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EQUINE VETERINARY EDUCATION / AE / MARCH 2015


Clinical Commentary The diagnostic challenge of scrotal enlargement in the stallion


A. N. J. Claes and T. A. E. Stout* Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. *Corresponding author email: t.a.e.stout@uu.nl


Introduction


Mature intact stallions are underrepresented in the domestic horse population, primarily because most stallions are castrated at a young age to ameliorate the expression of sexual or aggressive behaviour. Nevertheless, because the major reason for maintaining an intact stallion is for the purpose of breeding they are often valuable animals. For expensive or potentially popular breeding stallions, it is common practice to evaluate testicular size as an index of semen production capacity and, depending on the studbook with which the stallion is to be registered, further parameters of semen production and quality may also be evaluated prior to acceptance or insurance as a breeding stallion. Once at stud, the external genitalia of stallions used to mate significant numbers of mares are inspected regularly, albeit fairly superficially, either after semen collection for artificial insemination or after natural cover. In most cases, per cycle pregnancy rates are also monitored throughout the breeding season. The aim of the initial checks is to ensure that the stallion has normal genitalia and can be considered ‘breeding sound’, whereas subsequent monitoring is focused on ensuring optimal reproductive efficiency, but has the added advantage of helping to identify any reproductive problems at an early stage. By contrast, intact stallions that are not used for breeding or used only on occasional mares, such as the Hackney pony stallion described in this issue (Stoll et al. 2015), are not regularly inspected or closely monitored and, as a result, the detection of subtle or chronic reproductive tract abnormalities is likely to be delayed.


Differential diagnosis and diagnostic work-up of scrotal enlargement in the stallion


Unilateral or bilateral scrotal enlargement is one of the most common and readily apparent genital tract abnormalities in the stallion. In the case reported in this issue, Stoll et al. (2015) were able to restrict the differential diagnoses for scrotal enlargement based on the absence of scrotal or testicular pain. An enlarged, painful scrotum can result from various inflammatory, obstructive or traumatic conditions including periorchitis, orchitis, epididymitis, testicular abscessation, testicular haematoma, inguinal hernia and torsion of the spermatic cord. In parallel with the presence or absence of pain, differential diagnosis can be greatly assisted by identifying the anatomical structure or site affected, where this may include the scrotal skin, vaginal tunic or cavity, testis, epididymis, or spermatic cord (Table 1). In this respect, the list of potential causes of an enlarged scrotum is quite large and it is important to identify the underlying aetiology in order to implement an appropriate treatment regime, be it specific or symptomatic, and offer a reliable prognosis. Rapid diagnosis can also be critical because some of the underlying conditions


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can have a profound detrimental effect on spermproduction capacity, spermquality and fertility and/or the general health of the stallion; indeed, a strangulating inguinal hernia will need emergency surgery to avoid severe complications while it is advantageous to detect testicular tumours early because some, in particular seminomas, have the potential to metastasise and thereby become life-threatening (Sherman et al. 1990). The first step in the differential diagnosis of scrotal enlargement is to obtain a detailed medical and, if applicable, reproductive history followed by a general clinical examination (i.e. are there indications of systemic disease). Subsequently, the scrotum should be inspected and palpated noting the size, symmetry, consistency, orientation (as indicated by the position of the cauda epididymis), mobility, temperature and sensitivity to palpation of both testes and the other scrotal contents. Palpation of the scrotal contents of the Hackney pony stallion reported by Stoll et al. (2015) revealed a relatively large, firmmass attached to the left testis and several smaller masses connected to the right testis. The absence of pain as a presenting sign or during palpation helped significantly to narrow down the range of possible causes. Moreover, because the masses could be delineated from the testes during scrotal palpation, it was probable that they did not originate from the testicular parenchyma. Nevertheless, ancillary diagnostic techniques were quite rightly used to confirm the clinical impression. One ancillary technique that was not considered appropriate in the current case was semen evaluation. Semen evaluation can, however, be a valuable diagnostic and prognostic indicator in stallions with testicular or scrotal pathology, since it gives information about the number and normality of sperm cells and the presence of abnormal (e.g. inflammatory) cells, microorganisms etc. This can be particularly valuable during diagnosis and monitoring of the response to treatment in case of inflammatory conditions. Nevertheless, the most useful initial diagnostic tool for stallions with an enlarged scrotum is ultrasonography, which will assist localisation of any enlargements or abnormalities to anatomical locations within the scrotum and give an indication of the nature or composition of any abnormal structures. Indeed, scrotal ultrasonography in the Hackney pony stallion described by Stoll et al. (2015) confirmed that the masses were paratesticular, suggesting that they originated in either the tunica albuginea of the testis or the vaginal tunic, and further demonstrated that they were poorly vascularised soft tissue masses. The differentiation between a tumour originating from the testicular parenchyma as opposed to other tissues within the scrotum is of significance because, as mentioned above, some testicular tumours have been associated with a significant risk of metastasis. In cases of anticipated malignancy and indeed in stallions presenting with acute scrotal pain, ultrasonography of the spermatic cord


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