ASSOCIATION OF DEFICIENCY IN ANTIBODY-RESPONSE TO VACCINE AND HETEROGENEITY OF EHRLICHIA-RISTICII STRAINS WITH POTOMAC HORSE FEVER VACCINE FAILURE IN HORSES

Ehrlichia risticii is the causative agent of Potomac horse fever (PI-I F), which continues to be an important disease of horses, Commercial i nactivated whole-cell vaccines are regularly used for immunization of horses against the disease. However, PHF is occurring in large numbers of horses in spite of vaccination. In a limited study, 43 confirmed c ases of PHF occurred between the 1994 and 1996 seasons; of these, 38 ( 89%) were in horses that had been vaccinated for the respective season , thereby clearly indicating vaccine failure. A field study of horses vaccinated with two PHF vaccines indicated a poor antibody response, a s determined by immunofluorescence assay (IFA) titers. In a majority o f horses, the final antibody titer ranged between 40 and 1,280, in spi te of repeated vaccinations. None of the vaccinated horses developed i n vitro neutralizing antibody in their sera. Similarly, one horse expe rimentally vaccinated three times with one of the vaccines showed a po or antibody response, with final IFA titers between 80 and 160. The ho rse did not develop in vitro neutralizing antibody or antibody against the 50/85-kDa strain-specific antigen (SSA), which is the protective antigen of the original strain, 25-D, and the variant strain of our la boratory, strain 90-12. Upon challenge infection with the 90-12 strain , the horse showed clinical signs of the disease. The horse developed neutralizing antibody and antibody to the 50/85-kDa SSA following the infection. Studies of the new E. risticii isolates from the field case s indicated that they were heterogeneous among themselves and showed d ifferences from the 25-D and 90-12 strains as determined by IFA reacti vity pattern, DNA amplification finger printing profile, and in vitro neutralization activity. Most importantly, the molecular sizes of the SSA of these isolates varied, ranging from 48 to 85 kDa. These studies suggest that the deficiency in the antibody response to the PI-IF vac cines and the heterogeneity of E. risticii isolates may be associated with the vaccine failure.