Etiologic diagnosis of diarrheal disease of calves: frequency and methods for detecting enterotoxin and K99 antigen production by Escherichia cola.

Escherichia coli isolated from calves in Minnesota and Montana were tested for enterotoxigenicity via bio-assay of cell-free broth culture fluid and for K99 antigen via a serum agglutination test. Infant mice were used to assay for heat-stable enterotoxin (ST), and adrenal cells in culture were used to assay for heat-labile enterotoxin (LT). Forty-six of the 345 E coli isolates produced ST enterotoxin, but none produced LT enterotoxin. Thirty-five of the 46 enterotoxigenic isolates had K99 antigen, and only 9 of 66 nonenterotoxigenic isolates so tested had this antigen. The enterotoxigenicity of 28 additional E coli isolates known or suspected to be calf enteropathogens and provided by investigators from 3 different laboratories was also tested. All isolates from 2 laboratories produced ST but not LT. All isolates from the 3rd laboratory produced LT but not ST. Escherichia coli organisms that were positive in the infant mouse assay also caused positive ligated, jejunal-loop responses in calves and in 9-day-old (but not in 5-week-old) pigs. It was concluded that the infant mouse and adrenal cell tests for ST and LT, combined with the agglutination test for K99, would be useful in the diagnosis of enteric enterotoxic colibacillosis of calves.