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 hormones, triggering signs of heat in the dog.

In an intact female dog, the ovaries produce hormones, including estrogen and progesterone. Estrogen is the hormone 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. Because your spayed dog no longer has ovaries that produce estrogen, it does not go into heat.

If a previously spayed dog shows signs of heat, it may indicate that functioning ovarian tissue 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).

What are the clinical signs of ovarian remnant syndrome?

The most obvious clinical sign of ovarian remnant syndrome 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.

Occasionally, a dog may develop signs of false pregnancy after they ovulate. This is caused by the ovarian remnant producing progesterone. Signs of false pregnancy include behavioral changes (morning sickness, increased affection towards the owner, being more subdued) and the development of mammary tissue. If you think your dog may be showing signs of false pregnancy, contact your veterinarian.

Not to be mistaken for ovarian remnant syndrome, dogs will sometimes show signs of “false whelping” or lactation shortly after being spayed. This occurs if they are spayed during a particular time of their heat cycle and the sudden removal of progesterone triggers milk production.

How is ovarian remnant syndrome diagnosed?

Several tests (often in conjunction) can support a diagnosis of an ovarian remnant, but a definitive diagnosis requires finding the remnant and identifying it with a microscope (histopathology) as ovarian tissue. Your veterinarian may also want to confirm that your dog has had no exposure to anything containing estrogen, such as certain creams.

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 a specific type of vaginal cell called cornified cells. If cornified cells are found, it indicates that your dog is under the influence of estrogen and may have an ovarian remnant. Other conditions can also cause estrogen exposure, so your veterinarian may recommend additional testing. If cornified cells are not found, this could indicate your pet is not yet in full heat (estrus) and your veterinarian may recommend repeating the cytology.
2. Baseline hormone levels: There are several baseline hormone levels your veterinarian can use to support a diagnosis of ovarian remnant syndrome. These may include progesterone, anti-Mullerian hormone, or luteinizing hormone (LH). These tests have limitations, so your veterinarian may recommend running more than one test to increase accuracy.
3. Ultrasound: Ultrasound may be used to visualize ovarian tissue. 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. Ultrasound may be recommended prior to surgical intervention to identify the approximate location of the remnant, which can reduce the time your pet needs to be under anesthesia.

How is ovarian remnant syndrome treated?

Ovarian remnant syndrome is treated by surgically removing the remaining ovarian tissue. This surgery may be best performed while your dog is showing signs of heat, or for the two months afterwards, to maximize the visibility of the remaining ovarian tissue. It can be challenging to locate ovarian tissue, so referral to a specialist may be recommended. 

What happens if ovarian remnant syndrome is not treated?

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 and 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, called the uterine stump, is left above the cervix. The uterine stump can be susceptible to infection from the hormonal effects of the ovarian remnant.