Duration of serum antibody response to rabies vaccination in horses

Abstract

OBJECTIVETo investigate the impact of age and inferred prior vaccination history on the persistence of vaccine-induced antibody against rabies in horses. DESIGN Serologic response evaluation.ANIMALS 48 horses with an undocumented vaccination history. PROCEDURESHorses were vaccinated against rabies once. Blood samples were collected prior to vaccination, 3 to 7 weeks after vaccination, and at 6-month intervals for 2 to 3 years. Serum rabies virus-neutralizing antibody (RVNA) values were measured. An RVNA value of ≥ 0.5 U/mL was used to define a predicted protective immune response on the basis of World Health Organization recommendations for humans. Values were compared between horses < 20 and ≥ 20 years of age and between horses inferred to have been previously vaccinated and those inferred to be immunologically naïve. RESULTSA protective RVNA value (≥ 0.5 U/mL) was maintained for 2 to 3 years in horses inferred to have been previously vaccinated on the basis of prevaccination RVNA values. No significant difference was evident in response to rabies vaccination or duration of protective RVNA values between horses < 20 and ≥ 20 years of age. Seven horses were poor responders to vaccination. Significant differences were identified between horses inferred to have been previously vaccinated and horses inferred to be naïve prior to the study. CONCLUSIONS AND CLINICAL RELEVANCEA rabies vaccination interval > 1 year may be appropriate for previously vaccinated horses but not for horses vaccinated only once. Additional research is required to confirm this finding and characterize the optimal primary dose series for rabies vaccination. (J Am Vet Med Asssoc 2016;249:xxx-xxx) revaccination is currently recommended by vaccine manufacturers and in the vaccination guidelines of the AAEP. 3 Research in several animal species indicates that the primary correlate of protection against rabies is the presence of neutralizing antibody in serum. 4 The humoral immune response to rabies vaccination can be evaluated with the RFFIT or fluorescent antibody virus neutralization test, both of which are used for in vitro measurement of RVNA concentration in serum samples. The RFFIT is currently the gold standard and reference technique for most laboratories. [5][6][7] An RVNA titer of ≥ 0.5 U/mL is globally recognized as the threshold of seroconversion for humans. 7 Values reported in this manner (U/mL) are titers expressed as standard units. This value of 0.5 U/mL, when obtained via RFFIT or fluorescent antibody virus neutralization, is also recognized by regulatory authorities in most rabies-free areas as proof of adequate response to vaccination for importation of cats and dogs. 5 No general consensus currently exists regarding the RVNA titer needed to confer protection

Equids’ Core Vaccines Guidelines in North America: Considerations and Prospective

“…Assess the humoral response of vaccinated horses. Additional booster in poor responders might be beneficial (based on peer review cohort studies [28,29,42]) Footnote: AAEP-American Association of Equine Practitioners. A murine study, highlighted the development of neutralizing antibodies, was key in the protection against rabies [44].…”

“…Following the vaccination (Imrab 3) of naive horses or horses with low titers prior to vaccination, some were defined as poor responders by having titers below 0.5 IU/mL by one year post vaccination, with some horses that did not mount titers above 0.5 IU/mL after vaccination [ 42 ]. This suggested an interindividual response to the rabies vaccine and the need for adapting protocols for these horses; assessing post vaccination titers might require attention.…”

“…Previously vaccinated horses with titers above 0.5 IU/mL prior to administration of the vaccine, maintained titers above 0.5 IU/mL for two to three years [ 42 ]. This suggested annual revaccination might not be warranted and requires further studies, although this would require a clinical trial and such studies can be limited due to welfare concerns.…”

Viral Equine Encephalitis, a Growing Threat to the Horse Population in Europe?

“…Several inactivated adjuvant vaccines are commercialized in Europe and can be used in domestic mammals (see Table 2 for the list). Rabies vaccination intervals >1 year may be appropriate for previously vaccinated horses, but not in primed horses vaccinated only once [78].…”

Rabies prevention: the role of serology in parenteral vaccination of companion animals and livestock

“…Duration of immunity has been investigated over the years, with an early paper reporting immunity periods of five years for dogs and four years for cats following vaccination with the Evelyn-Rokitnicki-Abelseth strain of the virus (50). Longlived RVNA responses of up to three years have also been demonstrated in horses (51). The use of RVNA testing systems to measure the immune response to specific RABV antigens and the response to rabies vaccines should not only be accurate and precise, but also meaningful.…”