The volume of ejaculate and number of sperm are defined units that vary in various animal species. Correlation between scrotal circumference, testicular size and quantitative semen characteristics is high but is age and breed dependent. A standard semen scoring system in bulls considers spermatozoal morphology, spermatozoal motility and relative scrotal circumference. The majority of testicular tumors in domestic animals occur in dogs perhaps due to their longevity. Testicular tumors have been described in bulls and horses as well, but the overall incidence in these species is low when compared to the dog.
Testicular tumors are extremely rare in pigs and cats.
The gross appearance of three of the common canine testicular tumors are quite typical. These can often be diagnosed on gross examination with accuracy. Microscopic examination, however, is mandatory to assess malignant potential. In the dog, interstitial cell tumors are the most frequently occurring, while seminomas and Sertoli cell tumors develop with slightly less frequency. Of course, some of these tumors may be endocrinologically active.