Observations on vaccine and post-infection immunity in contagious equine metritis

The vaccination of four ponies on two occasions with a formolised culture of Haemophilus equigenitalis produced a high circulating antibody titre to the organism in each pony. Three out of four vaccinated and all of three unvaccinated ponies developed typical symptoms of contagious equine metritis (CEM) when subsequently challenged with a vaginal exudate containing H equigenitalis. Similarly, three ponies which had previously been infected with H equigenitalis and which had recovered spontaneously also developed contagious equine metritis when rechallenged with the organism. The clinical and bacteriological symptoms in the vaccinated ponies and in the rechallenged ponies were less severe than those observed in the unvaccinated ponies but H equigenitalis was still recovered 17 days after challenge from the three vaccinated ponies which had developed CEM. The vaccinated pony which remained free from infection did not exhibit the highest circulating antibody titre of the vaccinates before challenge.