While cryptorchidism (failure of a testicle or both testicles to descend into the scrotum) is encountered with relative frequency in companion animals, true monorchidism is rarely reported in any species and is limited to isolated case reports. Monorchidism (also known as unilateral anorchidism) is defined as the partial or complete absence of testicular tissue unilaterally, with or without spermatic cord or epididymal remnants and normal genitalia (Lamesch, 1994). The cause of this condition in humans is thought to result from testicular regression, usually as a result of ischaemia from intrauterine torsion (Lamesch, 1994). However, agenesis and ischaemic testicular necrosis have also been proposed as causes in the horse (Gardner et al, 2017). Clinical examination reveals the absence of one testicle and diagnosis in horses and other species is usually achieved through negative identification of the testicle at exploratory surgery/laparoscopy combined with blood hormone assays (Clements et al, 2020). To the author's knowledge, this is the first published case report of monorchidism in a pet rabbit.
Case presentation
A 1-year 11-month-old, 2.1 kg, male intact crossbreed rabbit presented to Abercorn Veterinary Clinic, Edinburgh after being fostered through a rabbit charity. Clinical examination by a veterinary surgeon confirmed the rabbit was male; however, only the left testicle was present (Figure 1). There was evidence of mild pododermatitis, and the foster carer reported intermittent sneezing; however, the rest of the clinical exam was unremarkable. The carer was instructed to keep a diary of the sneezing episodes to quantify frequency, and the rabbit was scheduled for re-evaluation to include conscious ultrasonographic scanning of the abdomen and inguinal canal for the absent testicle.
The rabbit was clipped sparingly to avoid heat loss and ultrasonography was performed by a veterinary surgeon. No testicle could be identified; therefore, the rabbit was scheduled for an exploratory laparotomy. The rabbit was premedicated with midazolam (0.5 mg/kg) and methadone (0.5 mg/kg) by intramuscular injection. An intravenous catheter was placed in the marginal ear vein and anaesthesia was induced with ketamine at 7.5 mg/kg by a further intramuscular injection. Anaesthesia was maintained with sevoflurane in oxygen at 1.5 L/min and Hartmann's solution was administered intravenously 10 ml/hour. The airway was maintained via a supraglottic airway device. The rabbit was monitored continuously by the author throughout the anaesthetic event. The rabbit was positioned in dorsal recumbency, and the abdomen was clipped and surgically prepared. A clear surgical drape was used to maximise visualisation of the thorax and monitor respiration.
As described by the veterinary surgeon, a 3 cm caudal midline incision was made through the skin and linea alba. The bladder was identified and reflected caudally to expose the dorsal surface (Figure 2). This allowed identification of the ductus deferens (on both sides). The ductus was then followed cranially to the point at which it crossed the ureter. At this point the ductus then wrapped around the ureter and travelled caudoventrally. On the side with the descended testicle, obvious testicular/spermatic blood vessels travelled with the ductus caudally towards the inguinal canal. On the undescended side these vessels were underdeveloped. They terminated with the ductus at what appeared to be a rudimentary piece of reproductive tissue, which at the time of surgery was thought to be the gubernaculum attached to the caudal pole, heading caudally into the inguinal canal. The vasculature was ligated using monofilament absorbable suture and the suspected rudimentary reproductive tissue was removed from the abdominal cavity and sent for histopathological analysis. Routine abdominal closure was performed and the descended testicle was surgically removed using a modified open to closed scrotal technique. An injection of meloxicam 1 mg/kg was administered subcutaneously at the end of surgery and the rabbit recovered uneventfully.
Meloxicam was prescribed at 0.6 mg/kg twice daily by mouth for 5 days and the rabbit was discharged the same day. No assisted feeding or use of pro-kinetic agents were required. Histopathology of the submitted tissue concluded it was consistent with uterinus masculinus, which is a vestigial embryonic remnant of the paramesonephric duct in males (Lim et al, 2015). In dogs, this has been associated with clinical signs such as dysuria, incontinence and urethral obstruction (Lim et al, 2015). This case represented with urine scalding approximately 10 days postoperatively, which resolved following another course of meloxicam at 0.6 mg/kg twice daily.
During this period, blood was taken for baseline kidney parameters. Blood urea nitrogen was normal at 5.5 mmol/l (reference range 3.3–8.1 mmol/l, Nationwide Laboratories), creatinine was normal at 106 µmol/l (reference range 71–256 µmol/l), manual packed cell volume was 33% and a titre for Encephalitozoon cuniculi was performed, which found no serological evidence of exposure via immunoglobulin G (IgG) and IgM. Urinalysis was unremarkable.
Due to the negative findings during exploratory surgery, it was decided blood hormone assays should be carried out to determine the likelihood of remaining testicular tissue, 6 weeks post-castration of the intact testicle. A blood sample was taken from the lateral saphenous vein for anti-mullerian hormone which yielded a negative result, identifying <0.01 ng/ml.
Consultation with the reporting pathologist advised that to strengthen certainty that there was no further testicular tissue present, baseline testosterone levels by radioimmunoassay and basal testosterone followed by human chorionic gonadotrophin (hCG) stimulation would be conclusive. Testosterone levels by radioimmunoassay were <0.1 nmol/l and a hCG stimulation test was performed with extrapolation from feline protocols (Nationwide Laboratories). A serum sample was taken from the lateral saphenous vein after topical application of local anaesthesic cream and using a considerate handling technique. An intravenous catheter was placed in the marginal ear vein and 450 iu of hCG was administered intravenously. The rabbit was returned to a quiet, predator free area of the practice and another serum sample was taken from the lateral saphenous 1 hour later. The results demonstrated no stimulation, with basal testosterone measuring 0.2 nmol/l and testosterone post-hCG 0.23 nmol/l. A note from the reporting pathologist explained that neutered male rabbits will still produce testosterone in the spring and summer but at a reduced level and these results are consistent with negative stimulation. No further action was taken, and the rabbit recovered uneventfully from all procedures.
Discussion
While single case studies represent one of the lowest levels of evidence in the evidence hierarchy, their value should not be dismissed as they still provide a structured way of disseminating knowledge and clinical experience (Morresey, 2018). This is of particular importance in cases which are identified infrequently and where there is a lack of literature. This case report describes the investigation into an unusual case of monorchidism in a pet rabbit. The possibility that this rabbit had been unilaterally castrated at a younger age was considered but deemed very unlikely by the veterinary surgeon, as there was no prior history of surgery or anaesthesia, the remaining testicle was normal and the rabbit lacked a scrotal sac on the affected side.
Diagnosis by a veterinary surgeon relies upon differentiation from cryptorchidism, which occurs more frequently. In this case, ultrasonography was used initially to screen for a second testicle before proceeding to an exploratory laparotomy. In dogs and cats, Felumlee et al (2012) found that the sensitivity for detecting abdominal testes was 96.6% and 100% for inguinal testes. The author is unaware of equivalent data for rabbits, and it is possible sensitivity is reduced in this species due to the volume of gastrointestinal content and associated gas obscuring small structures within the abdomen. An alternative imaging modality that could have been considered would be computed tomography; however, financial restraints eliminated this as a possibility for this case.
In this case, the rabbit underwent general anaesthesia and exploratory laparotomy where the structure sent for histopathology (uterinus masculinus) was surgically removed alongside the intact testicle. As exploratory surgery still did not identify testicular tissue and the rabbit was due to be rehomed as a neutered male, blood hormone assays were performed. Anti-mullerian hormone is a glycoprotein and is secreted in early fetal life of male animals by Sertoli cells. As a result of this, measurable amounts of anti-mullerian hormone in blood are indicative of testicular tissue (Murase et al, 2015). In this animal <0.01 ng/ml was detected which strengthened the suspicion this animal was monorchid. While there are data regarding reference ranges for anti-mullerian hormone in female rabbits (Böhmer et al, 2022), neither the clinical team involved with this case nor the reporting laboratory could identify similar data for males. Despite anti-mullerian hormone being used extensively in equine medicine to identify hemi-castrated unilateral cryptorchid horses, due to the rarity of the phenomenon the pathologist recommended testosterone and hCG stimulation tests. Sanni et al (2012) demonstrated that castration significantly decreased testosterone levels in the three groups under study (unilateral castration, bilateral castration and bilateral castration with testosterone replacement) in New Zealand White male rabbits. However, in the unilateral castration group there was complete recovery of testosterone levels to precastration values after only 2 weeks. This should ensure that the values obtained in this case were accurate and reflective of ongoing physiological state, as blood sampling was taken 6 weeks after unilateral castration, which allows sufficient time for testosterone levels to either decline or increase based on whether further testicular tissue is present.
Further strengthening the likelihood of monorchism was the negative hCG stimulation test. Berger et al (1976) demonstrated that even in juvenile rabbits (between 20–180 days of age), hCG stimulation at various doses (50, 150, 300 and 500 iu) resulted in significant increases of testosterone post-stimulation, with the most pronounced increase seen when rabbits of 180 days of age were given 500 iu. In this case, the rabbit was administered 450 iu of hCG as per a feline protocol by Nationwide Specialist Laboratories (2023). A specialist pathologist was consulted regarding this dose, and it was confirmed that hCG stimulation tests can be used in any mammalian species. In this animal, there was no stimulation following a dose at the higher end of the dose range (100–500 iu), allowing the veterinary surgeon to conclude that this animal did not have remnant testicular tissue.
Conclusions
In conclusion, a combination of investigations and tests have been used in this case which all provided agreement in results, allowing the veterinary surgeon to diagnose monorchidism in this case. However, further research in this area would be highly beneficial to gain species-specific protocols and reference ranges to confidently support the use of the aforementioned blood hormone assays in male rabbits. Despite this, to avoid invasive exploratory surgery where there is a suspicion of cryptorchidism/monorchism or where surgery has produced uncertainty regarding remnant tissue, hCG stimulation, basal testosterone and antimullerian hormone all appear to correlate positively, which may be useful when approaching cases following unilateral castration to exclude testicular remnants.