New-born calves receiving no colostrum, or else receiving it after some delay, may die of Bacillus coli septicemia, of which scours is a local manifestation.
Calves receiving an insufficiently protective dose of colostrum may become victims of various bacterial diseases such as arthritis, nephritis, omphalitis, and possibly pneumonia.
Calves receiving a sufficient dose of colostrum may still develop scours of various degrees of severity due to the local multiplication of various types of Bacillus coli in the small intestine.
There exists in the young calf a delicate balance between certain strains of Bacillus coli and the mucous membrane and digestive ferments, which, upset in favor of Bacillus coli, produces scours. The necessary conditions for such attacks are in part inherited defects of the digestive tract, both morphological and functional, and special types of Bacillus coli, resident in the herd and environment.
The immediate indications are a great increase in the number of Bacillus coli in the lowest third of the small intestine with a spreading of the invasion towards the duodenum as the disease gains headway. Under these conditions a general intoxication results. The bacilli form layers or films attached to the top plates of the epithelial cells. At this time morphological changes in the cells are not yet recognizable. The vacuolar or hydropic condition frequently found antedates the bacterial invasion and may be present in the absence of bacteria. In mild or recovered cases, during the 1st week a fatty infiltration of the epithelium is not infrequently present.
In a large number of clinically normal calves, from 1 day to 3 months of age, Gram-negative bacilli were not present in sufficient numbers to be detected in sections, although enriched cultures, when attempted, revealed them.