Development of neutralizing antibody in horses infected with Ehrlichia risticii.

The role of the humoral immune response in ehrlichial infection is unknown. Development of neutralizing antibodies during a course of Ehrlichia risticii infection in a pony was examined in vitro by determining the inhibition of E. risticii infection of P388D1 cells in the presence of the sera. The pony experimentally infected with E. risticii developed significant neutralizing activity in the sera by 15 days postinfection when parasitemia started to decline. Neutralizing activity continued to rise after recovery from the disease up to 34 days postinfection at which time the experiment was terminated. In vitro neutralizing activities in the sera from 3 additional ponies infected with E. risticii were lower at 2 weeks than at 4 weeks postinfection. The sera from vaccinated/challenged ponies had comparable neutralizing activity to those of the recovered ponies at approximately 3 to 4 weeks postchallenge. Equine sera from infected or vaccinated/challenged ponies were also effective in protecting mice from E. risticii infection. These studies demonstrated the significant development of neutralizing activity in the sera of recovered or vaccinated/challenged ponies.