Canine Oestrus Cycle

The bitch is a monocyclic, non-seasonal breeder; they have only one oestrus during each breeding season. The oestrus cycle occurs every 6 to 9 months.

There are 4 stages of the ovarian cycle:

• Proestrus
• Oestrus
• Dioestrus
• Anoestrus

Proestrus

Characterized by increased plasma concentration of oestrogen which causes:

• Vulval oedema and swelling
• Serosanguinous discharge

The bitch is attractive to males but will not accept them. This is the first indication that heat is approaching.

Typically lasts for 9 days (2-27 days), oestrogen levels are high and progestrone levels low. Follicles develop during this phase.

Vaginal cytology - gradual shift from intermediate and parabasal cells to superficial cells. Early in proestrus there are is a mixed cell population. Late in proestrus intermediate and superficial cells predominate. Red blood cells are present in large numbers (especially early proestus) and neutrophils can also be seen.

Oestrus

During oestrus the bitch will accept the male and adopts the breeding stance. Ovulation usually occurs 1 or 2 days after the onset of oestrus and ova are fertilisable from 2-5 days after the ovulation.

Typically lasts for 9 days (3-21 days), LH (lutenising hormone) surge occurs, oestrogen levels are low and progesterone levels rise rapidly.

Vaginal cytology - predominance of superficial cells, there may be a few red blood cells.

Dioestrus

End of the fertile period, the bitch refuses to accept the sexual advances of the dog, the cervix closes and the vulva reduces in size. Corpora lutea (CL) are present and fully functional. CLs secrete large quantities of progesterone.

Typically lasts for 2 months, oestrogen levels are low and progesterone continues to rise to maximal level at 3-4 weeks.The corpus luteum regresses at 70-80 days. Signs of pseudopregnancy can be seen in this phase.

Vaginal cytology - decreasing numbers of superficial cells and reaappearance of intermediate and parabasal cells.

Anoestrus

Bitch is not receptive to mating and the vulva is small. Lasts for around 4 months (1-12 months).Oestrogen low until end of anoestrus when starts to rise for coming in season and progesterone levels are low.

Vaginal cytology - Intermediate and parabasal cells predominate in smears taken during anestrus. Superficial cells are absent or very small numbers.