Ovarian Remnant Syndrome in Dogs

What is ovarian remnant syndrome?

Ovarian remnant syndrome is a condition that occurs when ovarian tissue remains inside the body after a female dog is spayed. This tissue can produce estrogen, triggering signs of heat in the dog.

In an intact female dog, the ovaries produce hormones, including estrogen. It is this estrogen that triggers your dog to go into heat. When your dog is spayed, the entire reproductive tract (including both ovaries and the uterus) is surgically removed. Therefore, your spayed dog no longer has ovaries, produces estrogen, or goes into heat.

If a previously spayed dog shows signs of going into heat, this may indicate that functioning ovarian tissue (known as an ovarian remnant) is still present and producing estrogen. Ovarian remnants may be left behind during surgery or may be caused by the presence of accessory ovarian tissue (a small piece of tissue that fragmented off the ovary and established enough of a blood supply to begin producing hormones).

"Ovarian remnants may be left behind during surgery or may be caused by the presence of accessory ovarian tissue..."

What are the clinical signs of ovarian remnant syndrome?

The most obvious clinical sign of ovarian remnant surgery is when a previously spayed dog goes into heat. This can happen at any time after spay surgery, with a delay of months to years.

The signs that a dog is in heat include swelling of the vulva and blood-tinged vaginal discharge. Additionally, a dog that is in heat may demonstrate behavioral changes, such as being more receptive to male dogs that she attracts. Dogs with functioning ovarian tissue typically go into heat every six to eight months.

Rarely, a dog may develop signs of false pregnancy after being spayed due to the sudden removal of hormones. Signs of false pregnancy involve behavioral changes and the development of mammary tissue. If you think your dog may be showing signs of false pregnancy, contact your veterinarian.

How is ovarian remnant syndrome diagnosed?

Your veterinarian will diagnose the presence of ovarian remnant syndrome using one or more of the following diagnostic tests:

1. Vaginal cytology. This preliminary test involves taking a swab from your dog’s vagina while she is showing signs of heat. Your veterinarian will assess the sample under the microscope for the presence of cornified cells. If cornified cells are found, this indicates that your dog is under the influence of estrogen.

2. Baseline hormone levels. Anti-Mullerian hormone (AMH) can be evaluated when your dog is in heat and the combination of elevated AMH and progesterone gives strong evidence for retained ovarian tissue. Rarely, very small amounts of ovarian tissue will fail to produce a positive test.

3. Ultrasound. Ultrasound may be used to visualize ovarian tissue if performed while your dog is in heat. The accuracy of this test, however, is influenced by the size of the ovarian remnant, the stage of the heat cycle, and the skill of the ultrasound specialist. This test is not frequently used.

"A blood test showing an increase in either estradiol or progesterone indicates the presence of functioning ovarian tissue."

4. Hormone stimulation test. This test is widely regarded as the most accurate test for diagnosing ovarian remnant syndrome. A synthetic hormone (hCG or GnRH) is administered while your dog is showing signs of heat. A blood sample is then taken either two hours later to evaluate your dog’s level of estradiol or drawn seven to fourteen days later to evaluate progesterone levels. A blood test showing an increase in either estradiol or progesterone indicates the presence of functioning ovarian tissue.

How is ovarian remnant syndrome treated?

Ovarian remnant syndrome is treated by surgically removing the remaining ovarian tissue. This surgery should be performed while your dog is in heat, or just after, to maximize the visibility of the remaining ovarian tissue.

What happens if ovarian remnant syndrome is not treated?

"The continued presence of estrogen predisposes dogs to mammary gland tumors, ovarian tumors, and pyometra."

Dogs with an untreated ovarian remnant are subject to the same risks as dogs that are not spayed. The continued presence of estrogen predisposes dogs to mammary gland tumors (breast cancer), ovarian tumors, and pyometra (infection of the uterus). Pyometra is a serious life-threatening condition that requires urgent veterinary care. (See handout “Pyometra in Dogs” for more information.)

How is pyometra possible if the uterus was removed as part of the spay surgery?

When a dog is spayed, a small part of the uterus, call the uterine stump, is left above the cervix. The uterine stump can be susceptible to infection from the hormonal effects of the ovarian remnant.