Estrous Cycle in Dogs (Bitches)

The estrous cycle in female dogs, commonly referred to as the heat cycle in bitches, is a recurring reproductive cycle that includes four distinct stages: proestrus, estrus, diestrus, and anestrus.

Female dogs typically experience two ovarian cycles per year. The timing of breeding seasons depends on both genetic and management factors. Most breeds enter their first heat between 6 and 10 months of age, though some dogs may not begin to cycle until 18 to 24 months of age.

The average interestrus interval is 7 months, ranging from 3.5 to 13 months. However, the Basenji breed typically cycles only once a year.

Phases of Estrous Cycle

There are four phases namely:

1. Proestrus
2. Estrus
3. Diestrus
4. Anestrus

1. Proestrus

Proestrus begins with the appearance of vaginal bleeding (spotting) and ends when the bitch allows a male dog to mount and breed. The average duration is 9 days, ranging from 2–3 to 25 days.

Proestrus is influenced by estrogen, which is synthesized by the developing follicles. Early proestrus is associated with estradiol concentrations of approximately 25 pg/ml, which may rise to 60–70 pg/ml in late proestrus. Thereafter, estrogen levels begin to decline, reaching basal levels (5–15 pg/ml) at the onset of diestrus.

Progesterone concentrations begin to rise from basal levels (<1 ng/ml) in late proestrus, reaching approximately 3 ng/ml at the onset of estrus and 5 ng/ml at the start of ovulation. This progesterone is secreted by the follicle, which becomes partially luteinized prior to ovulation and the development of the corpus luteum (CL).

2. Estrus

Estrus begins with the bitch’s first acceptance of the male and ends when she refuses further mating attempts. The average duration is 9 days, but it can range from as short as 1–2 days to as long as 18–20 days.

The bitch is unique in that standing estrus is exhibited when estrogen levels have begun to decline, while progesterone levels are rising. This hormonal shift triggers two key events:

• Exhibition of maximal estrus behaviour
• Initiation of LH surge in the bitch

Ovulation occurs 24 to 48 hours after the LH surge and takes place over a span of 72 to 96 hours. The eggs are released as primary oocytes and require 24 to 72 hours to mature and become fertile. Once mature, they remain viable for 2 to 4 days. At the time of ovulation, the serum progesterone concentration typically ranges from 4 to 10 ng/ml.

3. Diestrus

Diestrus begins with the cessation of standing heat and ends when blood progesterone levels return to basal levels. Progesterone levels continue to rise, reaching 50 to 60 ng/ml approximately 20 to 30 days after ovulation.

The bitch is unique in that, regardless of pregnancy status, the corpus luteum (CL) remains functional for a fixed duration. There is no pregnancy recognition mechanism in dogs; therefore, the CL functions throughout the normal gestational period, whether or not fetuses are present.

In a pregnant bitch, the luteal phase ends with the onset of parturition (around 65 days), whereas in a non-pregnant bitch, the CL remains functional for a longer period—typically 75 to 100 days—after which regression occurs due to natural aging of the CL.

Thus, the uterus does not play a role in regulating the lifespan of the CL. Apart from luteinizing hormone (LH), prolactin is the major luteotrophic factor, particularly in the second half of the luteal phase.

4. Anestrus

Anestrus is the phase during which the uterus undergoes involution. The average duration is 4.5 months, but it can vary.

Follicle-stimulating hormone (FSH) levels remain relatively high, similar to levels seen during proestrus. However, because the follicles do not fully mature and instead regress, estrus behavior is not exhibited.

Symptoms of Estrus in Bitch

Proestrus Phase Signs

• The bitch is playful.
• She appears sexually attractive to the male but refuses mounting attempts by moving away, baring her teeth, or snapping.
• She may keep her tail tightly against the perineum and cover her vulva. This behavior changes as proestrus progresses.
• The female becomes more receptive, seeks out males, engages in playful teasing, but still refuses mounting by crouching or lying down.
• In late proestrus, the bitch may sit or stand passively when mounted.
• The bloody discharge fades and becomes transparent to straw-colored, and the vulva, which is swollen and firm during early proestrus, becomes smaller and softer as estrus approaches.

Estrus Phase Signs

• The bitch may crouch and elevate her perineum toward the male.
• She attracts males over long distances due to the presence of potent pheromones.
• The vulva is soft and flaccid, and the vaginal discharge is often straw-colored or pink.