Association of Deficiency in Antibody Response to Vaccine and Heterogeneity ofEhrlichia risticiiStrains with Potomac Horse Fever Vaccine Failure in Horses

Abstract

Ehrlichia risticii is the causative agent of Potomac horse fever (PHF), which continues to be an important disease of horses. Commercial inactivated 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 cases 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, as determined by immunofluorescence assay (IFA) titers. In a majority of horses, the final antibody titer ranged between 40 and 1,280, in spite of repeated vaccinations. None of the vaccinated horses developed in vitro neutralizing antibody in their sera. Similarly, one horse experimentally vaccinated three times with one of the vaccines showed a poor antibody response, with final IFA titers between 80 and 160. The horse 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 laboratory, 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. risticiiisolates from the field cases indicated that they were heterogeneous among themselves and showed differences from the 25-D and 90-12 strains as determined by IFA reactivity 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 PHF vaccines and the heterogeneity ofE. risticii isolates may be associated with the vaccine failure.

Clinical and Clinicopathological Factors Associated with Survival in 44 Horses with Equine Neorickettsiosis (Potomac Horse Fever)

“…The apparent high rate of vaccination failure is consistent with previous reports of vaccine inefficacy. 16,38,39 Vaccine failure has been reported to be as high as 89%, and in some studies, vaccination was not found to reduce prevalence, cost, or severity of disease. 16,39 However, 1 study demonstrated resistance to homologous reinfection in vaccinated horses for at least 20 months postinoculation.…”

“…16,38,39 Vaccine failure has been reported to be as high as 89%, and in some studies, vaccination was not found to reduce prevalence, cost, or severity of disease. 16,39 However, 1 study demonstrated resistance to homologous reinfection in vaccinated horses for at least 20 months postinoculation. 40 Incomplete protection from vaccination is due in large part to extensive variability in the major surface antigens such as the 51-kDa and 55-kDa major antigens, and there would not be any cross-protection between strains.…”

“…15,16 Vaccine failure has been reported to be as high as 89% and is associated with relatively low levels of vaccine-induced antibodies. 16 Although the epidemiology of EN has been extensively investigated, [17][18][19][20][21][22] and the clinical and clinicopathological changes have been documented in 55 horses that were experimentally infected, 23 there are limited data regarding clinical and clinicopathological aspects of EN in naturally occurring cases. Therefore, the objectives of this retrospective study were to describe the signalment, history (including vaccination status), clinical signs, clinicopathologic values, antimicrobial use, and survival status in horses exhibiting clinical signs of EN, and to identify variables associated with survival.…”

Diagnosis and Treatment of Undifferentiated and Infectious Acute Diarrhea in the Adult Horse

“…Several killed and adjuvants vaccines have been marketed. However, vaccine failure rates are high, 33,34 likely caused by the antigenic differences present among greater than 14 N rickettsia strains isolated from clinical cases. 27…”

Cloning and Expression of 51‐kDa Antigenic Protein of Neorickettsia risticii NR‐JA1

“…6 However, it has been demonstrated that the strain-specific antigens from different N. risticii isolates vary in their sizes and sequences. 7 A 51-kDa protein (p51) is the major antigen recognized in horses with PHF. 8 P51 has been found in all N. risticii strains identified to date.…”