The theory of an intranasal vaccine for Strangles is that the site of entry and infection with Strangles is via the tonsils located in the nose and mouth. If we stimulate immunity at these sites by introducing a vaccine directly to those tonsils, we can limit the propagation of the bacteria at its site of entry. The Strangles vaccine most commonly used is a modified live bacterial vaccine, which is unable to replicate but mimics the immunity stimulated by a natural infection. However, its efficacy is dependent on an adequate amount of the vaccine reaching the tonsils deep in the head, so it must be administered via the nasal passageways.
Older vaccines against Strep equi were injectable, and there are a few newer injectable vaccines on the market in other countries. The older vaccines were notorious for a very high rate of injection site reactions and abscessation. Newer injectable vaccines typically contain extracts of the Strep equi bacteria, but their efficacy has been disappointing, with one study suggesting 50% of vaccinated horses developing clinical disease when exposed to the disease agent just a few weeks after a proper initial vaccination and booster.
As more information is learned from the genetic sequencing of the bacteria, it is likely that more effective and safer vaccines (hopefully ones that are intramuscularly administered) will be eventually developed.