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.     

Case-control study of factors associated with fibrinous pericarditis among horses in central Kentucky during spring 2001

OBJECTIVE: To identify factors significantly associated with an epidemic of fibrinous pericarditis during spring 2001 among horses in central Kentucky. DESIGN: Case-control study. ANIMALS: 38 horses with fibrinous pericarditis and 30 control horses examined for other reasons. PROCEDURE: A questionnaire was developed to solicit information regarding a wide range of management practices and environmental exposures from farm owners or managers. RESULTS: The following factors were found in bivariate analyses to be significantly associated with an increased risk of pericarditis: being from a farm with mares and foals affected by mare reproductive loss syndrome, exposure to Eastern tent caterpillars in or around horse pastures, younger age, shorter duration of residence in Kentucky and at the farm of current residence, being fed hay grown outside Kentucky, a lack of access to pond water, access to orchard grass for grazing, and a lack of direct contact with cattle. In multivariate logistic regression analyses, only variables related to caterpillar exposure and age were significantly associated with fibrinous pericarditis. CONCLUSIONS AND CLINICAL RELVANCE: Results suggest that fibrinous pericarditis in horses may be associated with mare reproductive loss syndrome. Exposure to Eastern tent caterpillars was the greatest risk factor for development of fibrinous pericarditis. The distribution of times of diagnosis of fibrinous pericarditis was consistent with a point-source epidemic.     

Evaluation of early fetal losses on four equine farms in central Kentucky: 73 cases (2001)

OBJECTIVE: To determine features of an early fetal loss (EFL) syndrome and evaluate potential risk factors for EFL in Thoroughbred broodmares on 4 farms in central Kentucky. DESIGN: Retrospective study. ANIMALS: 288 pregnant broodmares. PROCEDURE: Year-2001 breeding records for 288 Thoroughbred broodmares were examined. Early fetal loss was defined as loss of a fetus that was viable at > or = 40 days of gestation but was subsequently lost by 5 months of gestation. RESULTS: Overall 2001 EFL rate was 25% (73/288), median gestational age at time of fetal loss was 77 days, and median date of loss was May 7. Mares on 1 farm had significantly fewer fetal losses (5%) than mares on the other 3 farms (26 to 35%). Fetal losses were higher for maiden (42%) and barren (42%) mares than for foaling mares (18%). Fetal losses were greater in young than in older mares. Effects of broodmare farm, mare age, and reproductive status were all significant. Fetal losses were not associated with sire used for mating or stud farm. CONCLUSIONS AND CLINICAL RELEVANCE: Greatest risk for EFL occurred during the period from late April to May (ie, in mares bred during February through March). Higher incidence of EFL in maiden and barren mares and lower incidence of EFL on 1 farm suggest management or environmental influences may have affected outcome. Risk factors that should be investigated include environmental differences among farms and differences in management procedures used for lactating versus nonlactating mares.     

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.     

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.     

Equine amnionitis and fetal loss: mare abortion following experimental exposure to Processionary caterpillars (Ochrogaster lunifer)

Reasons for performing study: In Australia, there have been recent reports of unusual abortions in mid‐ to late‐gestation mares. These were clinically distinct from other recognised causes of pregnancy loss and the term ‘equine amnionitis and fetal loss’ (EAFL) was adopted to describe this syndrome. Initial investigations concluded that possible causal factors included the presence on affected stud farms of Processionary caterpillars (Ochrogaster lunifer). Objectives: To determine if exposure of pregnant mares to Processionary caterpillars or their shed exoskeletons can induce EAFL. Methods: Processionary caterpillars and their shed exoskeletons were collected and stored frozen. Mid‐gestation mares were dosed with a slurry of caterpillars or shed exoskeleton by nasogastric intubation. Their clinical responses and times to abortion were recorded. All aborted fetuses were autopsied and samples taken for bacteriological and virological culture and histopathology. Results: Intubating mares in mid‐pregnancy with preparations of either whole Processionary caterpillars or shed caterpillar exoskeletons induced abortion with few impending clinical signs. The gross pathological and bacteriological findings of the aborted fetuses were similar to those observed in field cases of EAFL. Potential relevance: Possible exposure to Processionary caterpillars should be considered when examining cases of fetal loss in the mare. The present results provide a starting point to further explore the aetiology and pathogenesis of EAFL.     

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.     

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.     

Serologic evidence of vesivirus-specific antibodies associated with abortion in horses

OBJECTIVE: To test horses for serologic evidence of an association between vesiviral antibodies and abortion. SAMPLE POPULATION: Sera from 141 horses. PROCEDURES: 2 experiments were conducted. Experiment 1 comprised sera obtained in 2001 and 2002 from 3 groups of horses (58 mares from farms with a history of abortion problems, 25 mares between 3 and 13 years of age with unknown reproductive histories that were sold at auction [breeding-age control mares], and 29 mixed-age males and yearling females sold at auction [negative control population]). Experiment 2 comprised sera from 3 groups of pregnant mares (10 pregnant mares fed Eastern tent caterpillars [ETCs], 9 pregnant mares fed ETC frass only, and 10 pregnant control mares). Sera were analyzed for antibodies against vesivirus by use of a validated recombinant vesivirusspecific peptide antigen in an indirect ELISA. RESULTS: For experiment 1, 37 of 58 (63.8%) mares from farms with abortion problems were seropositive for vesivirus antibodies, whereas 10 of 25 (40%) breeding-age control mares were seropositive. All 29 mixed-age males and yearling females were seronegative for vesivirus antibodies. For experiment 2, 17 of 29 mares aborted (some from each group). Seropositive status for vesivirus antibodies increased from 47.1% (8/17) to 88.2% (15/17) for the pregnant mares that aborted during the experiment. CONCLUSION AND CLINICAL RELEVANCE: Significant association was detected between seropositive status for vesivirus and abortion in mares; consequently, vesivirus appears to be a pathogenic virus associated with abortion in mares. These data support adding vesivirus antibody testing into diagnostic screening to determine the cause for abortion in mares.     

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.     

Evaluation of administration of West Nile virus vaccine to pregnant broodmares

OBJECTIVE: To determine whether administration of killed West Nile virus vaccine was associated with pregnancy loss among broodmares. DESIGN: Retrospective cohort study. ANIMALS: 595 mares. PROCEDURE: Records of pregnant mares with known vaccination history from 4 farms were reviewed. Information obtained from 595 mares included mare's identification; farm; age; breed; reproductive status; last breeding date; date last known pregnant; vaccination date; age of conceptus at vaccination; vaccination during the early embryonic, early fetal, and late fetal periods; and whether an early embryonic death (EED), early fetal loss (EFL), or late fetal loss (LFL) occurred. The relationships between the dichotomous outcomes of loss (eg, EED, EFL, LFL) and independent categoric variables (eg, vaccination during the early embryonic, early fetal, or late fetal periods) were examined. RESULTS: Vaccination of pregnant mares during any period of gestation was not associated with increased incidence of pregnancy loss. CONCLUSIONS AND CLINICAL RELEVANCE: Many mares are already pregnant at the onset of mosquito season, when mares are more likely to be vaccinated than at other times. Our findings provide evidence that vaccine administration will not compromise pregnancy in horses.     

The Pattern of Embryonic Fixation and its Relationship to Pregnancy Loss in Thoroughbred Mares

Ultrasonographic pregnancy records of 195 mares from six Thoroughbred stud farms, over a period of 7 years were retrospectively analysed to assess the effect of various factors on embryonic vesicle (EV) fixation pattern and pregnancy outcome. Of the total of 746 pregnancies analysed, significantly (p < 0.01) more EV fixations were evident in the right uterine horn than in the left (53.35% vs 46.65% respectively). There was no significant effect of either, the side of ovulation, or age of the mare, on the side of EV fixation. However, EV fixation, was significantly (p < 0.001) more likely to occur in the right uterine horn in maiden and barren mares (65.75% vs 57.45% respectively). The age and reproductive status of the mare as well as foal heat breeding failed to demonstrate a consistent effect on pregnancy loss relative to the side of EV fixation. In lactating and foal heat bred mares, EVs were significantly (p < 0.0001) more frequently established in the contralateral horn to the one from which the mare delivered her most recent foal. In lactating mares, significantly (p < 0.05) higher embryonic and pregnancy losses were observed in the ipsilateral horn. In conclusion, (a) side of EV fixation was (i) independent of the side of ovulation and mare age (ii) significantly (p < 0.001) affected by reproductive status, (b) neither age of mare nor reproductive status had any effect on pregnancy loss rates relative to the side of EV fixation and (c) in lactating mares the EV had a greater chance of fixation and survival in the horn contralateral to the one from which the mare delivered her most recent foal.     

Evaluation of early fetal loss induced by gavage with eastern tent caterpillars in pregnant mares

OBJECTIVE: To determine whether gavage of pregnant mares (housed without access to pasture) with starved eastern tent caterpillars (ETCs) or their excreta is associated with early fetal loss (EFL), panophthalmitis, or pericarditis. DESIGN: Randomized clinical trial. ANIMALS: 15 mares. PROCEDURE: 15 mares with fetuses from 40 to 80 days of gestation (dGa) were randomly assigned to 1 of 3 groups and received 2.5 g of ETC excreta, 50 g of starved ETCs, or 500 mL of water, respectively, once daily for 10 days. Mares were housed in box stalls, walked twice daily, and not allowed access to pasture for 12 days before or during the 21-day trial. RESULTS: 4 of 5 mares gavaged with starved ETCs (group 2) aborted on trial days 8 (2 mares), 10, and 13. No control mares or mares that received excreta aborted. Differences between the ETC group and other groups were significant. Abortion occurred on 49, 64, 70, and 96 dGa. Allantoic fluids became hyperechoic the day before or the day of fetal death. Alpha streptococci were recovered from 1 fetus and Serratia marcescens from 3 fetuses. Neither panophthalmitis nor pericarditis was seen. The abortifacient component of the ETCs was not elucidated. CONCLUSIONS AND CLINICAL RELEVANCE: These findings suggest that mares with fetuses from 40 to 120 days of gestation should not be exposed to ETCs because they may induce abortion.     

Use of a Bayesian risk-mapping technique to estimate spatial risks for mare reproductive loss syndrome in Kentucky

OBJECTIVE: To estimate spatial risks associated with mare reproductive loss syndrome (MRLS) during 2001 among horses in a specific study population and partition the herd effects into those attributable to herd location and those that were spatially random and likely attributable to herd management. Animals-Pregnant broodmares from 62 farms in 7 counties in central Kentucky. PROCEDURE: Veterinarians provided the 2001 abortion incidence proportions for each farm included in the study. Farms were georeferenced and data were analyzed by use of a fully Bayesian risk-mapping technique. RESULTS: Large farm-to-farm variation in MRLS incidence proportions was identified. The farm-to-farm variation was largely attributed to spatial location rather than to spatially random herd effects. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicate that there are considerable data to support an ecologic cause and potential ecologic risk factors for MRLS. Veterinary practitioners with more detailed knowledge of the ecology in the 7 counties in Kentucky that were investigated may provide additional data that would assist in the deduction of the causal factor of MRLS via informal geographic information systems analyses and suggest factors for inclusion in further investigations.     

Reproductive performance of Thoroughbred mares in the Waikato region of New Zealand: 2. Multivariable analyses and sources of variation at the mare,stallion and stud farm level

Abstract

AIM: The objective of this study was to utilise multivariable statistical methods appropriate for clustered data to identify mare-related explanatory variables that significantly affected the reproductive performance of Thoroughbred mares in the Waikato region of New Zealand. In addition, we aimed to determine the relative contribution of the mare, stallion and stud farm to reproductive performance.

METHODS: A prospective cohort study was performed involving five stud farms in the Waikato region of New Zealand during three consecutive breeding seasons (2006–2008). A total of 1,482 individual mares contributed 2007 mare years and 3,402 oestrous cycles over the three breeding seasons. Reproductive performance was measured using three parameters; (a) first-cycle pregnancy rate (FCPR), (b) end-of-season pregnancy rate (SPR), and (c) the start-of-mating to conception interval.

RESULTS: When controlled for the effects of serving stallion, stud farm and year of study the only significant mare-related variables included in the final models of FCPR, SPR and conception interval were the age of the mare and her reproductive status (classified as dry or foaling). Advancing mare age significantly reduced reproductive performance regardless of reproductive status and foaling mares had significantly poorer reproductive outcomes compared with dry mares when controlled for age. For each additional increase in year of age, the FCPR was reduced by a factor of 0.94 (95% CI=0.92–0.96) and the SPR was reduced by a factor of 0.91 (95% CI=0.88–0.93). Mares older than 14 years of age took longer to conceive after the start-of-mating compared with younger mares. The daily hazard of conception for mares 14 years and older was 0.64 (95% CI=0.47–0.83) times less than mares younger than 9 years of age. Determining the relative contribution of the mare, stallion and stud farm to the FCPR indicated that 95.9% of the variation was at the mare level, 4.1% was at the stallion level and 0% was at the stud farm level. For the SPR the variance components indicated that 92.5% of the variation was at the mare level, 6.7% was at the stallion level and 0.8% was at the stud farm level.

CONCLUSIONS: The reproductive performance of Thoroughbred mares in the Waikato region of New Zealand is influenced by two main mare-related factors; the age of the mare and her reproductive status (dry or foaling). The majority of variation in reproductive performance was associated with mare-level factors and the contribution of the stallion and stud farm was relatively minor.     

Late-term abortion associated with umbilical cord torsion in the mare: Case report

Torsion of the umbilical cord in the mare can be a cause of fetal death and abortion, commonly seen in mid-to late gestation. Length of the umbilicus is considered a contributing factor. Measured lengths of less than 84 cm (33 in) were considered normal in a study among Thoroughbred mares in 1 report.[1] Increased lengths have been associated with excessive torsion and allantochorionic necrosis. The case presented conforms to these indications of vascular obstruction as the cause of fetal death, with visible twisting and excessive cord length after delivery of the dead foal.     

Pregnancy and twinning rates in Thoroughbred mares following the administration of human chorionic gonadotropin (hCG)

AIM: To determine the effect of hCG administration to cycling Thoroughbred mares, on pregnancy and twinning rates and the number of serves in the treated cycle. METHODS: A retrospective case control approach was conducted involving 2119 mare ovulatory cycles, on 1110 mares over a 7-year period. Data were collected by 1 of the authors during routine stud work at 3 commercial Thoroughbred farms in the Waikato region of New Zealand. The hCG (1500 IU) was administered by intravenous injection to selected mares 24 h before the expected time of breeding. Mares were scanned for pregnancy (singleton or twins) 14 days after the onset of dioestrus or detection of ovulation. Multilevel logistic regression analyses were used to identify the risk factors associated with the outcomes of interest while simultaneously controlling for possible confounding factors. RESULTS: Treatment with hCG tended to improve the odds of pregnancy (p=0.06), produced a 3-fold increase in the odds of twins (p<0.001), and increased the odds of a mare having a single serve in the treated ovulatory cycle (p=0.036). The first ovulatory cycle of a season in which a mare was bred was associated with a lower odds of pregnancy (p=0.02), and a lower odds of twins (p=0.003), when compared with subsequent cycles. Lactating mares were less likely to be diagnosed with twins (p=0.005), and were more likely to have a single serve (p<0.001), in any one ovulatory cycle than non-lactating mares. CONCLUSIONS: This report supports the role of hCG as an important therapeutic tool in veterinary management of broodmares for optimal reproductive performance. Mares treated with hCG must be managed in the knowledge that they have an increased likelihood of twins.     

Factors Influencing the Frequency of Pregnancy Loss among Thoroughbred Mares in Hidaka,Japan

Pregnancy loss in mares is thought to be a main problem associated with reproductive efficiency. To clarify the situation of pregnancy loss in Thoroughbred mares in Japan, the occurrence of pregnancy loss before and after 35 days of gestation was investigated with 1,476 Thoroughbred mares in Hidaka, Japan, from 2007 to 2009. Pregnancy loss on days 17-35 was determined by ultrasound examination between 17 and 35 days after the last mating. Follow-up surveys were conducted between 35 days and foaling to determine pregnancy loss on day 35 until foaling in 843 of these mares. Using multiple logistic regression analysis, we assessed the influence of mare age, reproductive status, twin pregnancy reduction, body condition score (BCS), estrus type in foaling mares (foal heat or not), progesterone therapy, and endometrial cysts on pregnancy loss rates on days 17-35 and on day 35 until foaling in this population of mares. The pregnancy loss rates on days 17-35 and on day 35 until foaling were 5.8% and 8.7%, respectively. The overall pregnancy loss rate (day 17 until foaling, including parturient losses) was 14.7%. Risk factors for pregnancy loss included decrease in BCS between 17 and 35 days, <5 BCS at day 35, mating during foal heat, and endometrial cysts. In all, 14.7% of pregnancies were lost between day 17 and birth, contributing significantly to reduced reproductive efficiency in Thoroughbred mares in Japan. These observations indicate that mares should be maintained at high BCS and should be prevented from mating in foal heat to decrease the pregnancy loss rates.     

Quantifying the occurrence of early embryonic mortality on three equine breeding farms

This prospective field study was designed to describe the incidence of early embryonic mortality (EEM) and factors associated with the cause of EEM on three equine breeding farms in Ontario during the 1989 breeding season. Early embryonic mortality was defined as the loss of a single embryo during the first 40 days of pregnancy (day 0 = day of ovulation or last breeding). Pregnancy diagnoses and subsequent embryonic losses were observed by serial trans-rectal ultrasonography between days 12-20 (PD1) and 21-30 (PD2), and by trans-rectal ultrasonography or palpation per rectum between days 31-40 (PD3). Information on pregnancy status of a mare (or cycle) at 40 days after the last breeding was recorded when available. Nonpregnancy rates were calculated on a per cycle basis, to account for mares with no ultrasonic evidence of an embryo at the initial pregnancy examination. Embryonic mortality rates per cycle were calculated cumulatively (EMR₍₄₀₎) for the entire 40 day embryonic period and during the specific time periods when a pregnancy diagnosis took place (EMR_(PD1), EMR_(PD2), EMR_(PD3)). Embryonic mortality rates were also calculated on a per mare basis for mares experiencing EEM on either their first (EMR_(f)) or any (EMR_(a)) breeding cycle. Per cycle mare withdrawal rates were calculated cumulatively for the entire 40 day embryonic period (MWR₍₄₀₎), and at each specific pregnancy diagnosis time period (MWR_(PD1), MWR_(PD2), MWR_(PD3)) to account for those breeding cycles in which mares were not able to be observed for the entire forty days of the embryonic period. Records from a total of 699 mares involving 1014 breeding cycles were examined and analyzed. Per cycle risk rates for nonpregnancy (NP) were 36.4%, 45.0%, and 22.1%, for farms 1,2 and 3, respectively. Per cycle EMR₍₄₀₎ ranged from 8-17%. Per cycle MWR₍₄₀₎ ranged from 56.5-98.9%, indicative of a high rate of mare withdrawal from the study for the duration of the “embryonic” period. Significant differences (p < 0.05) in EMR_(f) and EMR_(a) per mare existed between two farms, indicative of a farm effect on EMR. Multivariable forward stepwise logistic regression analyses revealed that mares bred on foal heat were 1.9 times more likely than mares not bred on foal-heat to experience EEM (p = 0.008).     

Reproductive efficiency of Flatrace and National Hunt Thoroughbred mares and stallions in England

REASONS FOR PERFORMING STUDY: Previous surveys of reproductive efficiency in British Thoroughbreds included only mares and stallions standing on studfarms in and around Newmarket. The present study was widened to compare Flatrace (FR) (Group A) and National Hunt (NH) (Group B) mares and stallions on studfarms throughout England. OBJECTIVES: To assess the influences of mare type, status and age, and veterinary manipulations on reproductive efficiency parameters. To compare the inherent fertility of stallions, based on singleton and twin pregnancy rates and pregnancy loss rates, in Groups A and B Thoroughbred breeding stock. METHODS: Managers of 24 FR and 9 NH public studfarms were asked to complete a questionnaire for each mated oestrous cycle shown by 2321 Group A and 1052 Group B mares throughout the 2002 mating season. Parameters such as per cycle singleton and twin pregnancy rates, and pregnancy loss rates were noted, and the success of hormone treatments to induce oestrus and ovulation assessed. The number of matings per oestrus and per pregnancy were recorded, together with the incidence and effectiveness of uterine and other veterinary treatments. The inherent fertility of 84 Group A and 43 Group B stallions in the study, as measured by the singleton and twin early pregnancy rates and the pregnancy loss rates recorded in the mares they mated, was also estimated. RESULTS: Per cycle early pregnancy (Days 13-16) was 63.2% for Group A and 65.3% for Group B mares; and 10.3% and 13.1%, respectively, of those pregnancies were twins or triplets. Early, middle and late pregnancy loss rates were 7.2% vs. 8.0% (Days 15-42), 3.6% vs. 6.1% (Days 42-1st October) and 2.7% vs. 2.1% (October-foaling), respectively. Matings per oestrus and per early pregnancy were significantly higher in Group B vs. Group A mares. For stallions that mated > or = 30 mares, overall early pregnancy rates per cycle in mares mated ranged from 30-89% across the 2 groups. CONCLUSIONS: No major differences in reproductive efficiency were identified between FR and NH mares and stallions. Increasing mare age was the single biggest limiting factor to an otherwise high rate of fertility in well-managed English Thoroughbreds. POTENTIAL RELEVANCE: This study identified factors that influence reproductive efficiency in the Thoroughbred.