Risk factors associated with uterine fluid after breeding caused by Streptococcus zooepidemicus

Abstract

Infectious endometritis is a major cause of infertility in the mare and inflicts major losses on the equine breeding industry. The ability of the mare to eliminate uterine infections has been studied intensively for decades; however, despite identification of several factors contributing to the multifactorial pathogenesis and improved treatment, infectious endometritis remains a significant problem in a subpopulation of broodmares. Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is one of the most commonly isolated pathogens from the uterus of mares, suffering from infectious endometritis. Its ability to cause chronic latent infection by residing deep within the endometrial tissue has previously been described. The aim of the present study was to correlate different mare risk factors to infectious endometritis and pregnancy rates in broodmares and to investigate whether clonal or genetically distinct S. zooepidemicus strains isolated from mares with endometritis were associated with mare risk factors and the outcome of insemination. Mares (N = 152) were examined after natural cover, and 20% (31 mares) had intrauterine fluid (IUF) accumulation. Fifty-five percent (16 of 29) of the mares with IUF had infectious endometritis, and S. zooepidemicus was isolated in 81% (13 of 16) of these mares. Significantly more resting mares had IUF compared with foaling mares, and the foaling mares had the highest prevalence of positive bacterial growth from the uterine swab. The per-cycle pregnancy rate for the study was 63%. Pregnancy rate was significantly negatively affected by increased age (>12 years) and increased parity (>4), and the lowest pregnancy rate was observed in resting mares compared with foaling, barren, and maiden mares. Up to four S. zooepidemicus isolates were selected from each mare with growth of S. zooepidemicus from the uterine swab sample and further characterized by pulsed-field gel electrophoresis. In total, S. zooepidemicus isolates from 18 mares were analyzed with pulsed-field gel electrophoresis. In 13 mares, the isolates selected from each mare showed a high genetic relatedness within each individual mare, whereas two genetically distinct strains were isolated in five mares. A clonal S. zooepidemicus infection was associated with increased age, high parity, and poor vulvar conformation. Mares with clonal infection had a low pregnancy rate (38%) compared with mares with two strains isolated (80%). In conclusion, the results indicate that clonal S. zooepidemicus endometritis is associated with increased age, parity, and poor vulvar conformation in the broodmare.

Introduction

Endometritis inflicts major losses on the equine breeding industry, with a prevalence of 25% to 60% of barren mares [1], [2], [3]. The transient breeding-induced endometritis is a normal event in the immediate hours after breeding, and the process is considered necessary for effective removal of bacteria and excess spermatozoa introduced into the uterine lumen during breeding [4]. In the resistant mare, the uterine inflammation is cleared well before the fertilized oocyte descends from the oviduct into the uterine lumen at 5½ to 6 days after conception [5], whereas the uterus of the susceptible mare will remain persistently inflamed at this time point [4]. The ability of the mare to eliminate uterine infections has been studied intensively for several decades, and it is well accepted that the pathogenesis of persistent endometritis is multifactorial [6]. A key factor in the development of persistent uterine infection is the mare's susceptibility to infection and the pathogenicity of the bacteria.

Several risk factors associated with the individual mare have been identified. Increased age has been associated with altered systemic immune response in horses and other species, and it is well documented that age is a factor predisposing mares to develop persistent uterine infections after breeding [7], [8], [9]. Other factors associated with persistent infections after breeding include poor vulvar conformation [10], a defect in myometrial contractility [11] and thereby delayed uterine clearance [12], positive uterine culture before breeding [13], uterine fluid retention during estrus before breeding [14], and a ventrally positioned uterus [15].

The potentially pathogenic Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is commonly found on the mucus membranes of the caudal reproductive tract [16], [17] but is also the most frequently isolated pathogen from the uterus of mares with endometritis [13], [18], [19] and a significant contributor to abortion in mares [20], [21]. The prevailing hypothesis is that S. zooepidemicus from the caudal reproductive tract may cause an ascending infection primarily governed by compromised uterine defense mechanisms in the susceptible mare [22], but recent studies suggest that a genetic subpopulation of S. zooepidemicus is associated with infectious endometritis in the mare and that some strains are better adapted to survive and colonize in the endometrial environment than others [17], [23].

The aim of the present study was to evaluate the association between different mare risk factors and infectious endometritis in a population of thoroughbred broodmares. A second aim was to investigate whether clonal or genetically distinct S. zooepidemicus strains isolated from mares with endometritis were associated with mare risk factors and the outcome of insemination.