The most common cause of prostatitis is believed to be ascending infection from the urethra. The prostate can also become infected from infections in the bladder, kidneys or blood. If other forms of prostatic disease are present, such as cysts, neoplasia or squamous metaplasia, the prostate may be predisposed to developing a secondary infection. E. coli is the most common bacterium that causes infection.

There are actually two different clinical presentations of prostatitis in the dog: acute and chronic. These two forms of the disease often present very differently, and require a different work-up and different therapy. In acute prostatitis, animals are usually quite ill and may even require emergency care. Animals are usually febrile and may have significant abdominal pain. Some dogs may even present with a critical blood infection (septicemia). On the other hand, dogs with chronic disease are generally much more stable or and have no clinical symptoms.

Signs of chronic prostatitis are may be subtle and include: chronic intermittent urinary tract disease, intermittent discharge from the urethra, weight loss, and infertility in the breeding animal. Chronic prostatitis may develop after acute prostatitis is treated. Many times the diagnosis of the chronic disease is difficult to confirm and a prostatic biopsy is required for definitive diagnosis.

Other diseases that may cause similar clinical signs as prostatitis include:

Urinary tract infections. Bacterial infections of the urinary bladder or kidneys may cause blood in the urine, fever and abdominal pain. It is not uncommon for an animal to have a urinary tract infection together with prostatitis.

Prostatic abscess. An abscess is a walled off pocket of infection containing white blood cells, bacteria and cellular debris. Abscesses occasionally form within the prostate gland in cases of chronic prostatic infections. They may get to be quite large and cause compression of both the colon and urethra. Animals may straining when they defecate or urinate, and most animals feel ill.

Benign prostatic hyperplasia (BPH). BPH is the most common form of prostatomegaly (increased prostate size), with virtually all intact male dog acquiring the condition as they age. It is caused by an increase in number and size of the prostate cells as the intact dog ages and is exposed to normal hormonal influences. It is a benign condition that usually does not cause any clinical signs. Occasionally a urethra discharge is present, which can be bloody. The majority of the time this condition is found incidentally on routine physical examination. Dogs with chronic prostatitis sometimes have very similar signs.

Prostatic neoplasia (cancer). Prostatic neoplasia may closely mimic chronic prostatitis. Animals with prostatic neoplasia tend to be systemically ill, and have a history of weight loss. Tumors of the prostate are almost always malignant. The most common tumors involving the prostate are adenocarcinoma and transitional cell carcinoma. In contrast to most other types of prostatic disease, prostatic cancer occurs with the same frequency in both intact and neutered dogs. In a neutered male dog with significant prostatomegaly, prostatic neoplasia is high on the list of potential causes. Many times, a prostatic biopsy is required to differentiate chronic prostatitis from prostatic neoplasia.

Squamous metaplasia. Squamous metaplasia is a change in the prostate gland due to elevated blood estrogen levels. The main cause of this is an estrogen-producing tumor (Sertoli cell tumor). Long-term oral estrogen supplementation can also cause these changes. Sertoli cell tumors can also cause a chronic debilitating disease due to the suppressive effects of estrogen on the bone marrow.