Experimental Streptococcus equi infection in the horse: correlation with in vivo and in vitro immune responses.

This study analyzes how horses’ immune response to Streptococcus equi, a bacterial strain, correlates with their susceptibility to and recovery from the related infection. The research specifically juxtaposes horses already exposed to S equi (demonstrating pre-existing immunity) with naïve horses, with observations suggesting immune mechanisms underpinning different disease outcomes.

Research Method and Selection Criteria

• In the study, fourteen young outbred horses were selected and divided into two groups based on their skin-test reactions (measured at 18 or 24 hours) to Streptococcus equi.
• The research team focused on two different groups. Group I consisted of horses displaying evidence of prior exposure to S equi, indicated by a positive skin response and existing serum antibodies. Group II encompassed S equi skin-test-negative (not previously exposed) horses without serum antibodies against S equi, signifying naïve individuals.
• All the horses, regardless of the group, were nasopharyngeally inoculated with the virulent strain of S equi.

Observations and Results

• In group I, most horses, having displayed exposure evidence, developed mild or no signs of disease following inoculation, suggesting pre-existing immunity.
• However, in group II, where horses were S equi-naïve and lacked this immunity, predictable and severe clinical signs of infection arose post-inoculation. This included nasal discharge secretion of the bacteria and ruptured mandibular lymph nodes, common symptoms associated with streptococccal infections.
• These findings indicate a correlation between the horse’s existing immune responses (both humoral and cellular) to streptococcal antigens and their resistance level to a virulent S equi infection.

Recovery and Immunity Acquisition

• For susceptible horses (those in group II), their recovery from the infection was associated with the emergence of humoral antibodies and a transition to positive skin-test response, implying an adaptive immune response to S equi antigens.
• This indicates that these horses developed an immunity to S equi as a result of the infection, stressing the role of immune mechanisms against this bacterial strain.