Contagious equine metritis: Antibody response of experimentally infected pony mares

Abstract

Intrauterine inoculation of pony mares with the bacterium that is the causative agent of contagious equine metritis (CEM) resulted in clinical disease. A humoral immune response could be detected by agglutination and complement fixation (CF), and in some cases precipitating antibody was found by immunodiffusion tests. Agglutinating antibody was the most reliable serological indicator of overt infection and was detected in 8 of 28 mares after initial intrauterine inoculation of 3–4 × 10 5 bacteria. Seventy percent of mares given a second inoculation and all mares given a third inoculation of 3–4 × 10 5 bacteria produced detectable agglutinating antibody. Only two of five mares given the third inoculation developed detectable complement-fixing antibody. Only one mare showed evidence of reinfection after a second or third intrauterine inoculation. All of the mares given a single intrauterine inoculum of ≥ 8 × 10 8 bacteria produced agglutinating antibody 10 to 30 days postinoculation (DPI) and 86% gave a positive CF test 10 to 20 DPI. Only mares with an agglutination titer of 320 or more produced precipitating antibody. Sera were considered positive in agglutination tests if they were reactive at a dilution of greater than 4 and positive in CF tests if they were reactive at a dilution of 4 or greater. Pony serum frozen at −70°C was anticomplementary (AC). Treatment at 56°C abolished AC activity and revealed enhancing or procomplementary activity with guinea pig complement. Procomplementary activity could be abolished by treatment of heated pony serum with formaldehyde, which increased CF titers ≥ threefold in weakly reactive sera.