Usually, seminomas are found as incidental findings on a routine physical exam, or an owner may notice a swelling on the pet's testicle. Seminomas are often found together with other testicular tumors. They occur in both testicles 10 percent of the time. The majority of the time seminomas cause no clinical problems, but occasionally it may be malignant or cause other serious disease conditions.

Malignant seminomas usually spread to the lymph nodes of the inguinal (groin) or caudal (toward the tail) abdominal areas. Occasionally they may spread to the lungs. Rarely cryptorchid testicles with seminomas may become large enough to cause abdominal distention and pressure on other abdominal organs. Seminomas may also produce excessive estrogen or, more commonly, elevated androgen production. Changes in androgen or estrogen levels seen with testicular tumors have been associated with prostatic disease that includes prostatic hyperplasia, infection, abscesses, squamous metaplasia, and cyst formation. Elevated androgen levels may also cause perianal disease including perianal adenomas, carcinomas, and perianal hernias.

Seminomas have also been associated with a male feminizing syndrome. This is caused by changes in hormonal concentrations that result in an elevated estrogen levels that cause the male dog to take on some female characteristics. This occurs more frequently in animals that have Sertoli cell tumors.

A much more severe and potentially life-threatening condition associated with chronic (long standing) elevated estrogen levels and testicular tumors is estrogen-induced bone marrow hypoplasia. Elevated levels of estrogen have a toxic effect on the bone marrow, and bone marrow hypoplasia may develop where the cells in the bone marrow are damaged and cannot function properly. Since the cells in the bone marrow normally produce red blood cells, white blood cells and platelets that help in clotting, decreases in all three cell lines may be seen (pancytopenia). This may lead to anemia (from the decreased red blood cells), infections (from the decreased white blood cells), and bleeding tendencies (from the decreased platelets).

Sertoli cell tumors are the most common tumor associated with this condition, but seminomas can occasionally be responsible. Other diseases that have similar clinical symptoms as seminomas include:

Interstitial cell tumors and Sertoli cell tumors are other testicular tumors that cause masses on the testes. Both tumors are usually benign, and require aspiration or biopsy to confirm diagnosis.

Orchitis and epididymitis are inflammations of the testicle and epididymis, which is the tube-like structure along-side the testicle. They are usually caused by a bacterial infection. The condition generally is painful, and dogs tend to feel ill. Affected dogs may also have a fever. If the infection is severe, the swelling in the testicle may begin to spread up the scrotum and into the inguinal area. Epididymitis may sometimes be caused by Brucella infection

Testicular torsion is a twisting of the testicle where blood supply and or lymphatic drainage is compromised. The testicle is usually symmetrically enlarged and painful. The condition is often associated with testicular neoplasia. Testicular torsions occur with greater frequency in abdominal cryptorchid testes.

A spermatocele also called a sperm granuloma may occur due to a cyst-like dilation of the epididymis. Sperm may become trapped inside the dilation, and an inflammatory response may occur. This response may produce a small swelling in the epididymis. It is a benign condition, but may result in infertility.

An inguinoscrotal hernia occurs when abdominal contents pass into the scrotal sack. Most commonly, it is fatty tissue or a loop of bowel that enters through the inguinal canal. It is not a common condition.

If the tumor is located on a cryptorchid testicle and abdominal distention is present, other causes of abdominal swelling would need to be considered, such as other intra-abdominal masses or fluid accumulation.