Dystocia is the lack of normal progression of parturition, usually involving lack of or difficulty in expulsion of fetuses through the birth canal. The incidence of dystocia differs among animal species owing to differences in anatomic and physiologic characteristics. Typical signs in small animals include active uterine contractions or straining without progression of fetal expulsion or, in the case of uterine inertia, a lack of active contractions.1
Causes of dystocia encompass both maternal and fetal factors. Maternal factors include underlying reproductive abnormalities, small pelvic size, primary or secondary uterine inertia, malnutrition, and systemic disease.1,2 Fetal factors include fetal malformations, fetal oversize (macrosomia), cephalopelvic disproportion, fetal malposition, and fetal death.1,2 Sometimes, multiple factors are involved. Other factors that can contribute to dystocia include obesity, stress, poor husbandry, and nutritional deficiencies (particularly deficiencies in vitamins A, D, and E).3,4
The reproductive tract of female rabbits differs from the tract of other female mammals in that it consists of 2 separate uterine horns, each connected to the vaginal vault via a separate cervix. Rabbits have a brief gestation period of 30 to 32 days and produce a mean of 4 to 10 kits/litter.5 Sexual maturity occurs between 4 and 8 months of age, depending on breed.6 Because does are able to conceive within hours after giving birth, a single female may produce > 60 kits in a given year.5 Just prior to parturition, does will remove hair from the dewlap region and body to create a nest,4 and owners will often notice this. Parturition usually occurs in the morning and consists of stages similar to those of other mammals.5 The first stage consists of internal uterine contractions and dilation of the cervices. The second and third stages involve active expulsion of the fetus and fetal membranes, the duration of which varies by species.2 In rabbits, the second stage (active fetal expulsion) is brief (< 30 minutes), making it challenging to identify signs of dystocia and, therefore, identify abnormalities and when intervention may be necessary.6
Dystocia is uncommon in rabbits,5 and this species appears to be susceptible to true obstructive dystocia.4 One case report7 describes a 4-year-old Dwarf Lop rabbit with dystocia due to fetal oversize and secondary uterine inertia that was successfully treated with medical management and transabdominal manipulation for AVD. Other case reports describe successful surgical intervention to treat dystocia in a 7- to 8-month-old doe with suspected inappropriate uterine stimulation due to a small litter size8 and in another doe with a primary fetal abnormality (anasarca).9
The objective of the study reported here was to review clinical and pathological findings for rabbits examined for dystocia at a veterinary teaching hospital. Because dystocia is uncommon in rabbits, we believed that information about the medical history as well as diagnosis, treatment, and outcome of affected rabbits could help in the prevention and clinical management of future cases of dystocia in rabbits.
Materials and Methods
Case selection criteria
Electronic medical records of female rabbits examined at the William R. Pritchard Veterinary Medical Teaching Hospital, University of California-Davis, between January 1, 1996, and August 1, 2016, were searched for the keywords dystocia, inertia, fetal mismatch, uterine, and retention. Medical records identified in this search were reviewed by 1 investigator (MDG) to determine whether dystocia had been diagnosed. For this purpose, dystocia was defined as any event of abnormal parturition that required intervention. Records with a diagnosis of definitive or suspected uterine abnormalities without involvement of pregnancy were excluded from final evaluation. Rabbits were included in the study if they had been brought to the hospital because of clinical signs suggestive of dystocia, had dystocia diagnosed following diagnostic procedures, or had dystocia treated by owners at home immediately prior to examination.
Medical records review
Information extracted from each medical record included signalment (including age and breed when noted), month of evaluation, pertinent history, clinical signs, physical examination findings, laboratory test results, diagnostic imaging findings (including abdominal radiography and ultrasonography), treatment, identified cause of dystocia (when available), and outcome. Collected historical information included owner concerns at evaluation, immediate relevant history, husbandry, breeding history (when available), and previous litters. Housing was categorized as indoor, outdoor, both, or unknown. All diagnostic tests and procedures were performed at the teaching hospital. Treatment was classified as AVD, medical management, surgery, or a combination of these treatments or euthanasia without treatment. The cause of the dystocia was recorded if it could be determined from necropsy or other diagnostic findings.
Statistical analysis
Statistical softwarea and the Shapiro-Wilk test were used to evaluate age and body weight data for normality of distribution. Normally distributed data are reported as mean ± SD, and nonnormally distributed data are reported as median (range).
Results
Animals
Ten rabbits met the study selection criteria after review of 52 medical records of female rabbits with reproductive disease examined from 1996 through 2016. Pertinent case details were summarized (Supplementary Table S1, available at avmajournals.avma.org/doi/suppl/10.2460/javma.254.8.953). Rabbits included 9 client-owned domestic rabbits (Oryctolagus cuniculus) and 1 wild Riparian brush rabbit (Sylvilagus bachmani riparius), considered an endangered species in California.10 Four client-owned rabbits were of unidentified breeds; breeds of the remaining 5 included Californian, Holland Lop, Netherland dwarf, Mini Rex, and English Angora. Four client-owned rabbits were kept as show or breeding stock.
Median age at the time of evaluation for the 8 rabbits for which it was recorded was 1.25 years (range, 6 months to 11 years). Two of these 8 rabbits were < 1 year old, 5 were between 1 and 4 years old, and 1 was > 4 years old. Mean ± SD body weight at the time of evaluation for the 10 rabbits for which it was recorded was 2.09 ± 0.81 kg (4.6 ± 1.8 lb). Body condition score on a 9-point scale for the 9 rabbits for which it was recorded was 5 (ideal; n = 6), 6 (2), or 4 (1). The rabbit with the lowest body condition score was also the oldest of those with a known age (11 years).
Concerns at initial examination
All client-owned rabbits had multiple concerns as reported by the owner. Such concerns included bloody or colored vaginal discharge (n = 4); active straining or contractions (5); dead fetuses noted within 24 hours prior to evaluation (5); dead fetus visible within the vaginal canal (4); decrease in appetite, defecation frequency or amount, or both, suggestive of gastrointestinal stasis (3); preliminary signs of parturition such as nest building with no observed subsequent stages of parturition (2); increase in respiratory rate or effort (1); and unremarkable parturition but palpable abdominal mass (1). One rabbit with an abnormal appetite and defecation had a dead fetus successfully manually removed from the vaginal canal by the owners prior to evaluation. All 9 client-owned rabbits had signs of abnormal progression of parturition, but the wild Riparian brush rabbit had only hemorrhagic vaginal discharge, which was noted after it had been captured as part of an endangered species recovery program.
Husbandry, season, and breeding history
No specific seasonal distribution of dystocia was noted, with does brought for evaluation throughout the year in February (n = 1), March (2), August (1), September (2), November (3), and December (1). All 9 client-owned rabbits had bred in captivity, and the wild Riparian brush rabbit was presumed to have had normal wild mating behavior. Two client-owned rabbits were believed by the owner to be male when initially obtained. Six rabbits were primiparous, and 1 rabbit had an unremarkable parturition history, having successfully delivered multiple litters prior to the dystocia episode. Parity and parturition history of the remaining 3 rabbits were unknown. Rabbits were housed primarily outdoors (n = 5) or primarily indoors (2), or their housing environment was not specified in the medical record (3).
Clinical and laboratory findings
Physical examination findings included serosanguineous vulvar discharge (n = 3), evidence of gastrointestinal stasis (3), swollen vulva (2), straining or active contractions (1), perfusion estimates consistent with hypovolemic shock (1), and distended abdomen with a palpable fetus (1). A CBC and plasma biochemical analysis were performed for 6 rabbits, and urinalysis and abdominal fluid analysis were performed for 1 rabbit with abnormalities within the urinary tract and free peritoneal fluid detected on abdominal ultrasonography. Other diagnostic tests performed in the same 6 rabbits included measurement of plasma ionized calcium concentration (n = 4) and a neuromuscular panel (2). Clinicopathologic findings included mild to moderate anemia (n = 3), hypocalcemia (2; determined on the basis of plasma total calcium and ionized calcium concentrations), and an inflammatory leukogram (1). One rabbit had hypoproteinemia and azotemia.
Diagnostic imaging
Abdominal radiography was performed for 9 rabbits, 6 of which also received an abdominal ultrasonographic examination. When possible, the widest point of the fetal skull and the pelvic canal were measured on radiographs and compared. Ultrasonography was used to confirm the presence of live kits in 2 rabbits.
Fetuses were identified within the uterus only (n = 6 rabbits) or within the vaginal canal and uterus (3). Ultrasonography revealed no fetuses in 1 rabbit, and its owners had manually extracted a dead fetus wedged within the vaginal canal before taking the rabbit to the hospital. In 1 rabbit, radiography revealed gas within a single fetus, suggesting fetal nonviability11 (Figure 1), and this finding was confirmed via brief abdominal ultrasonographic examination. No radiographic evidence of fetal death was noted for 3 rabbits; however, no ultrasonography was performed to confirm that the fetuses were alive. Additional imaging findings in 1 rabbit included renal abnormalities with free peritoneal fluid. No rabbit had evidence of underlying uterine abnormalities.
Treatment and outcome
Two rabbits were euthanized without treatment following radiographic diagnosis of dystocia owing to owners' financial constraints. Multiple treatment approaches were used in the hospital for the remaining 8 rabbits, including medical management alone (n = 5), AVD alone (1), AVD plus medical management (1), and surgery alone (1). Assisted vaginal delivery consisted of gentle manual removal of a fetus that was partially protruding from the vaginal canal but unable to pass on its own. This approach was successful for all 3 rabbits in which it was used, including 1 rabbit in which AVD had been performed by the owner before it was brought to the hospital. The 1 rabbit that underwent AVD alone passed an additional mummified fetus afterward without the need for medical management. For the 1 rabbit that underwent AVD plus medical management, medical management consisted of IM administration of oxytocin (total of 4 U; 2.2 U/kg [1 U/lb]), which was provided to expel additional dead fetuses that remained within the uterus. This doe was 1 of the 2 rabbits with hypocalcemia (plasma total calcium concentration, 11.1 mg/dL [in-house reference limits, 13.7 to 16.2 mg/dL]); plasma ionized calcium concentration, 1.31 mmol/L [reference limits, 1.6 to 1.82 mg/dL]12), but whether the hypocalcemia was a primary cause of inertia or secondary to prolonged parturition and contractions could not be determined.
Medical management for the remaining 5 rabbits that received it consisted of IM administration of oxytocin (total of 2 to 8.85 U or 0.57 to 4.6 U/kg [0.26 to 2.1 U/lb] in 1- to 2-U increments) to promote uterine contractions with or without PO or IM administration of calcium gluconate. Specifically, oxytocin was administered 30 minutes after the calcium gluconate was, if both were administered, with a single dose of oxytocin administered to 2 rabbits and multiple doses administered 30 to 60 minutes apart to 3 rabbits. Additional supportive care included SC crystalloid fluid administration, heat support, an NSAID (meloxicam), an antimicrobial (trimethoprim-sulfamethoxazole), an analgesic (buprenorphine), and syringe-fed nutritional support.b Medical management alone was considered successful for 3 rabbits but unsuccessful for 1 rabbit, which was subsequently euthanized. The remaining rabbit passed 4 kits, but abdominal palpation revealed an additional fetus within the uterus. No information was available regarding the fate of this remaining fetus.
Surgery (cesarean section) was performed on the wild Riparian brush rabbit, yielding 1 live and 2 dead kits. Three live fetuses had been noted on abdominal ultrasonography prior to surgery, and the cause of death for 2 kits was unknown. This rabbit recovered without complication at a wildlife rehabilitation facility.
Overall, 7 rabbits were discharged from the hospital following successful management of dystocia. One rabbit (the one for which the owner had performed AVD) required no treatment other than basic supportive care given that imaging revealed no remaining fetuses within the reproductive tract. One rabbit underwent ovariohysterectomy 4 months after the dystocia episode because of secondary incontinence, and no uterine abnormalities were noted at that time. Because long-term follow-up information was only available for 1 rabbit, an overall morbidity and mortality rate could not be determined.
Dead fetuses delivered prior to initial examination or detected within the reproductive tract on examination were noted for all rabbits evaluated. Overall, 35 fetuses were accounted for, 25 of which were dead or later died. No primary uterine abnormality was identified in any rabbit at the time of dystocia, although thorough imaging of the uterus was not performed for all rabbits.
Cause of dystocia
The cause of the dystocia was determined for 8 rabbits. The other 2 rabbits lacked diagnostic or postmortem evaluation to determine a cause. One of these 2 rabbits was the one that had AVD performed by the owner, and abdominal ultrasonography revealed left renal pyelectasia and peritoneal effusion. These abnormalities, in addition to an inflammatory leukogram, could have been related to the underlying cause of dystocia or a consequence of it.
Identified causes of dystocia included fetal-maternal mismatch (n = 4; Figure 2), uterine inertia (2), fetal death or mummification (1), and stress-induced abortion (1). For the 2 rabbits with uterine inertia, this condition had been suspected on the basis of their response to medical management. Both rabbits had retained dead fetuses that may have contributed to secondary uterine inertia. Stress-induced abortion was suspected in the wild Riparian brush rabbit because of the circumstances surrounding its capture in the species recovery program. In this rabbit, viable fetuses were still evident within the uterus when it arrived. Stress-induced abortion would have prevented the normal progression of parturition, resulting in death of the remaining fetuses without intervention.
Postmortem examination
Two rabbits with fetal-maternal mismatch underwent necropsy following euthanasia, and fetal macrosomia and associated vaginal hemorrhage were identified. In 1 of these 2 rabbits, 2 oversized fetuses were present (Figure 3). No evidence of underlying uterine abnormality was noted on gross or histologic evaluation. This rabbit received a diagnosis of hepatic lipidosis, which can develop in periparturient does with a negative energy balance or secondary to hyporexia from any illness,3,4 but whether the hepatic lipidosis contributed to the dystocia was unknown. The other rabbit had a single large fetus within the vaginal vault and hemorrhage within both the uterus and vagina. Autolysis of the fetus indicated late-term fetal death. Again, no underlying uterine abnormality was noted on gross or histologic evaluation.
Discussion
Results of the present 21-year retrospective study of clinical features in a small case series of rabbits examined for dystocia suggested that fetal-maternal mismatch from fetal macrosomia was the most common cause. The included rabbits were most commonly young (≤ 4 years old), primiparous does. Reports of dystocia in this species are uncommon, compared with reports in other domestic animals, and the frequency of rabbits being examined for dystocia in the present study was low (approx 1 case/2 y). This low frequency may have been attributable to improvements in owner education leading to elective neutering of captive rabbits or a low predisposition of rabbits to this condition.
The reported incidence of dystocia in bitches, queens, and mares is approximately 5%,2 3% to 3.5%,2 and 4% to 10%,13,14 respectively, depending on the breed. Of the small mammal species, guinea pigs are most commonly examined for dystocia, but no specific incidence has been reported.5 The incidence of dystocia could not be estimated for the rabbits of the present study because such a statistic would have likely been an overestimate given the low frequency of rabbits with unremarkable parturition brought to the authors' referral hospital.
Because of the small number of rabbits with dystocia and the lack of rabbit patients with an unremarkable parturition, no breed or size predilection could be determined in the present study. We speculate that because most of the included rabbits were of small breeds (6 of the rabbits weighed < 2 kg [4.4 lb], but only 1 was considered a dwarf breed), size may have played a role in development of dystocia. In other species, dystocia can be more common in specific breeds, potentially owing to conformational or temperament differences. For example, brachycephalic breeds of dogs and cats such as Bulldogs, Siamese, and Persians appear to have a higher incidence of dystocia than other breeds.1 Friesians and Thoroughbreds may also have a higher incidence than other horse breeds.13,14
No seasonal pattern of dystocia was apparent for the client-owned rabbits of the present study, likely because rabbits in captivity often receive no natural environmental cues and breed year-round.5,6,15 Rabbits in natural conditions in the Northern Hemisphere have a long breeding season (between early February and late summer), with highest conception rates in the spring6 and a period of reproductive inactivity in the winter.5 The wild rabbit in the present study had dystocia in late summer, consistent with natural breeding cycles.
All client-owned rabbits in the study reported here had signs of an abnormal second stage of parturition, which included straining and vaginal discharge without appropriate progression of fetal expulsion. Several rabbits had dead fetuses or kits, including 4 rabbits that had immovable fetuses within the birth canal. Most of these rabbits were successfully managed and survived to discharge from the hospital. For rabbits that were euthanized, whether medical or surgical treatment would have been successful remains unknown. The brief second stage of parturition characteristic of rabbits is similar to that of horses, for which a mean duration of 16.7 minutes has been reported.14 However for horses, every 10-minute increase in the duration of this second stage is associated with a 10% increase in the risk of fetal death and a 16% increase in the risk of a foal not surviving to discharge from the hospital.16 These small increments of time are likely just as important for rabbits, and, therefore, rapid diagnosis and treatment are important for survival of both does and kits.
Most rabbits of the present study had visible signs of dystocia and parturition that were recognized by their owners, allowing treatment of the doe, and yet several (25/35) fetuses died. Although prolongation of second-stage parturition that went undetected until clinical signs were noted likely contributed to this high fetal mortality rate, fetal factors may have also played a role. Late-term fetal death was confirmed on postmortem examination of 1 rabbit, further supporting this supposition. The fact that most does give birth in the early morning6 further complicates detection of problems with parturition.
The most common cause (4/10) of dystocia in the rabbits in the present study was fetal-maternal mismatch. In those circumstances, fetal macrosomia was suspected on the basis of radiographic findings or confirmed on postmortem examination (Figures 2 and 3). This primary fetal abnormality secondarily caused obstruction of the birth canal and may have resulted in secondary uterine inertia in some does. Three of the does with fetal-maternal mismatch had a smaller than typical number of fetuses (< 4), which could have been related to the macrosomia. Guinea pigs, another small mammal species, appear more predisposed to dystocia owing to a combination of large fetal size, narrow pelvic canal, and fusion of the pubic symphysis at maturity than dystocia owing to other causes.17,18 No doe in the present study, including the 11-year-old doe, had any reproductive tract abnormalities noted that could have contributed to the dystocia. Neither the 11-year-old rabbit nor the wild Riparian brush rabbit had obstructive dystocia, and both produced live kits following treatment, suggesting that obstructive dystocia may be associated with a higher incidence of fetal death, whether it is the primary cause or the result of prolonged parturition.
Stress can induce abortion in multiple species, including mice and humans, owing to the action of substance P and an increase in cortisol release.19–21 Disruption of normal oxytocin cycles from irregular nursing patterns can also result in abnormal parturition and stillbirths.22 The wild rabbit of the present study was suspected to have had spontaneous stress-induced abortion, likely related to its capture. This is an important consideration during wildlife capture events, particularly when part of an endangered species recovery program, because of the potential impact on the relevant population. However, improper husbandry or outdoor housing of domestic rabbits can also potentially contribute to spontaneous abortion and subsequent dystocia. One client-owned rabbit in the present study had both a mummified fetus and a dead fetus in the vaginal canal.
The critical period for fetal death in rabbits begins at 21 days of gestation.3 Prior to this point, resorption can take place following fetal death and such an event is considered normal. However, if fetal death, whether stress induced or otherwise, occurs after 21 days, then abortion occurs, which can then lead to dystocia.4 Other than abnormal passage of the dead fetus, stillborn or aborted fetuses could potentially lead to delayed progression of parturition or abnormal passage of any remaining fetuses through obstruction or secondary uterine inertia. Knowledge of breeding date may then be particularly important when attempting to prevent dystocia or abortion in pregnant does.
Medical management was effective for most rabbits with dystocia in the present study, even for some rabbits with fetal-maternal mismatch or primary fetal abnormalities. In the 1 rabbit that received it, surgical management was also effective. Medical management was similar to that recommended for dogs and cats, which includes manual AVD when possible with or without oxytocin administration to facilitate uterine contractions, unless a definitive diagnosis of obstructive dystocia is made.2 Although not used in the present study, transabdominal manipulation to allow for AVD was successful in a previously reported case of dystocia in a rabbit7 and could assist in correction of positioning abnormalities if present. Compared with a single large dose, SC or IM administration of repeated small doses of oxytocin starting at 0.1 U/kg (0.045 U/lb) with a gradual increase to a maximum of 2 U/kg (0.9 U/lb) is more effective in dogs and cats and lowers the risk of sustained myometrial contractions. For dogs, the recommendation is to avoid exceeding a maximum of 20 U for any single dose.2 In rabbits, a dose of 1 to 3 U/rabbit is recommended when uterine inertia is suspected.5 Administration of calcium gluconate and potentially dextrose is also recommended to increase the strength of contractions and prevent further debilitation of the doe.2 To have the greatest impact on uterine contractions, these adjunct treatments are recommended to be administered prior to oxytocin. Surgical management is indicated when medical management is unsuccessful, evidence of fetal distress is present, or obstructive dystocia is identified.23
A major limitation of the study reported here was the variability in detail recorded in the electronic medical records over the 21-year study period, which was attributable to differences in attending clinicians and admitting services (emergency vs daytime) and changes in the electronic record system over time. Regardless of this limitation, findings of the present study and the previously published case reports7–9 suggested that rabbits may be more likely to have obstructive dystocia from fetal macrosomia and secondary uterine inertia than other causes of dystocia. Young, primiparous does were common among the included rabbits. Medical management alone was successful for many rabbits and should be considered, particularly when surgical management is not possible. Encouragement of initial wellness visits at the time of adoption could allow confirmation of a rabbit's sex and education of owners about possible future pathological conditions. A full diagnostic evaluation, including imaging, is recommended for any doe with evidence of dystocia.
ABBREVIATIONS
AVD | Assisted vaginal delivery |
Footnotes
Shapiro-Wilk normality test. Available at: sdittami.altervista.org/shapirotest/ShapiroTest.html. Accessed May 15, 2017.
Oxbow Critical Care, Oxbow Animal Health, Murdock, Neb.
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