Specificity of slide agglutination test for detecting bacterial fish pathogens

Abstract

The usefulness of the slide agglutination assay for a rapid diagnosis of fish diseases was evaluated using a total of 80 pathogenic bacteria and environmental isolates belonging to the genera Vibrio (28), Pasteurella (5), Aeromonas (26), Yersinia (6), Edwardsiella (8), Pseudomonas (6) and Lactobacillus (1). Selected strains from each bacterial group were used as antigens for rabbit immunization. The specificity of the reactions between whole-cell antigens and the different whole-cell antisera varied according to the bacterial group analyzed. In the case of V. anguillarum, it was found that by using two sera against serotypes 1 and 2, it was possible to detect most of the strains causing vibriosis regardless of their origin. Cross-reactions were not detected between either both serotypes or with other pathogenic or environmental vibrios. In general, the whole-cell antisera from P. piscicida, A. salmonicida, Y. ruckeri and E. tarda detected all the strains within each species. However, a more heterogeneous pattern was exhibited by the motile Aeromonas species. The majority of A. hydrophila and A. sobria isolates did not agglutinate with the anti A. caviae serum, indicating that this antiserum is not adequate for identifying all motile Aeromonas strains. The antiserum against A. salmonicida subsp. masoucida displayed weak cross-reactions with some A. salmonicida subsp. salmonicida and A. hydrophila whole-cell antigens. Strong cross-agglutinations occurred also between types I and II of Y. ruckeri as well as between E. tarda and E. ictaluri. These cross-reactions were eliminated by using the respective thermostable somatic “O” antigens, which indicates that common thermolabile antigens are shared by these strains. The slide agglutination test, using anti whole-cell sera and two antigen preparations (whole-cells and “O” antigen) for each strain, is useful for a rapid detection of fish pathogens, with the additional advantage of its applicability for serotyping studies. Furthermore, some cross-reactions using the whole-cell antigens are reliable in identifying a wide range of strains causing similar diseases with a small number of anti whole-cell sera.