EXPERIMENTAL-INFECTION OF CALVES AND ADULT CATTLE WITH ESCHERICHIA-COLI O157-H7

Preweaned calves and adult cattle were inoculated with 10(10) CFU of Escherichia coli O157:H7 strain 3081, a calf isolate which produces Shiga-like toxin, to define the magnitude and duration of fecal shedding of E. coli O157:1-17 for each age group. Fecal samples of eight of eight, eight of eight, three of eight, and two of eight calves were positive at 2, 7, 14, and 20 weeks, respectively. In contrast, nine of nine, two of nine, and one of nine steers were positive at 2, 7, and 14 weeks, respectively. The magnitude of shedding (CFU per gram) by individual animals at any one time postinoculation varied widely within each age group but was greater for calves as a group. The differences in shedding patterns between adults and calves were statistically significant. After inoculation, 25 of 29 animals remained healthy and 4 of 17 calves had transient diarrhea. Histologic sections of the brain, kidney, jejunum, ileum, cecum, and colon taken at necropsy from nine calves either 3, 14, or 18 days postinoculation or three adults either 2, 3, or 4 days postinoculation were normal. E. coli O157:H7 was recovered from the alimentary tracts of all of the animals necropsied, and there was no evidence of spread to the liver, spleen, or kidneys. Four calves that had ceased shedding were reinfected when inoculated again with the same strain. E. coli O157:H7 was recovered from none of five and two of five adults inoculated with 10(4) and 10(7) CFU, respectively. If one assumes that the E. coli strain and cattle used in this study are representative of the larger populations encountered in the field, then these observations suggest the following conclusions. (i) Fecal shedding of toxigenic E. coli O157:H7 varies widely among animals of the same age group but persists longer in calves than in adults. (ii) E. coli O157:H7 does not spread from the alimentary tract to other organs. (iii) Previous infection does not prevent reinfection by the same strain of E. coli O157:H7. (iv) The infectious dose of in vitro-grown E. coli O157:1-17 for normal adult cattle is high (>10(4) and probably greater than or equal to 10(7) CFU). (v) Most cattle infected with E. coli O157:H7 remain clinically normal.