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hormonal hazard: effects of estrogen exposure on pets

  • Dr. Kim
  • Posted by Dr. Kim Smyth on
    Staff Veterinarian and Pet Health Writer of Petplan


When your female dog or cat was spayed, her ovaries were removed, thus removing her ability to produce the hormone estrogen. It is this hormone which causes your pet to go into heat, so spayed female pets should not be showing those signs.

Because we typically spay our pets prior to their first heat cycle, many owners are not aware of the signs that female pets display when they are in heat (or ready to be bred). Typically, female dogs cycle every six to eight months. Female dogs who are in heat will attract males and may act playfully flirty with them. They usually have a bloody to straw colored vaginal discharge, and these signs will persist for about three weeks. 

Female cats who are in heat are very noticeable, to say the least. Their changes are behavioral—they will vocalize loudly, act very restless, and are unusually affectionate, often presenting their hind ends to anyone who will take notice. They may urinate outside of the litter box during this time as well. Cats tend to come into heat as the days start getting longer and will cycle in and out of heat through the season unless she is bred. Needless to say, this is a very trying period for owners.

If your pet has been spayed, you shouldn’t notice any signs of heat because the source of estrogen (the ovaries) has been removed. What happens, then, when a female pet has signs of heat but has already been spayed? These signs can range from behavioral (most often this is the case in cats) to changes in your pet’s appearance. Dogs who have been exposed to estrogen can have an unusually swollen vulva and mammary glands, and they may suffer from hair loss and have vaginal discharge.

Generally speaking, if your pet comes into the office with these symptoms, your vet will want to do a few tests to determine if estrogen exposure behind them. There is a blood test for estrogen, but a cheap and easy way to find out if estrogen is causing your pet’s signs is to look at some cells from your pet’s vaginal walls. Estrogen changes the appearance of these cells, and a vaginal smear is a quick test that can be run right there in your vet’s office. 

Once estrogen exposure is established, the next question is “Where is it coming from?” In these cases, it’s usually coming from one of two places. 

First, we have to consider that there is an ovarian remnant left inside your dog. This doesn’t necessarily mean that something went wrong when she was spayed and a piece of her ovary was left behind, although this certainly can happen from time to time. Sometimes, ovarian tissue exists somewhere in the abdomen separate from the ovary and becomes active only after the ovaries are removed. Ovarian remnants can cause clinical signs of heat months or years after a spay surgery.

In the case of an ovarian remnant, your veterinarian may suggest going back in to try to find the remnant and surgically remove it to reduce the chances of illness due to long term estrogen exposure (namely, mammary cancer). Sometimes, however, it is not possible to locate the ovarian remnant.

The second way that pets are commonly exposed to estrogen is when a human in their life uses topical estrogen.  Estrogen creams are a commonly used form of hormone replacement therapy in female humans, and often the cream is applied on the arm.  Pets are exposed to the cream through skin to skin contact or by licking the hands and arms of the user. Humans seeking to avoid exposing their pets to their estrogen cream should:

  • Wear gloves while applying the cream
  • Apply the cream to an area which is always clothed (abdomen, for example)
  • Remove the gloves and place them in the trash, out of reach of the pet
  • Wash hands thoroughly

Once removed, the effects of estrogen on your pet’s appearance may last for months or even up to a year.  These pets should be monitored for signs of bone marrow suppression in the short term and mammary tumors long term.

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Chris AshtonCo-Founder and Co-CEO of Petplan Pet Insurance
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