An assessment of mucosal immunisation in protection against Streptococcus equi (‘Strangles’) infections in horses.

This research investigates whether vaccines administered to the mucosa, or the lining of various body passages, can help protect horses against a bacterial infection called Streptococcus equi, more commonly known as ‘Strangles’. The study used four different potential vaccine methods and evaluated their effectiveness by observing how well they prompted the immune system to respond and defend the body.

Development of ELISA Test

• The researchers first developed an enzyme-linked immunosorbent assay (ELISA). This is a test that checks for antibodies in the blood, which can indicate if a horse has been exposed to the Streptococcus equi bacteria. This test was used to ensure that they selected horses for the study that had not previously been exposed to the bacteria.
• The ELISA test was also used throughout the study to monitor the immune response of the horses after they were given the potential vaccines.

Vaccine Candidates

• The four potential vaccines tested in this study included: paraformaldehyde-killed S. equi given orally, paraformaldehyde-killed S. equi given via injection into the body cavity (intraperitoneally) along with a substance to boost the immune response (an adjuvant), live but harmless S. equi given orally, and a microencapsulated streptococcal M protein also administered orally.
• The effectiveness of the latter three candidates was first tested in a rat model. The researchers gauged the effectiveness of the vaccines based on how quickly the rodents could clear their lungs of the bacteria after it was inoculated.
• The first two vaccine candidates were then evaluated in a horse model.

Results of the Study

• The results of the study showed that administering the potential vaccines intraperitoneally (directly into the body cavity) was effective in stimulating the production of specific antibodies that fight off the Streptococcus equi infection, in both the mucosal and systemic sites.
• Four weeks after the initial vaccination, the horses were exposed to the live Virulent S. equi bacteria through the nose. Despite the vaccination, both groups of horses showed some signs of infection, though to a lesser extent than the control group which was not vaccinated.
• It was observed that in the group of horses that received the vaccine intraperitoneally, only half showed clinical symptoms of Strangles. In contrast, all of the orally-vaccinated horses developed abscesses which are a common symptom of the infection.
• Despite these signs of infection, none of the vaccinated horses became as ill as the horses that were not vaccinated. The unvaccinated horses exhibited a higher degree of fever, loss of appetite, weight loss, and overall discomfort.