Table 1.

The main pathogens that cause diarrhea in calves and their mechanism of action.

Pathogens	Period of infection	Clinical symptoms	Mechanism of action	Citations
ETEC	Calves aged 4 days after birth	Watery diarrhea, passing pale yellow, gruel-like, foul-smelling stools	ETEC proliferates in the enterocytes of the intestinal villi, causing secretory diarrhea	Foster and Smith (2009)
Salmonella	Calves aged 10 days to 3 months	Watery and mucoid diarrhea, passing stools that are grayish-yellow liquid mixed with mucus and blood	Salmonella can invade the intestinal mucosa and proliferate in the lymphoid tissue, resulting in systemic disease	Tsolis et al. (1999)
BRV	1 to 2 weeks old calves	Severe watery diarrhea with undigested curds mixed in stool	BRV replicates in the cytoplasm of intestinal villous epithelial cells, which destroys intestinal epithelial cells and secretes viral enterotoxin, resulting in diarrhea caused by indigestion and absorption	Jang et al. (2019)
BCV	1 to 2 weeks old calves	Discharge yellow watery stool with respiratory symptoms	BCV attaches to intestinal epithelial cells through prickles and hemagglutinin glycoprotein, fusing the envelope of the virus with the cell membrane or endocytosis vesicle, leading to cell death	Hodnik et al. (2020)
C. parvum	Calves aged 4 days and 6 weeks	Severe diarrhea, gray white or yellow feces, containing a large amount of cellulose, blood and mucus	The direct cytotoxic effect of C. parvum on intestinal epithelial cells and apoptosis	Buret et al. (2003)