Variation in SeM genotype is associated with virulence of Streptococcus equi subspecies equi in mice.

Strangles is a common infectious disease caused by Streptococcus equi subspecies equi (S. equi) that primarily affects the upper respiratory system. To date, 271 alleles of the M protein (seM) have been identified that may be related to antigenic differences of isolates. This study evaluated the virulence of S. equi isolates from different alleles of the M protein in an experimental mouse model. Thirty-six Swiss mice were allocated into 12 groups (G1-G12) and each infected group received a different isolate of S. equi recovered from horses with strangles: G1: seM-117; G2: seM-61; G3: seM-123; G4: seM-115; G5: seM-271; G6: seM-124; G7: seM-158, and G8: seM-39, G9: no allele, G10: seM-28, G11: control (no infection - Brazil), G12: control (no infection - Texas). Mice were infected intranasally with 2 × 106 CFU/mL and monitored for clinical signs, weight, and nasal culture over 10 days. Clinical signs varied among mice inoculated with different isolates of S. equi, ranging from lethargy, serous ocular discharge, and rhinitis to tachypnea and neurological alterations. Isolates from alleles seM-158 (G7), seM-39 (G8), and seM-271 (G5) were classified as highly virulent, frequently resulting in death or euthanasia, along with consistent bacterial excretion and enlargement of lymph nodes. Mice in G4 (seM-115), G6 (seM-124), G9 (no allele), and G10 (seM-28) showed moderately severe clinical signs of disease, whereas clinical signs for mice in G1 (seM-117), G2 (seM-61), and G3 (seM-123) were mild or absent. Results demonstrate that isolates of S. equi with different M protein alleles exhibit varying levels of virulence in mice, ranging from asymptomatic infection to severe illness and mortality. Additional investigations should be conducted to assess whether virulence in horses is associated with S. equi M protein variability and whether the association of M protein genotype with virulence is causal.