An overview of calf diarrhea - infectious etiology, diagnosis, and intervention

Cho, Yong-il; Yoon, Kyoung-Jin

Escherichia coli can be classified into six pathogroups based on virulence scheme: enterotoxigenic E. coli (ETEC), shiga toxin-producing E. coli, enteropathogenic E. coli, enteroinvasive E. coli, enteroaggresive E. coli, and enterohaemorrhagic E. coli [65, 95] . Among these bacteria, the most common cause of neonatal diarrhea is ETEC stains that produce the K99 (F5) adhesion antigen (commonly referred to as E. coli K99 + ) and heat-stable enterotoxin [95] . It should be noted that other pathogroups of E. coli, which are usually identified by histopathology, can be missed if the diagnosis focuses on E. coli K99 + alone. Neonatal calves are most susceptible to ETEC infection during first 4 days after birth and develop watery diarrhea if infected [42] . Following ingestion, ETEC infects the gut epithelium and multiplies in enterocytes of the intestinal villi. The distal portion of the small intestine provides the most favorable environment for ETEC colonization due to the low pH (less than 6.5). Villous atrophy due to a loss of infected cells and damage to the laminar propria are commonly observed in affected small intestine. The bacteria express the K99 antigen for attachment [43] . After colonization of the gut epithelium, heat-stable toxin production induced by ETEC leads to the up-regulation of chloride secretion into the gut. This osmotically pulls water into the intestinal lumen and leads to the development of secretory diarrhea in calves.