CHAPTER 44 Postpartum Uterine Infections
Uterine infections are common disorders affecting dairy cows during the postpartum period. The literature is replete with case reports and retrospective and prospective studies suggesting that uterine infections decrease milk yield and subsequent reproductive performance. In these reports, however, no differentiation was made among the types of infection, and uterine infection was classified as a disease complex with clinical manifestations ranging from minimal or none to life-threatening sepsis. Consequently, a divergence of opinion continues to exist among veterinarians regarding diagnosis and treatment of uterine infections.1
The uterus normally is protected from bacterial contamination by the vulva, vestibular sphincter, and cervix. During and immediately after parturition, these mechanical barriers are breached and the uterus normally is contaminated by a variety of pathogenic and nonpathogenic microorganisms. Most of these bacteria are merely transient residents and are promptly eliminated by the uterine defense mechanism during the puerperium. In some cases, however, pathogens persist in the uterus and cause disease. The organism most commonly associated with uterine disease in cattle is Actinomyces pyogenes.2, 3 In addition, the gram-negative anaerobes Fusobacterium necrophorum and Bacteroides melaninogenicus frequently are associated with A. pyogenes. Bacteroides decreases chemotaxis and inhibits phagocytosis by neutrophils, allowing A. pyogenes to persist.4 A variety of other microorganisms occasionally are associated with uterine disease in cows and include coliforms, Pseudomonas aeruginosa, staphylococci, hemolytic streptococci, and others. Clostridium spp. occasionally infect the uterus and cause severe gangrenous metritis or tetanus. Some of the organisms that transiently contaminate the uterus during the postpartum period produce penicillinase; this should be a consideration in the selection of drugs and routes of administration used in treating the disease. In cows with a normal puerperium, the uterus is nearly free of bacterial contamination by 4 weeks after calving.5
Uterine infections are associated with retained fetal membranes (RFM), dystocia, delivery of twins, overconditioning, underconditioning, long-term feeding of urea to dry cows, and large herd size.6-11 In one study, severe uterine infections frequently followed manual removal of retained membranes.12 Unsanitary calving conditions and traumatic obstetric procedures predispose cows to uterine infections. Because of differences in their calving environment, postpartum uterine infections more commonly affect dairy cows than beef cows.
Definition of uterine infections has considered character of uterine discharge, days postpartum, clinical findings, and endocrine status.1, 13 Unfortunately, clinicians and researchers have vaguely applied terms such as metritis and endometritis when describing uterine infections, which has contributed to confusion among veterinarians in the definition and economic impact of uterine infections. Therefore, as suggested in the review by Bondurant,13 definitions used by theriogenologists and pathologists should be applied in describing uterine infections, in order to avoid confusion. Specific definitions of related entities are presented next.
Metritis is a result of a severe inflammation involving all layers of the uterus (endometrial mucosa and submucosa, muscularis, and serosa).13 It generally develops during the first week after calving and is associated with dystocia, RFM, and calving trauma. Affected cows may be septic and present with fever, depression, anorexia, and reduced milk yield (Olsen). In addition, a copious fetid vaginal discharge may be present. Other terms used to describe metritis have included “acute puerperal metritis” and “toxic metritis.”
Endometritis is characterized by inflammation of the endometrium extending no deeper than the stratum spongiosum.13 This condition may follow parturition, copulation, artificial insemination, or infusion of irritants into the endometrial cavity. Usually, presence of a purulent exudate is noticed on visual inspection of the vulva. The affected animal is rarely systemically ill, and the uterus is found to be normal on palpation. Acute endometritis typically is a temporary condition, and after several estrous cycles, the offending bacteria usually are eliminated. In chronic cases of endometritis, a purulent vaginal discharge may persist. Affected cows may have a lower first-service pregnancy rate than their herdmates and require more time to conceive.14
Pyometra is characterized by a collection of purulent exudate of variable amount within the endometrial cavity, persistence of a corpus luteum, and suspension of the estrous cycle.15 This condition is most likely to develop in cows that have their first postpartum ovulation before bacterial contamination of the uterus has been eliminated. The ensuing corpus luteum persists beyond its normal lifespan because intrauterine fluid prevents luteolysis. Progesterone continues to dominate the uterus and suppresses the uterine defense mechanism.16 Pyometra also is an occasional clinical sign of trichomoniasis, and Tritrichomonas fetus should be suspected as a cause of pyometra that develops during the breeding season.
The lactation incidence of postpartum uterine infection varies among reports and with criteria used for diagnosis. In a large retrospective study, metritis complicated 13.8% of lactations,17 whereas the authors of a review of published literature found a mean incidence of 17.4% and a range of 8.5% to 24.2%.18 Published reports are very difficult to interpret, however, because of the inordinate variety of criteria used to define uterine infections. Furthermore, it is likely that misinterpretation of the normal postpartum discharge from reproductive tracts of cows leads to inflated estimates of incidence of the disease.19
Clinical signs of uterine infection vary with the virulence of the causative organism and the presence of factors that predispose to the disease. Lochia normally is expelled from the reproductive tract during the first few weeks after calving, and the discharge may persist for up to 30 days if uterine involution is delayed. Normal discharges range in color from dark brown to red to white; discharge usually should not be considered abnormal unless the fluid is fetid or clinical signs of sepsis are present.
Examination of the uterus by palpation through the rectal wall commonly is used to evaluate the degree of involution before breeding. Palpation may not be useful to diagnose abnormalities of uterine involution during the first 2 weeks post partum, however. When involution is delayed, the uterine wall may be atonic and lack the longitudinal rugae (involution lines) typical of a normal uterus. In cases of metritis, the uterus is swollen and friable, and occasionally fibrin deposits and adhesions between the uterus and other organs or the body wall may be palpable. If involution is normal, fluid should not be palpable within the uterine lumen by 14 to 18 days after calving. In one series, cows in which a uterine lumen was palpable at the postpartum examination were likely to have pathologic changes that suggested delayed uterine involution or permanent uterine damage.20
A common method used for diagnosis of endometritis combines palpation of the uterus and visual inspection of the vulva for a purulent exudate. The size and consistency of the uterus and cervix, along with evaluation for the presence of fluid within the lumen of the uterus, are criteria used to diagnose endometritis. Visual inspection may reveal crust of a purulent nature formed in the perineal region. In addition, a purulent exudate may be present at the ventral commissure of the vulva.
Palpation per rectum is, however, neither a sensitive nor a specific method for accurate diagnosis of endometritis. In one trial, 157 cases of endometritis were diagnosed by palpation, but bacteria were isolated from uterine fluid in only 22% of the samples taken.21 By contrast, evaluation of cervical size and the presence of a purulent discharge may be of diagnostic value. In a study that involved lactating dairy cows, diagnosis of endometritis was based on the presence of a purulent uterine discharge or cervical diameter greater than 7.5 cm after 20 days post partum or a mucopurulent discharge after 26 days post partum.22 This definition was based on the negative effect that these criteria have on subsequent fertility.
Observation of purulent exudate with the aid of a vaginal speculum has been reported to be a useful tool for diagnosis of subacute and chronic endometritis and for evaluation of response to treatment.3, 22 A prevalence of 16.9% of endometritis was found in one study, and vaginoscopy was required to identify a purulent discharge in 44% of the cases.22 These findings suggest that visual inspection of the vulva or palpation of the uterus may not be sufficient to identify a purulent content for a confident diagnosis of endometritis. Unfortunately, the requirements for sterile and disposable specula and for aseptic preparation of the perineum and external genitals make this procedure cumbersome when cows are examined in free-stalls or lock-up stanchions.
Real-time ultrasonography has been used to demonstrate uterine changes associated with postpartum infections.23, 24 The intrauterine fluid associated with uterine infections contained echogenic particles and was easily distinguished from clear nonechogenic fluid associated with estrus and pregnancy. In addition, the uterine wall of cows affected with uterine infections was variably thickened.
In cows with septic metritis, a degenerative left shift and marked neutropenia may occur. Hypocalcemia, which can occur early post partum, may accompany metritis. An association between ketosis and metritis has been reported in dairy cows.25, 26 Furthermore, high blood concentrations of nonesterified fatty acids (NEFAs) in cows have been demonstrated to impair lymphocyte function in vitro.27 Thus, fat mobilization with increased blood concentrations of NEFAs may explain the higher incidence of infections observed in cows that experience an energy deficit early post partum.
Guarded culture instruments can be passed through the cervix and samples of intrauterine fluid obtained for bacterial culture. Samples should be cultured in both aerobic and anaerobic environments. Under most circumstances, however, samples from individual animals are rarely submitted for bacterial culture because a decision to treat usually is made before results of culture are available. The cost of the procedure can only rarely be justified. A. pyogenes and gram-negative anaerobes usually are assumed to be the most likely causative organisms. Bacterial culture and antibiotic sensitivity tests may be indicated on farms that have an unusually high incidence of uterine infection, or if cows fail to respond to treatment.28
Evaluation of endometrial biopsy specimens has not become as common in cows as in mares.29 An association between isolation of A. pyogenes and endometrial lesions has been demonstrated.30 Uterine biopsies at days 26 and 40, however, are reported to have a detrimental effect on future fertility.31
Neutrophils are the primary response against pathogenic bacteria of the postpartum uterus, resulting in an increase in polymorphonuclear (PMN) cells within the uterine lumen. Evaluation of the PMN count through endometrial cytology has been successful in identifying dairy cows with endometritis.32 In clinically normal cows (without evidence of an abnormal discharge), subclinical endometritis was defined as greater than 18% PMN leukocytes during days 20 to 33 post partum and as greater than 10% during days 34 to 47 post partum. Cows with subclinical endometritis were less likely to become pregnant than cows without subclinical endometritis.