If your dog's been neutered, his vulnerability to prostate disease is greatly decreased. A neutered dog's prostate is much smaller than an intact dog's because he lacks testosterone to develop the gland. Benign prostatic hyperplasia, infection and cancer are diseases of the canine prostate.
Looking at the Prostate
Your male dog's prostate surrounds the neck of his urinary bladder. The urethra -- the tube carrying urine outside his body -- passes through the bladder and his prostate. His prostate produces fluid that nourishes his sperm and facilitates their movement. The fluid passes through his urethra. It is part of his ejaculate when he mates. This organ is considered a secondary sex gland: It's important for mating but doesn't produce sperm. Neutering doesn't remove the prostate, but it takes away testosterone so the gland doesn't develop; it eventually shrinks to about a quarter of its original size. Its diminished presence is harmless.
Benign Prostatic Hyperplasia
Benign prostatic hyperplasia, an enlarged prostate gland, is the most common prostate condition in dogs. Only unneutered dogs can develop this condition, which progresses as a dog ages. Generally, the disease shows up around age 5; symptoms include bloody urine and straining during urination or defecation. Unless a dog's showing symptoms of BPH, he won't require treatment. If he is experiencing symptoms, neutering is the treatment of choice. Prescription medications can treat BPH; however, they're useful only for the short term, as they can cause diabetes with continued use. Medication is the preferred option for a dog who's intact for breeding purposes and should be used only long enough to manage symptoms until sperm can be collected for future breeding. After the dog's sperm have been frozen, he should be neutered to treat his BPH.
Neutered or intact, a dog can develop an infection of the prostate, known as prostatitis. The acute version of a prostate infection has a sudden onset and is marked by pain during ejaculation, urination or defecation. The dog may be lethargic and have little appetite. If the infection is longstanding, the dog may have chronic prostatitis, which often is difficult to diagnose because symptoms aren't as pronounced as in the acute version. Chronic prostatitis may present repeating urinary tract infections or diminished quality of semen. Prostatitis is usually secondary to other conditions, including BPH. Treatment requires several weeks of antibiotics. Neutering keeps infection at bay.
The most serious form of prostate disease for a dog is prostatic neoplasia, or prostate cancer. Any male dog, intact or neutered, is vulnerable to this aggressive disease. A dog with this malignant cancer may have difficulty urinating or defecating, may show weight loss and may experience coughing, fatigue or lameness. It's an aggressive cancer; by the time symptoms present and a diagnosis is made, the cancer has likely spread. Treatment is geared at making the dog comfortable with radiation and medication.
Other Prostate Diseases
Paraprostatic cysts are fluid-filled cysts developing next to the prostate. These are based on abnormal tissue left behind while the puppy was in utero; they begin to develop shortly after birth. They don't cause problems until the dog is mature, causing blood in the urine, difficulty urinating and gastrointestinal distress. They're surgically removed. Squamous metaplasia is caused by too much estrogen in the dog, leading to an enlarged prostate.