Subclinical endometritis by Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is a common finding in barren mares. Uterine instillation of bActivate, a bacterial growth medium, can induce active growth and allow detection of S. zooepidemicus residing within the endometrium1.
In the present investigation we aimed to determine if activation followed by antimicrobial treatment improved fertility.
To investigate the effect of activation and treatment, broodmares barren for at least three reproductive cycles despite having been managed by dedicated veterinarians and bred to fertile stallions, were included. A uterine diagnostic sample (pre-activation) was obtained followed by uterine instillation of bActivate. The following day a post-activation sample was recovered. In mares with bacterial growth treatment with uterine lavage and antimicrobials combined with systemic antimicrobials was initiated. The mares were bred in the following estrus. The pregnancy status was recorded during the two subsequent breeding cycles. The foaling rate was compared to data from a reference mare population2.
A total of 47 barren broodmares were instilled with bActivate. Active growth of S. zooepidemicus was induced in 57% (27/47) of the mares. Pregnancy was established in 77% (36/47) mares and 57% (27/47) had a live foal. The pregnancy and foaling rates were 81%(22/27) and 59%(16/27), respectively, if active growth of S. zooepidemicus was induced and treated. For mares negative for streptococcal growth: pregnancy 70%(14/20) and foaling rate 50%(10/20), respectively. For mares barren despite >5 breeding attempts at enrolment the foaling rate was significantly higher in the group where active growth of S. zooepidemicus was induced and treated compared to non-treated mares (P<0.0001).
In perspective we demonstrated that subclinical S. zooepidemicus infections are very common in barren mares; that they have a profound negative effect on fertility and that long standing barren mares can resume active breeding upon activation and clearing of the infection.