites, which invade epithelial cells in the gut mucosa.
• Infected endothelial cells become hypertrophic forming first generation (“giant”) meronts.
• These rupture and the resulting merozoites infect epithelial cells in the large intestine producing second generation meronts and then gamonts .
• The most severe pathological changes occur in the cecum, colon and terminal part of the ileum, and are due to the gamonts.
• The mucosa appears congested, edematous, and thickened with petechiae or diffuse hemorrhages and the gut lumen may contain large amounts of blood.
• Later in the infection the affected mucosa is destroyed and sloughs away and the sub-mucosa may also be lost.
• In areas where damage to the mucosa is widespread, the mucosa may ulcerate and granulate, whilst in other less affected areas, the sub-mucosa shows signs of regeneration with hyperplastic crypts.
• In July 2022, SRUC reported commonly diagnosing a chronic form of coccidiosis in the UK. The SRUC website reported that affected animals mount a partial immune response and excrete lower numbers of oocysts in the feces. Chronic scour and ill-thrift were observed.  PME of these SRUC cases revealed active intestinal lesions with partial repair. 

Timecourse

• First generation meronts develop appear about 2 weeks post infection and gamonts thereafter.
• The prepatent period (PPP) is 2-3 weeks for most species of coccidia. The PPP for E. alabamensis is only 6-8 days, much shorter than that of other coccidia species.
• Clinical signs generally appear 2-4 weeks post infection and following a period of stress.

Epidemiology

• High stocking densities and intensive husbandry systems with overcrowding in unhygienic yards or feedlots lead to a build-up infective oocysts and disease outbreaks. Feedlot cattle are susceptible to coccidiosis all year round.
• Stress factors, such a poor milk supply, weaning, cold weather and transport, may reduce any acquired resistance and exacerbate the condition.
• Year-round calving can present problems with constant use of calf pens with successive batches of young calves added to pens or buildings already housing older calves.
• The season of the year can play a role in the appearance of coccidiosis. In the UK, coccidiosis is common in spring when young calves are born and turned out onto permanent pastures close to the farm buildings. Inclement weather at this time may cause stress at this stage lowering immunity and precipitating disease.
• Cold winters favor survival of overwintering oocysts in large enough numbers to represent sufficient disease challenge at turn out in spring; conversely mild wet springs favor sporulation, and rapid accumulation of large numbers of infective oocysts.
• Autumn born calves may be born into an already heavily contaminated environment.