Investigations towards an efficacious and safe strangles vaccine: submucosal vaccination with a live attenuated Streptococcus equi

As part of a search for a safe and efficacious strangles vaccine, several d ifferent vaccines and different vaccination routes were tested in foals. Th e degree of protection was evaluated after an intranasal challenge with vir ulent Streptococcus egui by clinical, postmortem and bacteriological examin ations. Inactivated vaccines containing either native purified M-protein (5 00 mug per dose) or whole S equi cells (10(10) cells per dose) administered at least twice intramuscularly at intervals of four weeks, did not protect against challenge. Different live attenuated S equi mutants administered a t least twice at intervals of four weeks by the intranasal route were eithe r safe but not protective or caused strangles. In contrast, a live attenuat ed deletion mutant administered intramuscularly, induced complete protectio n but also induced unacceptable local reactions at the site of vaccination Submucosal vaccination in the inner side of the upper lip with the live att enuated mutant at greater than or equal to 10(8) colony-forming units per d ose, appeared to be safe and efficacious in foals as young as four months o f age. The submucosal vaccinations caused small transient swellings that re solved completely within two weeks, and postmortem no vaccine remnants or o ther abnormalities were found at the site of vaccination.