CHAPTER 21 Bacterial Causes of Subfertility and Abortion in the Mare
Bacterial infections of the uterus are the leading cause of subfertility and infertility in mares and represent a major economic loss to the equine industry.1 The equine breeding industry depends on the health of the uterus, and without a healthy uterus a mare cannot successfully conceive or carry a foal to term. The industry’s demand for early foaling dates is largely responsible for equine infertility. The mare’s natural breeding season is late May, June, July, and August. In the early spring mares are in reproductive transition and therefore estrus periods are irregular and prolonged. Ovulation is irregular and difficult to predict, and repeated examinations are required to detect it. More than one breeding is usually necessary, which results in added contamination of both the mare and stallion. Frequent uterine contamination and disease occur if a breeder does not implement a meticulous preventive management program to minimize uterine contamination. This program must include immediate recognition and treatment of infected mares.
Bacteria enter the uterus at foaling, breeding, during routine genital examinations, and through vulvar defects (pneumovagina). The uterus of a healthy mare is able to respond to contamination and evacuate transient bacterial contamination. If the uterus is contaminated at breeding, normal defense mechanisms eliminate bacteria without treatment and re-establish a normal environment before the embryo descends into the uterus. Older, multiparous mares and postpartum mares often require special attention and management because their natural defenses have been compromised, and uterine contamination in these susceptible mares often results in infection.2–6
Endometritis, which is inflammation of the endometrium, can occur with or without bacterial infection and can be acute or chronic. Most young mares normally resolve uterine contamination within 96 hours, and these mares are considered to be resistant to endometritis.7 Mares unable to clear bacterial contaminants from their uterus become susceptible, and this results in persistent inflammation. Susceptible mares may have other conformational defects, such as pneumovagina, urine pooling, and foaling injuries, that predispose them to endometritis.
Metritis is an inflammation or infection of the deeper layers of the uterus. Systemic signs of metritis are not seen with any degree of regularity but occur more frequently in postpartum mares. Draft mares seem to be particularly susceptible to septic metritis. Septic metritis may extend to the peritoneal surface of the uterus (perimetritis) and into the peritoneal cavity (parametritis). Clinical signs of septic metritis include depression, increased temperature (>39.4° C), loss of appetite, and laminitis. Septic metritis can be life-threatening and requires prompt treatment.
Pyometra is characterized by accumulation of purulent exudate in the lumen of the uterus with a closed cervix. The uterus is distended and the cervix usually has adhesions that prevent drainage. Progesterone concentrations are elevated as a result of a retained corpus luteum, and the uterus is unable to release prostaglandin F2 alpha (PGF2α). Mares do not show systemic signs. The majority of pyometra cases are diagnosed during routine examination.
Many different bacteria have been isolated from mares with uterine disease, but Streptococcus zooepidemicus followed by Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae are the most frequently isolated pathogens.8–10 Corynebacterium spp., Proteus spp., and Staphylococcus spp. are considered pathogenic when isolated from mares with cytologic evidence of uterine disease. Gardnerella vaginalis, an organism reported to cause vaginitis in humans, has been documented in mares11,12; however, biopsy samples indicated only mild endometritis, and the mares conceived and carried foals to term without antibiotic therapy, so the clinical significance is questionable.12 Taylorella equigenitalis is a pathologic, microaerophilic, gram-negative organism transmitted venereally and is the cause of contagious equine metritis (CEM). Bacteroides fragilis is an anaerobic organism that has been associated with acute endometritis at foal heat and in barren mares.13
Diagnosis of uterine disease is based on history, physical examination, vaginal examination, uterine cytologic examination, uterine culture, and uterine biopsy. It is imperative that the practitioner use all available tests and not rely on only one. Diagnosis of infectious endometritis is based on documentation of inflammation in the uterine lumen and accurate proof of an infectious organism.14 Endometritis may escape detection with vaginal examination, transrectal uterine palpation, and uterine ultrasonography. In these cases, the uterus is not enlarged and there is only a small amount of accumulated fluid or exudate. Endometrial cytologic examination is the diagnostic test of choice because it provides direct evidence of the presence of neutrophils in the uterine lumen. Any significant number of neutrophils confirms active inflammation.14 A uterine culture confirms the causative organism and the antibiotic sensitivity helps direct the treatment. The Accu-CulShure (ACCU-MED Corp., Pleasantville, NY) swab is an efficient tool for obtaining endometrial specimens. When culturing the endometrium, it is important to obtain a sample uncontaminated by the vestibule and vagina caudal to the urethral orifice because this area is usually heavily contaminated with normal flora. The Accu-CulShure swab is sealed inside the guard tube and resealed immediately following sample collection, so there is no contamination or exposure to air during entry or egress. When the instrument has been removed from the genital tract following specimen collection, the swab is slowly pulled into the self-contained transport medium. The protected swab can be snapped off at the score on the instrument, placed in a transport tube, and sent to a diagnostic laboratory. Metritis and pyometra are associated with uterine enlargement and fluid (purulent exudate) accumulation in the uterus. The fluid may extend to the cervical or vaginal area. Therefore, vaginoscopy, transrectal uterine palpation, and uterine ultrasonography clearly aid in diagnosis. A uterine culture identifies the causative organism, and a biopsy identifies the extent of the disease.
The goal of therapy is to remove or reduce the offending bacteria and enhance uterine defense mechanisms, thereby decreasing the inflammatory process within the uterus. This has been accomplished with intrauterine infusions of antibiotics, antiseptics, plasma, sugars, uterine lavage, and systemic antibiotic therapy. Therapy can be completed before or after breeding, depending on the severity of the infection. It is important to tailor the therapy to the individual case and to remember that no single treatment can be effective in all cases. With severe infections, multiple therapeutic approaches can be used to control the infection. This discussion attempts not to define a course of treatment but to illustrate successful and commonly used treatments. Every mare is an individual and the routine treatment of all mares with the same medication will not result in the best treatment.15
A common form of therapy is intrauterine infusion of antibiotics, chemicals (antiseptics), plasma,16 and sugars.17,18 Medications are infused by gravity directly into the uterus, thus contacting the entire surface of the endometrium. Intrauterine infusion concentrates medication locally in the infected endometrium as opposed to systemic treatment, which relies on blood concentration of the drug.
For intrauterine antibiotic therapy, a drug should be selected on the basis of sensitivity tests. Broad-spectrum antibiotics might not be as effective as bactericidal antibiotics. Bacteriostatic drugs require assistance from the mare’s defense system, which is likely compromised as indicated by the presence of infection.16 Drug combinations should be avoided because incompatibility often renders the drugs ineffective. The same is true of drug and antiseptic combinations.15 The antibiotics most commonly used for intrauterine therapy are penicillin, gentamicin, ampicillin, streptomycin, chloramphenicol, nitrofurazone, polymyxin B, neomycin, the sulfonamides, and amikacin.15,19 Gentamicin, amikacin, ampicillin, and chloramphenicol are the most effective against the majority of pathogenic organisms. Penicillin is thought to be one of the most effective antibiotics available for treating equine uterine infections.15
Antibiotics infused into the uterus are dissolved or suspended in sterile water or saline solution and infused directly into the uterus daily for 3 to 5 days or more during estrus.20 Antibiotic treatment after service is advised for mares with impaired uterine resistance that cannot cope with microorganisms introduced at breeding. Antibiotics can be instilled into the uterus up to 3 days after ovulation, but treatment may be wasted if the mares require a second breeding.21 The size of the uterus, determined by transrectal palpation, should be used to determine the volume infused. The uterine capacity of maiden mares is approximately 35 ml, and that of older mares may be between 60 and 150 ml.16 Traditionally, the antibiotic is suspended in large volumes of fluid (100– 500 ml) on the assumption that this volume is optimal for filling the uterus and coating the endometrial surface; however, the effects of such dilution on antibiotic efficacy are unknown. Present evidence suggests that most of the antibiotic introduced in this manner is expelled through the cervix soon after treatment.16,20,21 Intrauterine antibiotic therapy is not without risk. Indiscriminate use of antibiotics may alter the normal uterine flora and result in development of resistant strains of bacteria, yeasts, and fungi.5,15 Some mares are sensitive to particular antibiotics.20 Caution must be used to ensure that therapy does not pose a greater threat than infection. The use of products not approved for intrauterine infusion can cause harmful effects in mares.15
For uterine lavage, large volumes of saline solution or sterile water are infused into the uterus and then allowed to drain. The rationale for this therapy is enhancement of cellular and mechanical aspects of uterine defense mechanisms.2 The therapeutic effects of lavage are mechanical removal of bacteria and debris from the uterus; stimulation of uterine contractions, thus aiding expulsion of foreign material; and mild endometrial irritation resulting in migration of neutrophils and perhaps serum-derived opsonins to the uterine lumen.4,20 Uterine lavage has also been reported to stimulate blood flow, improve tone, and decrease uterine size.6 When the fluid is heated to 40° to 50° C, lavage has proved beneficial in increasing myometrial tone and uterine circulation in older, multiparous mares with thick-walled or pendulous uteri.2,6,20,22 Lavage solutions are infused into the uterus in 1-L increments either by gravity or mechanically and then siphoned into a clear, graduated container.20 Inspection of the recovered fluid provides immediate information concerning uterine health or the success of uterine treatments. The degree of cellularity and concentration of other inflammatory components correlates well with the appearance of the recovered fluid.2
Antiseptics used historically for treatment of uterine disease included acriflavine, bismuth subnitrate, boric acid, charcoal, chlorine, iodine and iodine solutions, iodoform, perboric acid, silver oxide, hot saline and hypertonic saline, sodium hypochlorite, gentian violet, and hydrogen peroxide. Weak solutions of hydrogen peroxide have been used as treatment for acute endometritis and also appear helpful when exudate has been found in the uterine lumen.16 After phagocytosis by neutrophils, bacteria are destroyed in part by oxidative metabolism, which includes hydrogen peroxide.22 Lugol’s solution (10%) has been reported to be successful for chemical curettage of the endometrium.6,23
Care should be taken when infusing antiseptics not to inflict further damage with harsh or concentrated chemicals. A mare’s uterus is sensitive to irritating substances.6,18 Chlorhexidine suspension should be avoided or used cautiously because it causes tissue necrosis.6,16,24 Oxytetracycline powder can also induce severe tissue necrosis. The endometrium recovers well from some irritants, but repeated use may leave a fibrotic endometrium and possibly adhesions.24
Antibiotics for treatment of uterine infections can be administered systemically. Opinions are divided on the success of systemic administration compared to intrauterine therapy. The majority appear to favor intrauterine infusion, especially for treatment of endometritis. Infused antibiotics are believed to be more effective because of direct contact of drugs with the infected endometrium. Systemic therapy has its supporters, and there are reports of success in treating endometritis systemically.8,15,23 However, objective comparison between systemic and local antibiotic therapy is difficult without controlled trials.8 The main question to be addressed when considering antibiotic therapy is which tissues are involved. If the infection includes deeper layers of the uterus or other genital organs, then systemic therapy might be indicated. Some researchers believe that systemic administration results in higher antibiotic concentrations in infected tissues. Metritis, pyometra, and perimetritis may warrant systemic treatment.15,16,23
Systemic antibiotic therapy does have advantages. It can be conducted without invading the reproductive tract and treatment can be continued into the diestrous phase of the cycle, whereas postbreeding intrauterine therapy must stop before the embryo enters the uterus. Systemic therapy also enjoys an ease of application.20,25 A disadvantage is the cost. Systemic therapy requires a higher dose to achieve effective blood concentrations.20,24 Based on satisfactory results reported by practitioners, antibiotics suitable for systemic treatment include amikacin sulfate, ampicillin, gentamicin, procaine penicillin, and trimethoprim sulfa.20
Plasma or colostrum augmentation of a mare’s natural defense mechanism is another avenue of uterine disease treatment. Plasma therapy is often indicated when no causative agent is identified, when sensitivity tests indicate impractical drugs, or when previous therapy has been unsatisfactory.20 The goal of plasma therapy is to enhance the local uterine immune response and natural defense mechanism. Failure or incompetence of these responses probably accounts for mares that are unable to cope with contaminating organisms.6 These natural mechanisms are defined as cellular (phagocytes) and noncellular (opsonins, thermal stabile factors, and the leukocytic tide).6 The uterus of resistant mares contains opsonins, which function to allow efficient phagocytosis of contaminating bacteria by neutrophils. It is believed that susceptible mares are deficient in opsonins, which allows bacteria to become established.8 Plasma is the major source of proteins involved in opsonization of bacteria in the uterus.20 It has been suggested that a practical method to overcome ineffective uterine defense mechanisms is to infuse plasma or colostrum, substances that contain antibodies, into the uterine lumen.6
Studies have utilized both homologous and heterologous plasma. The use of heterologous plasma would be convenient, but the possibility of uterine sensitization to foreign protein cannot be ruled out.22 Antibiotic and plasma therapy can be used in concert, but some antibiotics are more compatible with phagocytic processes than others. Studies have found that in concentrations commonly used for intrauterine infusion, amikacin sulfate and gentamicin sulfate inhibit neutrophil phagocytosis, and potassium penicillin and ticarcillin do not.14,22 More work is needed in this area.