USE OF A VIRULENCE-ASSOCIATED PROTEIN-BASED ENZYME-LINKED-IMMUNOSORBENT-ASSAY FOR RHODOCOCCUS-EQUI SEROLOGY IN HORSES

An enzyme-linked immunosorbent assay (ELISA) was developed against Rho dococcus equi using Triton X-114 detergent extracted whole cell materi al, in which the virulence associated protein (VapA) predominated. Enz yme-linked immunosorbent assay titres corresponded to antibody reactin g with VapA on Western blots. There was considerable variation in anti body titres of nonimmunised mares and in the time when the colostrally derived antibody of their foals had declined to low or undetectable t itres. In general, antibodies in foals declined to their lowest levels at age 4-8 weeks. Seroconversion occurred in foals age 8-10 weeks, bu t the precise time depended on maternal titre and the month in which t he foal was born. Foals reaching age 8 weeks in late summer showed mor e marked seroconversion than foals born earlier. The ELISA was used to follow the response to immunisation with the same Triton X-114 extrac ted material. Six mares immunised before parturition with the antigen in aluminium hydroxide adjuvant developed high titres, up to >102,400 and transferred them to their foals through colostrum. Their foals res ponded to immunisation with 0.5-1.0 mg antigen 3, 5, 7 and 9 weeks aft er birth. Antibody titres following immunisation with similar dosage r eached up to >102,400 in a separate group of foals of nonimmunised mar es. Nonvaccinated control foals seroconverted at age 6-8 weeks, The Va pA based ELISA is useful to follow the course of natural infection wit h R. equi or immunisation with VapA based antigen.