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Short Communication
Short Communication Five cases of consecutive posterior (caudal) presentation of the fetus in two mares J. R. Newcombe, G. M. M. Kelly Introduction
Caudal or posterior presentation is an uncommon cause of dystocia in the mare. Postural abnormalities of the hindlimbs in conjunction with this presentation can result in major complications (Byron and others 2002). Breech presentation is a specific form of posterior presentation where there is bilateral flexion of the foal’s hips. No cases of breech presentation were reported in this paper. Reports of the incidence of posterior presentation are limited and are mostly from referral cases. Blanchard and others (1989) reported 6 (12 per cent) breech presentations in 50 referrals, Frazer and others (1997) reported caudal presentation in 14 per cent of 141 referrals, and Byron and others (2002) gave an incidence of 9.8 per cent in 161 mostly Thoroughbred (TB) mares referred for dystocia. The incidence in the field is harder to ascertain, although Platt (1979) in a survey of foaling Thoroughbred mares found 98 cases of dystocia of which 3 were posterior presentations. McCue and Ferris (2012) reported only 4 out of 1047 non-referral foalings that were not in cranial presentation. These accounted for 3.8 per cent of dystocias of which only 2 (1.9 per cent) were in caudal presentation (R. A. Ferris, personal communication, Colorado State University). Vanderplassche and others (1972) estimated that 1 in 500 births (0.2 per cent) involved posterior presentation. Also, Ginther and Williams (1996) found no cases of posterior presentation at eight farms in 517 spontaneous Thoroughbred, Standardbred, Quarterhorse and miniature horse births. Mobility of the fetus in the uterus and the ability to freely rotate to either anterior or posterior presentation persists up to five (Bergfelt and Adams 2011) or eight months (Ginther and others 1994, Bucca and others 2005). However, during late gestation the increasing size of the fetus means that any change in presentation in the uterus becomes increasingly difficult and less likely. To ensure that the fetus is in a cranial presentation at parturition when rotation is no longer possible, a complex mechanism involving entrapment of the fetal hindlimbs within one of the uterine horns has been suggested as the reason why almost all equine fetuses are in cranial presentation at parturition (Ginther and others 1994). This mechanism involves a temporary isolation of the fetus and allantoic fluid to the uterine body after which neurological signals from fetal maturation of the inner ear in conjunction with the acute angle of the uterine horns result in the hindlimbs being trapped in the gravid horn so the fetus is in dorsal recumbency with its head directed towards the mare’s cervix.
Veterinary Record (2014) J. R. Newcombe, BVetMed MRCVS, G. M. M. Kelly, BVM&S MRCVS, Equine Fertility Clinic, Warren House Farm, Brownhills, West Midlands WS8 6LS, UK
doi: 10.1136/vr.101532 E-mail for correspondence: [email protected] Provenance: not commissioned; externally peer reviewed Accepted May 13, 2014
Case reports
This report documents five instances of posterior presentation in consecutive foaling years in two different mares, one Thoroughbred and one Gypsy Cob.
Case 1
A multiparous, 15 years old (yo) Thoroughbred mare that had foaled spontaneously the previous year presented posteriorally in second stage labour with extended fetal hindlimbs in a dorsosacral position. Traction was applied to assist the birth. The foal was born alive but subsequently suffered respiratory distress and died. Bilateral fracture of several ribs at the widest part of the chest and resultant severe bruising of the myocardium had presumably caused death. The following year, when the mare was running milk at term, it was electively induced with 500 µg dl-cloprostenol subcutaneously (Estrumate) (Rossdale and others 1979). Parturition commenced 100 minutes later and a large foal was presented posteriorally in dorsosacral position with hindlimbs extended. It too sustained rib fractures and died. No obvious placental abnormalities were observed.
Case 2
A primiparous, 3 yo Gypsy Cob mare was presented with dystocia. The foal was presented posteriorally in a dorsosacral position with bilateral hock flexion. It was small and already dead and was delivered easily by traction after correction of the hock flexion. The placenta was normal in appearance and conformation. The following season the same mare was found in dystocia, and presented at the clinic at which point the foal was already dead. This presentation was also posterior but in contrast with the previous year the foal was in a dorsoiliac position with three feet presented without hock flexion. Manipulation of the forelimb was unsuccessful until the mare was anaesthetised and the hind quarters elevated. The dead foal was then delivered by traction on the hindlimbs. The placenta was recovered and although torn it showed no obvious abnormality in appearance or conformation. The foal in this case appeared of a normal size for a Gypsy Cob mare, the dystocia being due to malpresentation rather than fetal oversize. Visual inspection of the foal revealed no obvious congenital abnormalities. The Gypsy Cob mare was presented again two seasons later with dystocia, having failed to produce a foal the previous season. On examination the foal was already dead and in posterior presentation in a dorsoiliac position with bilateral hock flexion. Traction was applied and the dead foal was delivered. The placenta was also removed intact and examination revealed unusual placental morphology. Namely, the portion of the placenta corresponding to the uterine body appeared minimally distended while the pregnant horn was grossly enlarged to the tip of the horn (see Fig 1). The foal weighed 33.2 kg and the placenta 3.55 kg; making the latter 9.4 per cent of the foal birth weight indicating a normal foal to placenta ratio of approximately 11 per cent (Whitwell and Jeffcott 1975). The foal had no external congenital abnormalities. Postmortem examination and histopathology was not performed on the any of the foals or placentae in these cases.
Discussion
The senior author can recall only four other cases of posterior presentation during more than 40 years of equine clinical practice, one in a Shire, one in an Irish Draught, one Thoroughbred and the other in a 17 hands high (hh) Warmblood. Of two large stud farms serviced by the authors’ practice, one farm could recall only a single case in approximately 2700 foalings over 18 years (0.037 per cent) in Thoroughbred mares and the other farm, only two cases in approximately 1000 foalings in mixed breed mares including Irish Draught (0.2 per cent) over 20 years. This very low incidence and the fact that five cases in the case load occurred in two mares, stimulated the request to colleagues August 2, 2014 | Veterinary Record
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Short Communication
FIG 1: Placenta from third posterior presentation in Case 2: allantoic surface exposed. Black arrow: non-gravid horn; White arrow: gravid horn; Black star: cervix.
for their experiences. All the practising stud veterinary surgeons approached could not recall attending either any or more than one case. Seven very experienced stud managers in the Hunter Valley of Australia collectively recalled no more than 12 cases out of an estimated total of 23,000 Thoroughbred foalings (0.052 per cent) during a total of 150 observational years (A. Gunn, personal communication, Charles Sturt University, New South Wales). A similar incidence was recalled from four managers and stud grooms on large Newmarket stud farms of 3 cases in an estimated 5500 Thoroughbred foalings (0.055 per cent) over 100 observational years (I. Henderson, personal communication, Newmarket Equine Hospital, Newmarket, Suffolk, UK). Two cases were reported by a single Standardbred farm in Australia from approximately 2600 foalings (0.077 per cent; S. Litchwark, personal communication, Egmont stud Toowoomba, Queensland, Australia). Although these are only recollections from estimated numbers of foalings, the combined incidence in Thoroughbreds and StandardBreds of only 0.056 per cent suggests that the real incidence in the field is a lot lower than previously reported. Vanderplassche (1993) reported an incidence of 1 per cent and estimated 0.2 per cent (Vanderplassche and others 1972), a frequency of some 4–20 times more than the reports presented here from Thoroughbred and Standardbred mares. A large proportion of mares reported by Vanderplassche would be of the Belgian and Dutch draught breeds. Taken together these would appear to indicate a breed non-disposition in Thoroughbreds and light horses. It may be no coincidence that transverse presentation is not infrequent in draught mares (Vanderplassche 1993) but extremely rare in Thoroughbred and other reports of referrals: 0 per cent (Ginther and Williams 1996); 1.9 per cent (McCue and Ferris 2012); 0 per cent (Platt 1979). Frazer and others (1997) reported 18 per cent posterior presentation in draught breed referrals, compared with 8 per cent in light breeds. No data was found for ponies. It may be more than a coincidence that the only non-Thoroughbred farm reported by the authors here, had a threefold to fourfold higher incidence of 0.2 per cent, and four of the six cases attended by the authors were Veterinary Record | August 2, 2014
in non-Thoroughbreds. Even the TBmare reported here was a large National Hunt type of TB. This apparent increased incidence in mares of larger breeds would suggest that an increased uterine to abdominal size ratio may be a factor in posterior presentation. The increased size of the uterus to abdomen ratio may allow increased fetal movement and perhaps increases the likelihood of the fetus attaining an abnormal position. However, Holland (1950) reported a mare to foal weight ratio at birth of 6.8:1 for Shires while Willoughby (1975) reported ratios of 10:1 for Thoroughbreds and 13.3:1 for Shetland ponies. This data implies that the Shire may have relatively less ‘room’ for the foal than the Shetland and, hence, the incidence should be highest in small ponies. Many mares are examined at the end of September in the UK for certification of pregnancy or non-pregnancy. The author can recall only once out of nearly a thousand examinations, detecting a fetus in posterior presentation by transrectal ultrasound after about five months of gestation. More than coincidently, this Irish Draught mare was one of the four instances referred to earlier, to foal in posterior presentation. This observation is at odds with the studies of Ginther and others (1994) who reported that presentational changes do not cease until after the ninth month of gestation (only one mare found in posterior presentation on one occasion in the 10th month) and that of Bucca and others (2005) who found none in posterior presentation out of 150 mares after the eighth month. Although the fetus develops initially at the base of one uterine horn, it soon gravitates into the anterior portion of the uterine body which is normally the most dependent portion, the horns being elevated by the ovarian ligaments and the body by the cervix and pubis. Closure (absence of fluid) of both horns is normal (Ginther and others 1994). From then on development continues in the uterine body with both horns closed (Bergfelt and Adams 2011) until such time as increasing size causes extension of the rear end and hindlimbs of the fetus into one horn, usually (82.5 per cent) the horn in which nidation occurred initially (Allen and Newcombe 1981). However, the hindlimbs do not extend into a horn in all cases since the ‘dog-sitting’ presentation is not rare (8 per cent of dystocias due to presentational abnormalities (Platt 1979)). During fetal development, the ventral surface of the uterine body comes to lie against the ventral wall of the mare’s abdomen so that the fetus comes to lie more vertically with its head above its body and directed towards the cervix (Ginther 1998). One could speculate that although bathed in fluid, the fetal organs of balance would be sufficiently developed to enable the fetus to appreciate gravitational forces and be aware of its orientation. It is not unreasonable to assume that it would prefer to remain with its head higher than its body. Indeed, Ginther (1994) demonstrated a clear preference of newborn foals to lie with the cranial aspect of their thorax uppermost on a 40° incline. On the other hand, while orientated posteriorally, its head would be lower than much of its body and presumably, while still able to re-orientate, it would move around until it attained a more ‘comfortable’ position with its head above the level of its body. Such a situation could explain why almost every foal is presented anteriorly at parturition. It would not of course explain the rare occurrence of a posterior presentation. It is highly likely that such a presentation could have been present for several months and could therefore be diagnosed with ultrasound (Bucca and others 2005). It seems a mystery to us that according to reports, the fetus is apparently able to correct its presentation as late as nine months, considering that by then it is a large, relatively elongated structure, contained within a relatively narrow tube. However should the foal be in transverse presentation until later in pregnancy, then in an attempt to align itself anterior-posteriorally, its head rather than its back legs could become trapped in the uterine horn. In all but one of the cases described in this report no obvious abnormalities of the placenta were noticed nor were there any obvious congenital abnormalities in any of the foals. The lack of obvious placental abnormalities is unusual as it would be expected that a foal lying in an abnormal position for an extended period of time would lead to changes in the surrounding placenta (Wilsher and others 2012). However, in the cases described in this report only a visual examination of the placentae was undertaken and it is possible that some of the linear dimensions and weight of the placentae were abnormal.
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Short Communication In view of the obvious abnormal, gross morphological appearance of the placenta, in the third posterior presentation from Case 2 (see Fig 1) the authors must admit that the placentae in these other cases may well have had morphological abnormalities that were not noticed either due to damage to the placenta or failure on the authors’ part to observe. In the third posterior presentation from Case 2 the placental dimensions were grossly abnormal with the allantochorion not conforming to the typical F-shape when laid out postpartum. However, it was very difficult to compare its linear dimensions with the standard measurements set out by Whitwell and Jeffcott (1975) although an attempt was made to do this. The morphology of the placenta from Case 2 suggests that the bulk if not all of the fetus remained in the pregnant horn in the latter part of pregnancy after being fixed in posterior presentation. Although no histopathology was performed the authors did notice that the chorionic villi appeared sparse over an estimated 40 per cent of the gravid horn. An ultrasound diagnosis of posterior presentation at pregnancy diagnosis should be regarded as having the potential for a posterior presentation at birth. However early it was made, it should always be repeated later to confirm this continued presentation. Elective caesarean section might then be considered since parturition is likely to result in dystocia and possible death of the foal due to fluid aspiration, limb malposition or rib fracture. Elective caesarean has been reported to lead to 90 per cent fetal survival (Freeman and others 1999) although the complications of caesarean section should be considered including peritonitis, haemorrhage, retained placenta, laminitis, herniation, evisceration and incisional infection (Embertson 2002). The chance occurrence of two mares foaling with posterior presentation in two and three successive years is likely to be more than coincidental. Therefore, any mare which exhibits a posterior presentation at foaling should be considered at a high risk of doing so again in successive pregnancies and be monitored accordingly. Clearly, more investigation is required to determine why posterior presentations should occur.
Acknowledgements
To Allan Gunn and Imogen Hendersen for their efforts in accumulating the data presented from stud farms in Newmarket and the Hunter Valley NSW and for Sue Lichtwark’s data from Egmont Farm, Queensland.
BLANCHARD, T. L., VARNER, D. D., ELMORE, R. G., MARTIN, M. T., SCRUTCHFIELD, W. L. & TAYLOR, T. S. (1989) Management of dystocia in mares: Examination, obstetrical equipment, and vaginal delivery. Compendium: Equine 11, 745–753 BUCCA, S., FOGARTY, U., COLLINS, A. & SMALL, V. (2005) Assessment of fetoplacental well-being in the mare from mid-gestation to term: Transrectal and transabdominal ultrasonographic features. Theriogenology 64, 542–557 BYRON, C. R., EMBERTSON, R. M., BERNARD, W. V., HANCHE, S. R., BRAMLAGE, L. R., & HOPPER, S. A. (2002) Dystocia in a referral hospital setting : Approach and results. Equine Veterinary Journal 35, 82–85 EMBERTSON, R. M. (2002) Indications and surgical techniques for caesarian in the mare. Equine Veterinary Education. Manual 5, 60–64 FRAZER, G. S., PERKINS, N. R., BLANCHARD, T. L., ORSINI, J. & THRELFALL, W. R. (1997) Prevalence of foetal maldispositions in equine referral hospital dystocias. Equine Veterinary Journal 29, 111–116 FREEMAN, D. E., HUNGERFORD, L. L., SCHAEFER, D., LOCK, D. F., SERTICH, P. L., BAKER, G. J., VAALA, W. E. & JOHNSTON, J. K. (1999) Caesarian section and other methods for assisted delivery: Comparison of effects on mare mortality and complications. Equine Veterinary Journal 31, 203–207 GINTHER, O., WILLIAMS, D. & CURRAN, S. (1994) Equine foetal kinetics: Entry and retention of foetal hindlimbs in a uterine horn. Theriogenology 41, 795–807 GINTHER, O. J. (1994) Equine Physical Utero-Fetal Interactions: A challenge and a wonder for the Practitioner. Journal of Equine Veterinary Science 14, 313–317. GINTHER, O. J. (1998) Equine pregnancy: Physical interactions between the uterus and conceptus. Proceedings of the 44th Annual Convention of the American Association of Equine Practitioners. Vol 44, 73–104 GINTHER, O. J. & WILLIAMS, D. B. (1996) On-the-farm incidence and nature of equine dystocias. Journal of Equine Veterinary Science 16, 159–164 HOLLAND, A. G. (1950) British breeds of livestock: iv. The Shire horse. British Agricultural. Bulletin 3, 160–163 LITCHWARK, S., Egmont Stud Toowoomba, Queensland, Australia. McCUE, P. M. & FERRIS, R. A. (2012) Parturition, dystocia and foal survival: a retrospective study of 1047 births. Equine Veterinary Journal 44(Suppl 41), 22–25 PLATT, H. (1979) A survey of perinatal mortality and disorders in the thoroughbred. Newmarket, UK: Bulletin of the Equine Research Station, Animal Health Trust. p 15 ROSSDALE, P. D., PASHEN, R. L. & JEFFCOTT, L. B. (1979) The use of synthetic prostaglandin analogue (fluprostenol) to induce foaling. Journal of reproduction and fertility. Supplement 27, 521–529 VANDERPLASSHE, M. (1993) Dystocia. In Equine Reproduction. Eds A. O. McKinnon, J. Voss. Philadelphia: Lea and Febiger. pp 578–587 VANDERPLASSCHE, M., SPINCEMAILLE, J., BOUTERS, J. & BONTE, P. (1972) Some aspects of equine obstetrics. Equine Veterinary Journal 4, 105–109 WHITWELL, K. E. & JEFFCOTT, L. B. (1975) Morphological Studies on the fetal membranes of the normal singleton foal at term. Research in Veterinary Science 19, 44–55 WILLOUGHBY, D. P. (1975) Growth and Nutrition in the Horse. A.S. Barnes & Co. WILSHER, S., OUSEY, J. & ALLEN, W. R. (2012) Observations on the placentae of Thoroughbred foals born with flexural limb deformities. Equine Veterinary Education. 25, 84-85.
References
ALLEN, W. E. & NEWCOMBE, J. R. (1981) Relationship between early pregnancy site in consecutive gestations in the mare. Equine Veterinary Journal 13, 51–52 BERGFELT, D. R. & ADAMS, G. P. (2011) Pregnancy. In Equine Reproduction, 2nd edn. Eds A. McKinnon, E. Squires, W. Vaala, D. Varner. Blackwell Publishing Ltd. 2065-2079.
August 2, 2014 | Veterinary Record
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Five cases of consecutive posterior (caudal) presentation of the fetus in two mares J. R. Newcombe and G. M. M. Kelly Veterinary Record 2014 175: 120 originally published online June 4, 2014
doi: 10.1136/vr.101532 Updated information and services can be found at: http://veterinaryrecord.bmj.com/content/175/5/120.1
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Short Communication
Short Communication Five cases of consecutive posterior (caudal) presentation of the fetus in two mares J. R. Newcombe, G. M. M. Kelly Introduction
Caudal or posterior presentation is an uncommon cause of dystocia in the mare. Postural abnormalities of the hindlimbs in conjunction with this presentation can result in major complications (Byron and others 2002). Breech presentation is a specific form of posterior presentation where there is bilateral flexion of the foal’s hips. No cases of breech presentation were reported in this paper. Reports of the incidence of posterior presentation are limited and are mostly from referral cases. Blanchard and others (1989) reported 6 (12 per cent) breech presentations in 50 referrals, Frazer and others (1997) reported caudal presentation in 14 per cent of 141 referrals, and Byron and others (2002) gave an incidence of 9.8 per cent in 161 mostly Thoroughbred (TB) mares referred for dystocia. The incidence in the field is harder to ascertain, although Platt (1979) in a survey of foaling Thoroughbred mares found 98 cases of dystocia of which 3 were posterior presentations. McCue and Ferris (2012) reported only 4 out of 1047 non-referral foalings that were not in cranial presentation. These accounted for 3.8 per cent of dystocias of which only 2 (1.9 per cent) were in caudal presentation (R. A. Ferris, personal communication, Colorado State University). Vanderplassche and others (1972) estimated that 1 in 500 births (0.2 per cent) involved posterior presentation. Also, Ginther and Williams (1996) found no cases of posterior presentation at eight farms in 517 spontaneous Thoroughbred, Standardbred, Quarterhorse and miniature horse births. Mobility of the fetus in the uterus and the ability to freely rotate to either anterior or posterior presentation persists up to five (Bergfelt and Adams 2011) or eight months (Ginther and others 1994, Bucca and others 2005). However, during late gestation the increasing size of the fetus means that any change in presentation in the uterus becomes increasingly difficult and less likely. To ensure that the fetus is in a cranial presentation at parturition when rotation is no longer possible, a complex mechanism involving entrapment of the fetal hindlimbs within one of the uterine horns has been suggested as the reason why almost all equine fetuses are in cranial presentation at parturition (Ginther and others 1994). This mechanism involves a temporary isolation of the fetus and allantoic fluid to the uterine body after which neurological signals from fetal maturation of the inner ear in conjunction with the acute angle of the uterine horns result in the hindlimbs being trapped in the gravid horn so the fetus is in dorsal recumbency with its head directed towards the mare’s cervix.
Veterinary Record (2014) J. R. Newcombe, BVetMed MRCVS, G. M. M. Kelly, BVM&S MRCVS, Equine Fertility Clinic, Warren House Farm, Brownhills, West Midlands WS8 6LS, UK
doi: 10.1136/vr.101532 E-mail for correspondence: [email protected] Provenance: not commissioned; externally peer reviewed Accepted May 13, 2014
Case reports
This report documents five instances of posterior presentation in consecutive foaling years in two different mares, one Thoroughbred and one Gypsy Cob.
Case 1
A multiparous, 15 years old (yo) Thoroughbred mare that had foaled spontaneously the previous year presented posteriorally in second stage labour with extended fetal hindlimbs in a dorsosacral position. Traction was applied to assist the birth. The foal was born alive but subsequently suffered respiratory distress and died. Bilateral fracture of several ribs at the widest part of the chest and resultant severe bruising of the myocardium had presumably caused death. The following year, when the mare was running milk at term, it was electively induced with 500 µg dl-cloprostenol subcutaneously (Estrumate) (Rossdale and others 1979). Parturition commenced 100 minutes later and a large foal was presented posteriorally in dorsosacral position with hindlimbs extended. It too sustained rib fractures and died. No obvious placental abnormalities were observed.
Case 2
A primiparous, 3 yo Gypsy Cob mare was presented with dystocia. The foal was presented posteriorally in a dorsosacral position with bilateral hock flexion. It was small and already dead and was delivered easily by traction after correction of the hock flexion. The placenta was normal in appearance and conformation. The following season the same mare was found in dystocia, and presented at the clinic at which point the foal was already dead. This presentation was also posterior but in contrast with the previous year the foal was in a dorsoiliac position with three feet presented without hock flexion. Manipulation of the forelimb was unsuccessful until the mare was anaesthetised and the hind quarters elevated. The dead foal was then delivered by traction on the hindlimbs. The placenta was recovered and although torn it showed no obvious abnormality in appearance or conformation. The foal in this case appeared of a normal size for a Gypsy Cob mare, the dystocia being due to malpresentation rather than fetal oversize. Visual inspection of the foal revealed no obvious congenital abnormalities. The Gypsy Cob mare was presented again two seasons later with dystocia, having failed to produce a foal the previous season. On examination the foal was already dead and in posterior presentation in a dorsoiliac position with bilateral hock flexion. Traction was applied and the dead foal was delivered. The placenta was also removed intact and examination revealed unusual placental morphology. Namely, the portion of the placenta corresponding to the uterine body appeared minimally distended while the pregnant horn was grossly enlarged to the tip of the horn (see Fig 1). The foal weighed 33.2 kg and the placenta 3.55 kg; making the latter 9.4 per cent of the foal birth weight indicating a normal foal to placenta ratio of approximately 11 per cent (Whitwell and Jeffcott 1975). The foal had no external congenital abnormalities. Postmortem examination and histopathology was not performed on the any of the foals or placentae in these cases.
Discussion
The senior author can recall only four other cases of posterior presentation during more than 40 years of equine clinical practice, one in a Shire, one in an Irish Draught, one Thoroughbred and the other in a 17 hands high (hh) Warmblood. Of two large stud farms serviced by the authors’ practice, one farm could recall only a single case in approximately 2700 foalings over 18 years (0.037 per cent) in Thoroughbred mares and the other farm, only two cases in approximately 1000 foalings in mixed breed mares including Irish Draught (0.2 per cent) over 20 years. This very low incidence and the fact that five cases in the case load occurred in two mares, stimulated the request to colleagues August 2, 2014 | Veterinary Record
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Short Communication
FIG 1: Placenta from third posterior presentation in Case 2: allantoic surface exposed. Black arrow: non-gravid horn; White arrow: gravid horn; Black star: cervix.
for their experiences. All the practising stud veterinary surgeons approached could not recall attending either any or more than one case. Seven very experienced stud managers in the Hunter Valley of Australia collectively recalled no more than 12 cases out of an estimated total of 23,000 Thoroughbred foalings (0.052 per cent) during a total of 150 observational years (A. Gunn, personal communication, Charles Sturt University, New South Wales). A similar incidence was recalled from four managers and stud grooms on large Newmarket stud farms of 3 cases in an estimated 5500 Thoroughbred foalings (0.055 per cent) over 100 observational years (I. Henderson, personal communication, Newmarket Equine Hospital, Newmarket, Suffolk, UK). Two cases were reported by a single Standardbred farm in Australia from approximately 2600 foalings (0.077 per cent; S. Litchwark, personal communication, Egmont stud Toowoomba, Queensland, Australia). Although these are only recollections from estimated numbers of foalings, the combined incidence in Thoroughbreds and StandardBreds of only 0.056 per cent suggests that the real incidence in the field is a lot lower than previously reported. Vanderplassche (1993) reported an incidence of 1 per cent and estimated 0.2 per cent (Vanderplassche and others 1972), a frequency of some 4–20 times more than the reports presented here from Thoroughbred and Standardbred mares. A large proportion of mares reported by Vanderplassche would be of the Belgian and Dutch draught breeds. Taken together these would appear to indicate a breed non-disposition in Thoroughbreds and light horses. It may be no coincidence that transverse presentation is not infrequent in draught mares (Vanderplassche 1993) but extremely rare in Thoroughbred and other reports of referrals: 0 per cent (Ginther and Williams 1996); 1.9 per cent (McCue and Ferris 2012); 0 per cent (Platt 1979). Frazer and others (1997) reported 18 per cent posterior presentation in draught breed referrals, compared with 8 per cent in light breeds. No data was found for ponies. It may be more than a coincidence that the only non-Thoroughbred farm reported by the authors here, had a threefold to fourfold higher incidence of 0.2 per cent, and four of the six cases attended by the authors were Veterinary Record | August 2, 2014
in non-Thoroughbreds. Even the TBmare reported here was a large National Hunt type of TB. This apparent increased incidence in mares of larger breeds would suggest that an increased uterine to abdominal size ratio may be a factor in posterior presentation. The increased size of the uterus to abdomen ratio may allow increased fetal movement and perhaps increases the likelihood of the fetus attaining an abnormal position. However, Holland (1950) reported a mare to foal weight ratio at birth of 6.8:1 for Shires while Willoughby (1975) reported ratios of 10:1 for Thoroughbreds and 13.3:1 for Shetland ponies. This data implies that the Shire may have relatively less ‘room’ for the foal than the Shetland and, hence, the incidence should be highest in small ponies. Many mares are examined at the end of September in the UK for certification of pregnancy or non-pregnancy. The author can recall only once out of nearly a thousand examinations, detecting a fetus in posterior presentation by transrectal ultrasound after about five months of gestation. More than coincidently, this Irish Draught mare was one of the four instances referred to earlier, to foal in posterior presentation. This observation is at odds with the studies of Ginther and others (1994) who reported that presentational changes do not cease until after the ninth month of gestation (only one mare found in posterior presentation on one occasion in the 10th month) and that of Bucca and others (2005) who found none in posterior presentation out of 150 mares after the eighth month. Although the fetus develops initially at the base of one uterine horn, it soon gravitates into the anterior portion of the uterine body which is normally the most dependent portion, the horns being elevated by the ovarian ligaments and the body by the cervix and pubis. Closure (absence of fluid) of both horns is normal (Ginther and others 1994). From then on development continues in the uterine body with both horns closed (Bergfelt and Adams 2011) until such time as increasing size causes extension of the rear end and hindlimbs of the fetus into one horn, usually (82.5 per cent) the horn in which nidation occurred initially (Allen and Newcombe 1981). However, the hindlimbs do not extend into a horn in all cases since the ‘dog-sitting’ presentation is not rare (8 per cent of dystocias due to presentational abnormalities (Platt 1979)). During fetal development, the ventral surface of the uterine body comes to lie against the ventral wall of the mare’s abdomen so that the fetus comes to lie more vertically with its head above its body and directed towards the cervix (Ginther 1998). One could speculate that although bathed in fluid, the fetal organs of balance would be sufficiently developed to enable the fetus to appreciate gravitational forces and be aware of its orientation. It is not unreasonable to assume that it would prefer to remain with its head higher than its body. Indeed, Ginther (1994) demonstrated a clear preference of newborn foals to lie with the cranial aspect of their thorax uppermost on a 40° incline. On the other hand, while orientated posteriorally, its head would be lower than much of its body and presumably, while still able to re-orientate, it would move around until it attained a more ‘comfortable’ position with its head above the level of its body. Such a situation could explain why almost every foal is presented anteriorly at parturition. It would not of course explain the rare occurrence of a posterior presentation. It is highly likely that such a presentation could have been present for several months and could therefore be diagnosed with ultrasound (Bucca and others 2005). It seems a mystery to us that according to reports, the fetus is apparently able to correct its presentation as late as nine months, considering that by then it is a large, relatively elongated structure, contained within a relatively narrow tube. However should the foal be in transverse presentation until later in pregnancy, then in an attempt to align itself anterior-posteriorally, its head rather than its back legs could become trapped in the uterine horn. In all but one of the cases described in this report no obvious abnormalities of the placenta were noticed nor were there any obvious congenital abnormalities in any of the foals. The lack of obvious placental abnormalities is unusual as it would be expected that a foal lying in an abnormal position for an extended period of time would lead to changes in the surrounding placenta (Wilsher and others 2012). However, in the cases described in this report only a visual examination of the placentae was undertaken and it is possible that some of the linear dimensions and weight of the placentae were abnormal.
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Short Communication In view of the obvious abnormal, gross morphological appearance of the placenta, in the third posterior presentation from Case 2 (see Fig 1) the authors must admit that the placentae in these other cases may well have had morphological abnormalities that were not noticed either due to damage to the placenta or failure on the authors’ part to observe. In the third posterior presentation from Case 2 the placental dimensions were grossly abnormal with the allantochorion not conforming to the typical F-shape when laid out postpartum. However, it was very difficult to compare its linear dimensions with the standard measurements set out by Whitwell and Jeffcott (1975) although an attempt was made to do this. The morphology of the placenta from Case 2 suggests that the bulk if not all of the fetus remained in the pregnant horn in the latter part of pregnancy after being fixed in posterior presentation. Although no histopathology was performed the authors did notice that the chorionic villi appeared sparse over an estimated 40 per cent of the gravid horn. An ultrasound diagnosis of posterior presentation at pregnancy diagnosis should be regarded as having the potential for a posterior presentation at birth. However early it was made, it should always be repeated later to confirm this continued presentation. Elective caesarean section might then be considered since parturition is likely to result in dystocia and possible death of the foal due to fluid aspiration, limb malposition or rib fracture. Elective caesarean has been reported to lead to 90 per cent fetal survival (Freeman and others 1999) although the complications of caesarean section should be considered including peritonitis, haemorrhage, retained placenta, laminitis, herniation, evisceration and incisional infection (Embertson 2002). The chance occurrence of two mares foaling with posterior presentation in two and three successive years is likely to be more than coincidental. Therefore, any mare which exhibits a posterior presentation at foaling should be considered at a high risk of doing so again in successive pregnancies and be monitored accordingly. Clearly, more investigation is required to determine why posterior presentations should occur.
Acknowledgements
To Allan Gunn and Imogen Hendersen for their efforts in accumulating the data presented from stud farms in Newmarket and the Hunter Valley NSW and for Sue Lichtwark’s data from Egmont Farm, Queensland.
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August 2, 2014 | Veterinary Record
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Five cases of consecutive posterior (caudal) presentation of the fetus in two mares J. R. Newcombe and G. M. M. Kelly Veterinary Record 2014 175: 120 originally published online June 4, 2014
doi: 10.1136/vr.101532 Updated information and services can be found at: http://veterinaryrecord.bmj.com/content/175/5/120.1
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