Calf diarrhea (scours) is the primary cause of death in calves from 2 to 30 days of age. And, while advances in scours treatments may bring improved survival rates, the economic benefit of improved preventive measures far exceeds that of treatment.

Scours pathogens are viruses, bacteria and microscopic parasites. Most of these pathogens do their damage in the intestinal tract. Several mechanisms go to work there, all of which cause diarrhea or scours.

  • They cause the cells of the intestinal lining to malfunction.
  • They can kill the intestinal lining cells.
  • They can invade the deeper layers of the intestinal lining and rapidly destroy it.

The K99 strain of Escherichia coli (E. coli) causes diarrhea by impairing the ability of the intestinal lining to digest and absorb the milk nutrients. This K99 strain, however, is only capable of causing scours in calves under a week old. After that, the calf's intestine becomes resistant to its effects.

The cells lining the intestine can be killed by viruses such as rotavirus and coronavirus and a protozoan parasite called Cryptosporidium. The scours tend to be yellow and watery. Because viruses and cryptosporidia aren't susceptible to antibiotics, the only treatment of this type of scours is nutritional and fluid support of the calf until the intestine lining is regenerated.

Carriers And Shedders

Rotavirus, coronavirus and crypto are carried by healthy heifers and cows and are shed into the environment in their manure. Cryptosporidium, which is also a threat to humans, is particularly troublesome because no effective treatment nor vaccine is available, despite extensive research.


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Bacteria, such as certain strains of E. coli and Salmonella, invade the deeper layers of the intestinal lining, rapidly destroying it. Generally, this group causes release of blood and mucus into the diarrhea.

Calf scours are usually caused by two or more of these pathogens working together. The severity and duration of a scours episode is related to the following factors:

  • The number (or dose) of each agent involved (i.e., sanitation).
  • The extent of intestinal destruction or malfunction created by each pathogen -- their effects are additive. Different strains of each pathogen exist, and differ in their capacity to induce severe disease.
  • The amount and quality of colostrum consumed by the calf from the dam. *The severity of other stresses (wetness, cold, need to fight off other diseases and poor nutrition caused by poor maternal milk production or deficiencies of certain trace minerals or vitamins).