Contagious equine metritis: antibody response of experimentally infected pony mares.

Researchers have studied the immune response of pony mares after deliberate infection with the bacterium causing contagious equine metritis (CEM). The study confirms the onset of the disease due to the bacterium, and identifies the presence of immune response in varying degrees, depending on the number of times an inoculation is given and the number of bacteria in each inoculation.

Research Method

• The researchers injected pony mares intrauterinely (directly into the uterus) with the bacterium that causes CEM. They tracked the development of the disease and the mares’ immune response through several tests.
• The main test to detect antibodies was agglutination (clumping of cells), and complement fixation (an immunological response involving cells destroying infected cells). In some instances, they detected precipitating antibody (antibodies that react with soluble antigens to form a precipitate) with immunodiffusion tests.

Result of Initial Inoculation

• Out of the 28 mares subjected to the initial intrauterine inoculation with 3-4 x 10(5) bacteria, 8 developed and showed detectable agglutinating antibodies. These agglutinating antibodies are treated as the most reliable serological indicator of overt infection.

Result of Multiple Inoculations

• When given a second inoculation, 70% of mares developed these antibodies. All mares given a third showed detectable agglutinating antibodies.
• However, complement-fixing antibodies were much less frequent, with only two out of five mares given the third inoculation producing detectable antibodies.
• Only one mare showed evidence of reinfection after a second or a third intrauterine inoculation.

Impact of Bacterial Load and Response Time

• All mares that were given a large dose of bacteria (greater than or equal to 8 x 10(8)) developed agglutinating antibodies 10 to 30 days after being inoculated.
• In those given this dosage, 86% tested positively for a CF test 10 to 20 days postinoculation.

Antibody Titer

• Only mares with an agglutination titer (concentration) of 320 or more generated precipitating antibodies, highlighting the importance level of the immune response.
• Sera were considered positive in agglutination tests if they were reactive at a dilution greater than 4 and positive in CF tests if they were reactive at a dilution of 4 or greater.