Monorchidism in an appaloosa stallion

THE term ‘monorchidism’ has been used to describe the absence of a descended testicle but should, more strictly, be used to describe the complete absence of one testicle (Parks and others 1989, Santschi and others 1989, Cox 1993, Strong and others 1997). This condition may result from unilateral testicular agenesis or may follow a unilateral vascular accident. ‘Cryptorchidism’ is the correct term to define the retention of a testicle that does not descend into the scrotum (Parker and others 1997). The presurgical evaluation and surgical approach to cryptorchid and monorchid horses are complex diagnostic and surgical challenges (Schumacher 1992). A definitive diagnosis can only be made after surgical exploration of the abdomen, removal of the other testis and then hormonal testing. The human chorionic gonadotropin (hCG) stimulation test in horses over 18 months of age is the most reliable indicator for the presence of vital testicular tissue (Silberzahn and others 1989, Searle and others 1999). This short communication describes the evaluation and treatment of a case of monorchidism in an appaloosa stallion. An eight-year-old appaloosa stallion was referred to the Department of Veterinary Clinical Sciences of the University of Teramo for castration, with no history of previous surgery. Clinical examination showed the absence of the left testicle in the scrotal or inguinal regions. Laparoscopy in dorsal recumbency and under general anaesthesia was planned to diagnose left abdominal cryptorchidism or monorchidism. The horse was fasted for 36 hours and water was withdrawn 12 hours before surgery. Preoperative intravenous flunixin meglumine (1·1 mg/kg), 20,000 iu/kg bodyweight intramuscular penicillin and 6 mg/kg intravenous gentamicin were administered. Premedication with 0·01 mg/kg acepromazine intramuscularly 30 minutes before induction was followed by general anaesthesia induced with 1·1 mg/kg xylazine intravenously and 2·2 mg/kg ketamine intravenously. Anaesthesia was maintained with isoflurane in oxygen. Laparascopic examination according to the technique of Fischer (2002) did not identify the left testicle or vaginal ring, but a cylindrical structure, approximately 1 cm wide, was seen ending near the left internal inguinal ring. It was connected to the peritoneal surface by a short fibrous band. A piece of this structure was collected for histopathological examination. A left parainguinal laparotomy was then performed for inspection of the left dorsal and sublumbar abdominal region, but no testicle was found. Thereafter the right testicle was removed. Histopathology showed the sample from the cylindrical structure to be dense fibrous tissue with no signs of testicular tissue or scarring, suggesting the infarction of a testis. Postoperative therapy included tetanus prophylaxis, and 20,000 iu/kg bodyweight penicillin G given intramuscularly every 12 hours, 6 mg/kg gentamicin administered intramuscularly every 24 hours, and 1·1 mg/kg flunixin meglumine administered intravenously every 24 hours for three days. Eight days after surgery, a hCG stimulation test was performed to determine whether functional testicular tissue was present. This test was repeated 60 days later. Blood samples were collected for determination of plasma testosterone concentration before and one and 24 hours after intravenous administration of 10,000 iu hCG (Searle and others 1997). The blood was stored in sterile vacutainers containing lithium heparin (Venoject; Terumo), and plasma was obtained after centrifugation for 10 minutes at 1200 g. The plasma was then put into Eppendorf tubes at 20°C. A testosterone assay was performed with a radioimmunological assay technique using a commercial kit (Direck RIA; Sorin Biomedica) with an analytical sensitivity of 0·02 ng/ml (Arighi and Bosu 1989, Strong and others 1997). Table 1 shows the values of plasma testosterone before and after hCG administration. Following the testosterone results monorchidism was definitively diagnosed, and six months after surgery the horse showed no stallion-like behaviour. The tubular, fibrous structure detected during laparoscopy may have been the distal part of the undifferentiated ductus deferens, or, more likely, the gubernaculum testis in a stallion with testicular agenesis. It was decided to use a hCG stimulation test rather than the simple measurement of conjugated oestrogens as suggested by Schumacher (1992) to avoid the possibility of false positive results. The basal hormonal values for oestrogens and androgens would be expected to be high in the stallion soon after castration. However, according to Silberzahn and others (1989), the production of oestrogen is almost unresponsive to hCG treatment, and it was therefore decided to use the stimulation test to evaluate the changes in the androgen levels. The first hCG stimulation test was performed eight days after the orchiectomy, as suggested by Santschi and others (1989) and Strong and others (1997). The basal value of the testosterone was considered relatively high, despite the decrease seen one and 24 hours after the administration of hCG. The high basal value during the first test may have been due to the short time after orchiectomy. However, eight weeks after castration, the basal level of androgens was low, and it is suggested that at least this time should be allowed between surgery and testing to achieve the maximum accuracy of any hormonal test. As previously mentioned, a definitive diagnosis of monorchidism in the horse can be achieved only after a complex procedure. Moreover, laparoscopic examination of the abdomen in dorsal recumbency cannot be exhaustive because, even using the Trendelenburg position, dorsal abdominal structures cannot be thoroughly inspected (Galuppo 2002). After removal of the descended testicle, the only reliable way to make a diagnosis is to determine the blood testosterone level and before one and 24 hours after stimulation with 10,000 iu hCG (Arighi and Bosu 1989). In horses with no vital testicular tissue, there should be no increase above basal testosterone concentrations (Searle and others 1997).