DETECTION OF ANTI-LISTERIOLYSIN-O IN DAIRY-CATTLE EXPERIMENTALLY INFECTED WITH LISTERIA-MONOCYTOGENES

A dot-blot assay and an enzyme-linked immunosorbent assay (ELISA) to d etect listeriosis in dairy cattle were developed that detected anti-li steriolysin O antibodies in the serum of cows experimentally infected with Listeria monocytogenes. The tests utilized purified listeriolysin O (LLO) as the detection antigen and streptolysin O (SLO) to absorb c ross-reacting antibodies. The two tests were compared with an agglutin ation test that used formalin-killed whole L. monocytogenes cells. Blo od samples were collected periodically from 17 cows after intramammary gland infection, and the development of anti-LLO antibodies was follo wed by an agglutination test, the dot-blot test, and the ELISA. In gen eral, an agglutination titer of >640 was needed for a positive dot-blo t anti-LLO test for nonpregnant cows. However, 1 pregnant cow with an agglutination titer of 20 was positive in the dot-blot test. The ELISA was as sensitive as the dot-blot assay but gave a quantitative measur ement to distinguish serum samples of positive reactors from cross-rea ctors. The specificity of the LLO-based tests was further evaluated us ing serum from cows that had been experimentally infected with Staphyl ococcus aureus, 17 of which had agglutination titers for L. monocytoge nes >640. These elevated agglutination titers were probably due to cro ss-reacting bacterial antigens because serum from 9 of 17 of these ani mals did not react to the purified LLO antigen. A positive response to the LLO-based dot-blot and ELISA assays is indicative of previous or current infection with L. monocytogenes.