Studies of the pathogenesis of enteric E. coli infections in weaned pigs: bacteriological and immunofluorescent studies.

Qualitative and quantitative, bacteriological studies were performed on spontaneous cases of post weaning E. coli diarrhoea (PWD). The pigs derived from a herd, D, in which the disease had persisted for a period of almost 2 years. Orally vaccinated healthy pigs from herd D and from herds A and M without the disease were also examined. The results showed that haemolytic E. coli were frequently isolated from faecal samples which had been collected 5--7 days after weaning but seldom from samples from the same pigs collected before weaning. Haemolytic E. coli dominated the aerobic intestinal flora at 3--5 days after weaning in pigs from herd D with PWD. Oral vaccination using a formalinized vaccine delayed and suppressed the occurrence of haemolytic E. coli in pigs from herd D (Table I). Intestinal counts of the bacteria showed that the number of haemolytic E. coli present in the anterior portion of the jejunum was 10(-3)--10(-5) times higher in pigs which suffered from PWD than in weaned pigs of the same age which did not show symptoms of the disease (Table II). The lowest bacterial counts in various portions of the intestine were consistently observed in the sections obtained from the orally vaccinated healthy pigs. Pure cultures of K88-negative enteropathogenic E. coli serotype O149:K91 were consistently isolated from all the diseased pigs. Fluorescent antibody studies showed that the specific strain of bacteria adhered to the villous epithelium of the jejunum in a layer which covered the villi from the tip to the base and sometimes continued down into the crypts (Figure 1). The bacterial adhesion coincided with an intensive colonization of the jejunum with the homologous E. coli serotype and was nerve observed in apparently healthy pigs which did not have symptoms of PWD. It was concluded that characteristic intestinal colonization by adhesion may occur with enteropathogenic strains of E. coli O149:K91 which lack the K88 antigen (Figure 2).