Testicular Enlargement in a Stallion

A 19 year-old stallion presented for the complaint of testicular enlargement. On physical exam, the left testes palpated firm, non-painful and was approximately 30-40% larger than the right testes (Figure 1). The potential differentials for an enlarged testes in a stallion include testicular neoplasia (cancer), hydrocele, scrotal hernia, testicular torsion, and orchitis. Definite diagnosis is based on ultrasound exam and histologic exam of the abnormal tissue. Ultrasound exam of the stallion's testes was performed during the initial visit.

Figure 1

In Figures 2 and 3, the left testes is imaged via ultrasound. The testicle appears to be divided by two tissues-types which vary in density or echogenicity (degrees of brightness). The normal tissue appears brighter or hyper-echogenic whereas the abnormal tissue appears darker or less echogenic. The edges of the abnormal tissue are distinct and there are multiple, small "stars" or areas of increased echogenicity throughout the abnormal tissue. 

Figure 2

Figure 3

Ultrasound exam of the right testes revealed a homogeneous tissue consistent with normal testicular structure (Figure 4). The echogenicity did NOT vary throughout the testicle and the over-all size of the testicle was less than the left testicle. In Figure 5, the left and right testicles were imaged simultaneously and the difference in testicular architecture becomes more clear when comparing the "normal" right tests to the "abnormal" left testes. 

Figure 4

Figure 5

Testicular neoplasia or cancer is relatively rare in stallions compared to dogs. The types of cancer include seminomas, sertoli cell tumors, and leydig cell tumors. Histopathologic exam is required to determine the type of testicular cancer. All three types of cancer will result in compromise of normal sperm-producing tissue and thus reduction in fertility. Seminomas are the most common of the three types of testicular cancer in stallions and may have a high degree of malignancy and invasiveness. Scrotal hernia, testicular torsion and orchitis are additional causes of testicular enlargement which are typically associated with varying degrees of pain and inflammation. Ultrasound findings of these conditions may include small intestinal loops, dilated vascular supply and pockets of purulent debris. Similar to testicular cancer, these conditions typically alter the conditions for normal sperm production and will reduce fertility. A more benign cause of testicular enlargement is known as a hydrocele. This corresponds to an accumulation of abdominal fluid within the scrotal sac. Although benign, the increase in fluid may cause change in temperature and pressure which will also affect sperm production. This condition is more easily confirmed with ultrasound exam alone!.

The stallion in this case is scheduled for surgical removal of the affected testes for definitive identification of the cause for the testicular enlargement. Stay tuned!!