Use of immunofluorescence, Gram's staining, histologic examination, and seroagglutination in the diagnosis of porcine colibacillosis.

An indirect fluorescent antibody (IFA) test of ileal impression smears for K88, K99, and 987P pilus antigens was compared with histologic examination and seroagglutination of Escherichia coli isolates for efficacy in determining colibacillosis in pigs. Histologic examination appeared to be more effective than the IFA test in revealing colonization of the ileum by bacteria. However, histologic examination revealed little about the nature of the colonizing bacteria. Correlation between bacterial adherence, as observed in histologic sections of ileum, and the presence of bacteria with adherence pili, as determined by IFA testing, was 91%. Results of seroagglutination for pilus antigens correlated with results of histologic examination in only 84% of the cases. Pilus antigens were not identified by IFA testing or seroagglutination in 5% of the cases in which adherent bacteria were observed in histologic sections of the small intestine. Because little preparation time was required for the IFA test, results were available within 2 hours of necropsy. In contrast to histologic examination, the IFA test made possible identification of the colonizing organism as E coli and revealed the type of pilus antigen present. Adherence pili were identified more frequently by IFA testing than by seroagglutination. Examination of Gram's-stained ileal impression smears was useful in screening for colibacillosis in pigs. Bacterial adherence was found in positive correlation with the number of gram-negative bacilli observed. Bacterial adherence rarely was observed in histologic sections of ileum when smears contained less than or equal to 10 gram-negative bacilli/1,000 X microscopic field. Each diagnostic test compared offered advantages and disadvantages over the other tests. Seemingly, concurrent use of several of these tests, rather than one, should be used in the diagnosis of porcine colibacillosis.