USE OF RHODOCOCCUS-EQUI VIRULENCE-ASSOCIATED PROTEIN FOR IMMUNIZATIONOF FOALS AGAINST RHODOCOCCUS-EQUI PNEUMONIA

Objective-To evaluate use of the virulence-associated protein of Rhodo coccus equi in immunizing foals against R equi pneumonia. Animals-Eigh t (experimental group) and 6 (controls) mares with their foals. Proced ure-Virulence-associated protein extracted from R equi was used to pre pare an acetone-precipitated, Triton X-extracted (APTX) antigen. After determination of the efficacy of passive immunization, in untreated f oals or in foals given plasma from a horse vaccinated with APTX antige n or from a nonvaccinated horse, a field trial was done to evaluate th e efficacy of vaccination of 8 mares, twice with APTX before parturiti on, and of their foals at ages 3 and 5 weeks; 6 mares and their foals served as unvaccinated controls. All 2-day-old foals were given plasma from local donor horses inoculated with a locally produced bacterin. Serum opsonizing activity produced by vaccination with APTX was determ ined. Passively immunized foals were challenge exposed with an aerosol of virulent R equi Foals of the field trial were exposed to enzootic infection. Results-inoculation with APTX resulted in high IgG antibody titers with opsonizing activity. Passive immunization of foals with p lasma from an immunized horse enhanced bacterial clearance from the lu ngs, compared with that in foals not given plasma or given plasma with out APTX antibodies. Vaccination of mares and foals exposed to natural infection resulted in development of R equi pneumonia in 4 of 8 vacci nated foals, but in only 1 of 6 unvaccinated foals. Conclusions-Vaccin ation with APTX antigen led to high-titer, opsonizing antibody. Plasma from a vaccinated horse appeared to enhance clearance of R equi from the lungs of foals. Paradoxically, vaccination of mares and their foal s with APTX antigen did not protect foals and may have enhanced R equi pneumonia in the foals.