Persistent Heat (prolonged estrus) in Dogs

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Last updated on
3 min read

Key takeaways

The normal heat cycle of dogs has 4 periods; estrus is the period during which female dogs show symptoms such as accepting mating behavior, vulvar swelling, and bloody vaginal discharge. Prolonged estrus is defined as signs of estrus lasting longer than 21 days without ovulation.

  • Causes include exposure to hormones and ovarian cysts or tumors
  • Prolonged estrus is diagnosed by perception of prolonged estrus signs, confirmed by vaginal cytology, bloodwork, and diagnostic imaging (ultrasound)
  • Treatment is hormone therapy or surgery to remove tumors
  • Prognosis depends on the underlying cause but is typically good following treatment
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A closer look: Persistent Heat (prolonged estrus) in Dogs

Most mammals have consistent and predictable reproductive cycles. The period of this cycle when females are most fertile and likely to mate successfully is referred to as estrus, or heat. In dogs, symptoms of estrus include accepting mating behavior, vulvar swelling, nesting, and bloody vaginal discharge. Estrus is normal and expected in intact females, but it should come and go regularly. If estrus begins and does not stop, it is a symptom of medical concern.

Prolonged estrus is uncommon and not an emergency, but another cause of unexpected vaginal bleeding, pyometra, is life-threatening. Any time a dog is showing signs of unexpected vaginal bleeding accompanied by fever, lethargy, anorexia, depression, polydipsia/polyuria, and/or weakness, emergency veterinary care is required.

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Possible causes

Most cases of exposure to synthetic hormones are related to pets getting into human estrogen medications, including topical formulations.

Risk factors

There are 4 phases to the canine female reproductive cycle. Estrus is the second phase, during which females begin accepting males for mating and is associated with the symptoms that are visible to owners. This phase normally lasts 5-10 days; in prolonged estrus it lasts at least 3 weeks. The length of time signs of estrus continue, as well as the number of symptoms present may vary.

Intact female dogs are at highest risk of prolonged estrus. Female dogs living with humans who are using synthetic estrogen supplementation are also at risk. Spayed dogs are at the lowest risk. Male dogs are not at risk, but exposure to synthetic estrogen is a matter of medical concern for all dogs regardless of sex. Any dog with known accidental exposure to human medications requires prompt veterinary attention.

Testing and diagnosis

Diagnostics may include

  • Physical exam
  • Blood work
  • Vaginal cytology
  • Diagnostic imaging

In some cases, exploratory abdominal surgery may be necessary to diagnose the cause and/or excise tumors if present.

Treatment for ovarian cysts is hormone therapy. Spaying (sexual alteration) to remove the ovaries and uterus is almost always performed in cases that do not respond to medical treatment, and in cases of ovarian tumors.

Similar symptoms

Associated symptoms


No Author - Writing for MSD Veterinary Manual
John A. Bukowski , DVM, MPH, PhD; Susan Aiello , DVM, ELS - Writing for MSD Veterinary Manual
Shirley D. Johnston, DVM, PhD, DACT - Writing for Veterinary Partner
Malcolm Weir, DVM, MSc, MPH; Catherine Barnette, DVM - Writing for VCA Animal Hospitals
Ali Risvanli, Halis Ocal and Cahit Kalkan - Writing for Canine Medicine - Recent Topics and Advanced Research
No Author - Writing for MSD Veterinary Manual
Wendy Brooks, DVM, DABVP - Writing for Veterinary Partner
Wendy Brooks, DVM, DABVP - Writing for Veterinary Partner

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