Case-control study of late-term abortions associated with mare reproductive loss syndrome in central Kentucky

OBJECTIVE: To identify factors associated with abortions of mares during late gestation attributed to mare reproductive loss syndrome (MRLS). DESIGN: Case-control study. ANIMALS: 282 broodmares from 62 farms in central Kentucky, including 137 mares that had late-term abortions (LTAs) associated with MRLS, 98 mares from the same farms that did not abort, and 48 mares that aborted from causes other than MRLS. PROCEDURE: Farm managers were interviewed to obtain data on a wide range of management practices and environmental exposures for the mares. Data for case and control horses were compared to identify risk factors for a mare having a MRLS-associated LTA (MRLS-LTA). RESULTS: Several factors increased the risk of mares having MRLS-LTAs, including increased amount of time at pasture, less time in a stall, feeding concentrate on the ground, higher proportion of diet derived from grazing pasture, being fed in pasture exclusively during the 4-week period prior to abortion, access to pasture after midnight during the 4-week period prior to abortion, and drinking from a water trough or not having access to water buckets or automatic waterers. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis indicates that exposure to pasture predisposed mares to having MRLS-LTAs and stillborn foals. Methods for limiting exposure to pasture (keeping mares in stalls longer) during environmental conditions similar to those seen in 2001 should reduce the risk of mares having MRLS-LTAs.     

Descriptive epidemiology of late-term abortions associated with the mare reproductive loss syndrome in central Kentucky.

Epidemiological and pathological findings of 433 late-term abortions associated with the mare reproductive loss syndrome (MRLS) in central Kentucky were identified by reviewing the records of the University of Kentucky Livestock Diseases Diagnostic Center. The distribution of dates of abortion was clustered during a brief period of time, presumably from a simultaneous environmental exposure. The most common pathological findings were microscopic pulmonary lesions consisting of squamous epithelial cells present in alveoli with or without concurrent infiltration of inflammatory cells (neutrophils, macrophages, or monocytes) in the interstitium or within alveoli. Isolation of a non-beta-hemolytic Streptococcus (52% of fetuses) or an Actinobacillus sp. (19% of fetuses) was common. Placentitis or funisitis was identified in 44% of fetuses. No single pathological finding, however, was pathognomonic for MRLS-associated late-term abortion. This report describes the pathological findings characterizing the MRLS-associated abortion. A cause of MRLS could not be determined from necropsy findings.     

REVIEW paper: mare reproductive loss syndrome

An epidemic of early fetal loss (EFL), late fetal loss (LFL), fibrinous pericarditis, and unilateral uveitis which occurred during the spring of 2001, are together now known as the mare reproductive loss syndrome (MRLS). A similar epidemic with less intensity was reported during the same period of time from southern Ohio, West Virginia, and Tennessee. The same syndrome with lesser intensity recurred in 2002. The estimated economic loss from the syndrome in 2001 and 2002 together was approximately $500 million. Both EFL and LFL were characterized by the absence of specific clinical signs in aborting mares. Nonhemolytic Streptococcus spp. and Actinobacillus spp. accounted for 65% of the organisms isolated from fetuses submitted for a postmortem during the MRLS period in 2001 and 2002. The pathologic findings in fetoplacental units of LFL included bronchopneumonia and funisitis, and there were no findings in EFL. Epidemiologic studies conducted in 2001 suggested an association between the presences of eastern tent caterpillars (ETC) in pastures with MRLS. Experimental studies in pregnant mares by exposure to ETC, or administration by stomach tube or with feed material, reproduced EFL and LFL. Similar experimental studies in mouse, rats, and goats with ETC were unsuccessful. Currently, 2 hypotheses are proposed for MRLS. One hypothesis proposes that an ETC-related toxin with secondary opportunistic bacterial invasion of the fetus leads to MRLS. The second hypothesis suggests that a breach of gastrointestinal mucosal integrity by hairs of ETC leads to a bacteremia and results in MRLS. In 2004, a similar equine abortion storm was reported from Australia and caterpillar exposure was identified as a risk factor for the abortion. In 2006, the syndrome was observed in Florida and New Jersey.     

Case-control study of early-term abortions (early fetal losses) associated with mare reproductive loss syndrome in central Kentucky

OBJECTIVE: To identify factors associated with abortions during early gestation classified as mare reproductive loss syndrome (MRLS). DESIGN: Case-control study. ANIMALS: 324 broodmares from 43 farms in central Kentucky, including 121 mares from 25 farms that had early-term abortions (ETAs) associated with MRLS (case horses), 120 mares from the same farms but that did not abort, and 83 mares from 18 farms that were not severely Impacted by MRLS. PROCEDURE: Farm managers were interviewed to obtain data on various management practices and environmental exposures for the mares. Data for case and control horses were compared to identify risk factors for mares having MRLS-associated ETAs. RESULTS: Several factors increased the risk of MRLS-associated ETAs, including feeding hay in pasture, greater than usual amounts of white clover in pastures, more eastern tent caterpillars in pastures, abortion during a previous pregnancy, and sighting deer or elk on the premises. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis indicates that certain characteristics of pastures predisposed mares to MRLS-associated ETAs. Methods for limiting exposure to pasture (keeping mares in stalls longer) during environmental conditions similar to those of 2001 (ie, sudden freezing in mid-April following warmer-than-usual springtime temperatures and larger-than-usual numbers of eastern tent caterpillars in and around pastures) should reduce the risk of mares having MRLS-associated ETAs. It was not possible to determine whether exposure to white clover or caterpillars were causal factors for MRLS or were merely indicators of unusual environmental conditions that resulted in exposure of mares to a toxic or infectious agent.     

An alternative model for fetal loss disorders associated with mare reproductive loss syndrome

Fertile chicken eggs were used as an alternative model for large animals to evaluate suspect toxic dietary ingredients for fetal loss disorders associated with mare reproductive loss syndrome (MRLS) and fetal losses in other livestock. Nitrate, ammonia, and sulfate may react with proteinaceous compounds to enable the formation of abiotic pathogenic nanoparticles which were constant findings in pathognomonic placental lesions associated with non-infectious fetal losses of previously unknown etiology in mares, chickens and other livestock. The pathogenic nanoparticles may be produced naturally by toxic elements associated with air pollution that affect pasture forages or crops, unintentionally by reactions of these elements in protein-mineral mixes in dietary rations, or endogenously within tissues of fetuses and adult animals. The nanoparticles may form niduses in small vessels and predispose animals to a host of secondary opportunistic diseases affecting the reproductive, respiratory and gastrointestinal tracts of animals. The newly recognized abiotic pathogenic micro and nanoparticles are associated with MRLS. The discovery of the pathogenic nanoparticles led to the identification of nitrate, ammonium, and sulfur, in the form of sulfate, that seemingly enable the formation of the pathogenic nanoparticles in embryonic and fetal tissues.     

Temporality of early-term abortions associated with mare reproductive loss syndrome in horses

OBJECTIVE: To characterize the temporality of dates of breeding and abortion classified as mare reproductive loss syndrome (MRLS) among mares with abortions during early gestation. ANIMALS: 2,314 mares confirmed pregnant at approximately 28 days after breeding from 36 farms in central Kentucky, including 515 mares that had early-term abortions. PROCEDURE: Farm veterinarians and managers were interviewed to obtain data for each mare that was known to be pregnant to determine pregnancy status, breeding date, last date known to be pregnant, and date of abortion. RESULTS: Mares bred prior to April 1, 2001, appeared to be at greatest risk of early-term abortion, both among and within individual farms. Mares bred in mid-February appeared to be at greatest risk of abortion, with an estimated weekly incidence rate of abortion of 66% (95% CI, 52% to 80%). CONCLUSIONS AND CLINICAL RELEVANCE: Mares in central Kentucky bred between mid-February and early March were observed to be at greatest risk of early-term abortion, and risk gradually decreased to a background incidence of abortion of approximately 11%. Mares bred after April 1, 2001, appeared to be at markedly less risk, indicating that exposure to the cause of MRLS likely occurred prior to this date.     

Case-control study of factors associated with excessive proportions of early fetal losses associated with mare reproductive loss syndrome in central Kentucky during 2001

OBJECTIVE: To identify factors associated with excessive proportions of early fetal losses associated with mare reproductive loss syndrome in central Kentucky during 2001. DESIGN: Case-control study. PROCEDURE: Questionnaires were used to collect information on farm-, pasture-, and individual animal-level factors purportedly associated with mare reproductive loss syndrome. Data were collected for 133 farms (97 with excessive proportions of early feta losses and 36 control farms) representing 6,576 mares. RESULTS: Factors significantly associated with an increased risk of excessive early fetal losses were exposure to moderate to high concentrations of Eastern tent caterpillars, exposure to cherry trees, farm size > or = 50 broodmares, being bred during February 2001, and frequent exposure to waterfowl. Feeding hay to mares outside was associated with a decreased risk of excessive proportions of early fetal losses. Pasture composition and management factors were not significantly different between affected and control pastures. Individual animal-level factors were investigated on 6 farms representing 340 mares, and age, parity, and pre- and postbreeding treatments were not significantly associated with risk of early fetal loss. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that limiting exposure to Eastern tent caterpillars and cherry trees and feeding hay to mares outside may help decrease the risk of excessive proportions of early fetal losses associated with mare reproductive loss syndrome.     

Classification of Actinobacillus spp isolates from horses involved in mare reproductive loss syndrome

OBJECTIVE: To identify Actinobacillus spp isolates recovered from fetuses and pericardial fluid from horses affected with mare reproductive loss syndrome (MRLS) and determine whether these bacterial species are the same as those isolated from clinically normal horses. SAMPLE POPULATION: Isolates of actinobacilli recovered from 18 horses with pericarditis and 109 fetuses aborted by mares affected by MRLS. Procedures-Actinobacillus spp isolates were identified to the level of species or subspecies by use of conventional phenotypic tests and biochemical and enzyme test kits. The 16S rRNA gene from selected isolates was amplified, purified, and sequenced. Sequence data were compared with sequence data for actinobacilli in GenBank. RESULTS: Of the 109 isolates obtained from fetuses, 14 were Actinobacillus equuli subsp equuli, 65 were A equuli subsp haemolyticus, 28 were Bisgaard taxon 10-like bacterium, and 2 were Actinobacillus genomospecies 1. Of the 18 isolates from horses with pericarditis, 4 were A equuli subsp equuli, 13 were A equuli subsp haemolyticus, and 1 was Bisgaard taxon 10-like bacterium. Comparisons with published data and GenBank data revealed that the isolates recovered from horses with MRLS were the same as those isolated from the oral cavity or alimentary tract of healthy horses. CONCLUSIONS AND CLINICAL RELEVANCE: Actinobacillus spp isolates recovered from fetuses and pericardial fluid samples of horses affected by MRLS in 2001 to 2003 were identical to Actinobacillus spp found in the oral cavity and alimentary tracts of healthy horses.     

母马生殖损失综合征研究现状及其在中国流行的可能性

母马生殖损失综合征(MRLS)是指由天幕毛虫相关因素引起的孕马早期胎儿流产(EFL)、晚期胎儿流产(LFL)、纤维素性心包炎(FP)和单侧葡萄膜炎(UU)等综合症状的疾病。2001年,美国首次报道该病,该病的暴发对美国养马业造成了巨大的经济损失。流行病学调查研究表明,母马生殖损失综合征在牧场间的传播与东方天幕毛虫(ETC)有关。其原因可能为:天幕毛虫毒素中毒并发细菌二次感染或天幕毛虫通过胃肠道黏膜进入血液导致菌血症,从而引发母马生殖损失综合征。在中国,天幕毛虫分布广泛,尤其是每年的春夏季节是天幕毛虫大量繁殖的季节。据大量报道表明,中国母马流产多发生在4~6月份天幕毛虫活动频繁的季节,而这些马流产病例中是否存在由天幕毛虫引起的母马生殖损失综合征尚未知。因此,我们对母马生殖流产综合征的流行现状以及临床症状的研究现状做了首次中文综述,希望对国内马流产的临床诊断、流行病学研究有所借鉴。     

Theory of ammonia toxicity as the mechanism of abortion in the mare reproductive loss syndrome

In late April/early May of 2001, for the equine industry in Kentucky, a previously unknown factor led to the loss of up to 40% of the new pregnancies (20-60 days) for that year, with a following significant number of late abortions and ill new born foals. In addition, encephalopathy, ocular lesions, pericarditis, laminitis, and other diseases were suspected to be associated with the syndrome. This occurrence was known as the mare reproductive loss syndrome (MRLS). On examination of the weather data, this period was seen to have an unusual pattern of severe late spring frost. Acute changes in the pasture diet affected the horses gut microflora, allowing the overgrowth of opportunistic pathogens and resulting in the excessive absorption of nitrogen and ammonia from the large bowel. Clinical blood chemistries from the horses on pasture at this time showed abnormally high levels of blood urea nitrogen (BUN). Elevated blood ammonia levels were found in mares and newborn foals affected with MRLS. Ammonia is metabolically one of the most toxic substances to the body. It is present naturally as a breakdown product of nitrogen turnover. If the body nitrogen balance is disturbed for any reason, with nitrogen accumulation exceeding the body's ability to excrete it as urea (BUN), ammonia will accumulate in toxic levels. Ammonia toxicity can cause all the symptoms that were observed in the horses exhibiting MRLS.     

Use of a hand-assisted laparoscopic technique for removal of a uterine leiomyoma in a standing mare

A 20-year-old Thoroughbred mare was evaluated because of a 2-year history of infertility. The mare had normal estrous cycles and had been bred 7 times by different stallions. Ultrasonographic examination revealed a homogeneous hyperechoic intramural mass in the tip of the right uterine horn; the mass was also detected via hysteroscopy Unilateral ovariectomy and partial hysterectomy were performed by use of a hand-assisted laparoscopic technique. Leiomyoma was diagnosed via histologic examination of the mass. Unilateral ovariectomy and partial hysterectomy are recommended in mares with leiomyoma in a uterine horn, especially if the tumor is associated with infertility. The hand-assisted laparoscopic technique allows direct visualization of abdominal structures and accurate placement of ligatures without applying tension on the broad ligament, and eliminates the risks and costs of general anesthesia.     

Impact of reproductive efficiency over time and mare financial value on economic returns among Thoroughbred mares in central Kentucky

Reason for performing study: There have been no studies reporting the impact of reproductive efficiency and mare financial value on economic returns. Objective: To explore the economic consequences of differences in reproductive efficiency over time in the Thoroughbred mare. Methods: Complete production records for 1176 mares were obtained. Production history and drift in foaling date were calculated. Multiple logistic regression was used to identify factors influencing the probability of producing a registered foal in 2005. The ‘net present value’ and ‘internal rate of return’ were calculated for economic scenarios involving different initial mare financial values, levels of reproductive efficiency, and durations of investment. Results: Among mares that did not produce a foal every year (63%), the mean time before failing to produce a registered foal was 3.4 years. The majority of mares drifted later in their foaling dates in subsequent foaling seasons. Increasing mare age, foaling after 1st April, needing to be mated multiple times during the season, and producing a lower number of foals in continuous sequence during previous years decreased the probability of producing a registered foal. Over a 7 year investment period, live foals must be produced in all but one year to yield a positive financial return. Profitability was highest among mares of greatest financial value. Conclusions: Mares are long‐term investments due to the extended period before there is a return on the investment. Improving our understanding of mare, stallion and management factors that affect the likelihood of producing a live foal are critical to ensuring a positive financial return. Additional work is needed to test the robustness of the study's conclusions when the cost and revenue assumptions are varied. Potential relevance: This information can assist in assessing mare profitability and developing management strategies to maximise profitability.     

The mare reproductive loss syndrome and the eastern tent caterpillar: a toxicokinetic/statistical analysis with clinical, epidemiologic, and mechanistic implications

During 2001, central Kentucky experienced acute transient epidemics of early and late fetal losses, pericarditis, and unilateral endophthalmitis, collectively referred to as mare reproductive loss syndrome (MRLS). A toxicokinetic/statistical analysis of experimental and field MRLS data was conducted using accelerated failure time (AFT) analysis of abortions following administration of Eastern tent caterpillars (ETCs; 100 or 50 g/day or 100 g of irradiated caterpillars/day) to late-term pregnant mares. In addition, 2001 late-term fetal loss field data were used in the analysis. Experimental data were fitted by AFT analysis at a high (P <.0001) significance. Times to first abortion ("lag time") and abortion rates were dose dependent. Lag times decreased and abortion rates increased exponentially with dose. Calculated dose x response data curves allow interpretation of abortion data in terms of "intubated ETC equivalents." Analysis suggested that field exposure to ETCs in 2001 in central Kentucky commenced on approximately April 27, was initially equivalent to approximately 5 g of intubated ETCs/day, and increased to approximately 30 g/day at the outbreak peak. This analysis accounts for many aspects of the epidemiology, clinical presentations, and manifestations of MRLS. It allows quantitative interpretation of experimental and field MRLS data and has implications for the basic mechanisms underlying MRLS. The results support suggestions that MRLS is caused by exposure to or ingestion of ETCs. The results also show that high levels of ETC exposure produce intense, focused outbreaks of MRLS, closely linked in time and place to dispersing ETCs, as occurred in central Kentucky in 2001. With less intense exposure, lag time is longer and abortions tend to spread out over time and may occur out of phase with ETC exposure, obscuring both diagnosis of this syndrome and the role of the caterpillars.     

Testicular feminization syndrome in a mare.

Testicular feminization syndrome was diagnosed in a mare with aggressive, stallion like behavior and a history of infertility. She was found to have a high baseline testosterone concentration suggesting that testicular tissue was present, and ovarian-like structures examined by use of transrectal ultrasonography had the appearance typical of testicular tissue. Although her external female genitalia appeared normal, her vagina ended in a blind sac, and no cervix or uterus were identified. Surgery was performed, and structures removed from the abdominal cavity were determined to be hypoplastic testicles. Removal of the testicular tissue resulted in complete resolution of her aggressive behavior. Chromosomal evaluation revealed that the mare had 64X,Y (normal male) karyotype. Testicular feminization syndrome is a condition characterized by insensitivity of reproductive tissues to androgens during development because of an abnormality in androgen receptors. This androgen insensitivity results in development of normal external female genitalia, with high testosterone concentrations being released from developing testicles. Testicular feminization syndrome has not been commonly diagnosed in horses, but should be considered as a differential diagnosis for overly aggressive mares with a history of infertility.     

Recent advances in reproductive endocrinology of the mare

The recent development of radioimmunoassay techniques to determine changes in blood hormone levels has increased our understanding of the endocrinology of the oestrous cycle and pregnancy in the mare. This review describes recent findings concerning the endocrine basis and treatment of some common types of infertility.

Further evidence supporting the concept that prostaglandin F2∝ is the uterine luteolysin in the mare is discussed. Persistent corpus luteum is now recognized as an important cause of infertility, which may be treated by uterine distension or prostaglandin; recommendations for avoiding treatment failures are presented.

The clinical significance of recent findings on the effectiveness of HCG in hastening ovulation, and on the increase in anti-HCG antibodies after repeated injections of this hormone are also discussed.

Although antibiotics successfully control the specific pathogen causing contagious equine metritis, the treatment of common opportunistic organisms with antibiotics appears to give indifferent results. The increased understanding of the role of sex steroids in susceptibility to uterine infection seems likely to lead to improved methods of control.

Although stimulation of folliculogenesis and ovulation in seasonally acyclic mares is possible using extracts of equine pituitary glands, this treatment is impractical on a clinical scale. Gonadotrophin-releasing-hormone provides a more physiological method of stimulation, and results of recent experiments using this hormone are reviewed.

In the pregnant mare, recent studies have shown that the “pregnancy signal” to the mare may be the production of oestrogens from the developing blastocyst from Day 14. Measurement of PMSG as an aid to pregnancy diagnosis is evaluated, and the use of progesterone supplementation to prevent abortion is discussed.     

Use of n-alkanes to estimate feed intake in ruminants: a meta-analysis

Precise techniques to estimate feed intake by ruminants are critical to enhance feed efficiency and to reduce greenhouse gas emissions and nutrient losses to the environment. Using a meta-analysis, we evaluated the accuracy of the n-alkane technique to predict feed intake in cattle and sheep and assessed the relationships between feed intake and fecal recovery (FR) of n-alkanes. The database was composed of 28 studies, including 129 treatments (87 and 42 for cattle and sheep, respectively) and 402 animals (232 cattle and 170 sheep) fed at troughs, from published studies. Relationships between observed (in vivo measurement) and predicted feed intake by C31:C32 and C32:C33 n-alkane pairs were evaluated by regression. Meta-regression addressed the relationships between the difference in FR of n-alkane pairs and the error in intake estimation, as well as the amount and duration of C32 n-alkane dosing. Regression of observed intake on n-alkane-based estimates revealed good relationships in cattle (adjusted R² = 0.99 for C31:C32, and adjusted R² = 0.98 for C32:C33; P < 0.0001) and in sheep (adjusted R² = 0.94 for C31:C32, and adjusted R² = 0.96 for C32:C33; P < 0.0001). FR of natural n-alkanes showed a coefficient of variation of about 15% and 16% for C31 and C33, respectively, in cattle. In sheep, the coefficient of variation was 8% and 14% for C31 and C33, respectively. The relationships between the difference of FR of n-alkane pairs and the error in feed intake estimation in cattle were characterized by an adjusted R² = 0.83 for C31:C32 (P < 0.0001) and adjusted R² = 0.93 for C32:C33 (P < 0.0001). In sheep, they were characterized by an adjusted R² = 0.69 for C31:C32 (P < 0.001) and adjusted R² = 0.76 for C32:C33 (P < 0.001). The n-alkane technique provided the reliability for estimating feed intake in cattle and sheep in barn experiments. The present meta-analysis demonstrated that without correction for differences in FR of n-alkane pairs, deviation in feed intake prediction would occur. However, further research is necessary to determine the relationship between the n-alkane dosing procedure (daily amount and duration of dosing) and FR of n-alkane.