Indirect Fluorescent Antibody and Serum Neutralization Response in Pre-exposure Prophylaxis Against Rabies

Abstract

Rabies pre-exposure immunization in a group of 40 Peace Corps Volunteers for Pakistan (PCV) and Undergraduate-Year-in-India students (UGYI) was evaluated by the indirect fluorescent rabies antibody (IFRA) and the serum neutralization (SN) tests. The former is the standard reference for antibody response, the latter under experimental trial. Proof of antibody response has to be determined as not all individuals show evidence of protection regardless of immunization schedule. Because time was a limiting factor, the following immunization schedule was used: Four injections of 1 ml of Duck Embryo Vaccine (DEV) were given at weekly intervals with a booster at 8 weeks; bleedings were made prior to the booster and 7 or 10 days later. The more usual schedule is 3 injections at weekly intervals with a booster at 6 to 10 months and an antibody study 30 days later. The longer schedule allows for greater antibody response. Results showed that by the SN test 32 of 38 (84%) were positive; by the IFRA all tested (37) were positive. Two subjects were dropped from the project because of allergic reactions: one with angioneurotic edema; one with a severe local reaction. The SN results compared favorably with other similar studies, especially when the shortened schedule is considered. Comparison of the IFRA and SN antibody response to those of a large study done at the Communicable Disease Center (CDC) revealed the great accuracy of the IFRA test with no false positives. The advantages of the IFRA over the SN test are its greater simplicity, sensitivity and rapidity of performance (1 day as compared to 14). And, with results of antibody response quickly available the need for additional booster injections can be immediately determined. Whether the IFRA can replace the Sn test is still uncertain. Studies are currently underway at the CDC attempting to evaluate the significance of IFRA-SN-titer elevation relative to protection against rabies challenge. The direct fluorescent rabies antibody test (FRA) is being increasingly used in the diagnosis of rabies in animals. It is especially valuable in the Negri negative brain, and in studying the distribution of virus particles within cells and in various body tissues. Indications for rabies pre-exposure immunization (for certain high risk groups) can be assessed when certain pertinent factors of prevalence and pathogenesis are considered. Rabies in wild species is increasing. The natural reservoir of the infection is in many small carnivores found through the world, and in the western hemisphere, the bat. Even the dog is a healthy carrier of the disease in some areas (e.g., 30% of the dogs in Ethiopia). And, transmission of infection by contact with a rabid animal is not essential. Studies suggest the aerosol route is possible also.