Antigen-extinction profile in pregnant cows, using a K99- containing whole-cell bacterin to induce passive protection against enterotoxigenic colibacillosis of calves
Authors: Acres S. D. , Forman A. J. , Kapitany R. A. 
Published: February 2025

SUMMARY
A cow-vaccination, calf-challenge trial was conducted to determine the concentration of K99 antibody in colostrum which would protect calves against diarrhea and death caused by 2 strains of K99-positive enterotoxigenic Escherichia coli (etec), and the dose of a whole-cell bacterin (wcb) containing K99-positive E coli which would induce protective levels of colostral immunity in pregnant cows. Cows (groups 1, 2, 3, and 4) were vaccinated, during the 3rd trimester of gestation, by subcutaneous inoculation of a formalin-killed wcb (undiluted, or diluted 1:5,1:25, or 1:125, 2 doses with 20 days between doses). A 5th group of cows (controls) was given placebo comprised of phosphatebuffered saline solution and adjuvant. After the cows had calved, their calves were allowed to nurse and at 12 to 14 hours of age were orally challenge inoculated with 5 to 7 × 1011 viable cells of 1 of 2 heterologous serotypes of etec: strain B44 (O9:K30:K99:H-) or strain B117 (O8:K85:K99:H-).
Before the cows were vaccinated, 61 of 68 (90%) cows did not have detectable serum K99 antibody titers and the 7 remaining cows had titers ≤ 1:4. There was no increase in serum titers in cows inoculated with the placebo. In contrast, all cows vaccinated with dilutions of the wcb developed titers which ranged between 1:8 and 1:256 at 20 days after the 1st bacterin dose and between 1:32 and 1:256 at 21 days after the 2nd dose. At the time of parturition of the control cows, K99 titers in colostral whey did not exceed 1:4, whereas at parturition by the vaccinated groups, this titer was 1:32 to 1:4,096. The K99 colostra! titers appeared to be related to the interval between the 2nd vaccination and the parturition date, and the highest titers occurred in vaccinated cows which calved 7 to 40 days after their 2nd vaccination was given. There were no significant differences in the K99 titers in serum or colostra! whey of cows given different dilutions of the wcb.
After challenge exposure of calves with etec strain B44, 9 of the 10 (90%) calves born to the control cows had severe diarrhea and died. In contrast, only 3 of 25 (12%) similarly exposed calves born to the vaccinated cows became diarrheic (0 of 6 calves of cows given wcb undiluted, 1 of 7 calves of cows given wcb 1:5, 1 of 5 calves of cows given wcb 1:25, 1 of 7 calves of cows given wcb 1:125) and none died. Of the 9 calves born to control cows that were challenge exposed with the other etec strain (B117), 8 (89%) became diarrheic and 4 (44%) died. Among the 19 calves that were born of vaccinated cows and that were exposed to etec B117, 9 (47%) became diarrheic (2 of 4 calves of cows given wcb undiluted, 2 of 6 calves of cows given wcb 1:5, 5 of 7 calves of cows given wcb 1:25, 0 of 2 calves of cows given wcb 1:125), but none died. There was a correlation between the K99 colostral titer of cows and the severity of diarrhea after challenge exposure of their calves. None of 44 calves which were nursing cows which had a colostral K99 titer ≥ 1:4 developed severe diarrhea, as evidenced by clinical dehydration, cns depression, and marked loss in body weight. In contrast, 14 of 18 (78%) calves nursing cows with colostral K99 titers < 1:4 had severe diarrhea and 12 of these died. However, there was no correlation between K99 titer in dam's colostral whey and the presence of mild transient diarrhea after calf's challenge exposure.