Margaret V. Root Kustritz, DVM, PhD, DACT
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Vaginal epithelial cells are induced to divide by high serum estradiol concentrations present during proestrus in bitches.
The rapid increase in the thickness of the vaginal epithelium leads to the death of the cells at the vaginal lumen; these
appear as cornified or keratinized cells in vaginal cytology samples. The percentage of cornified vaginal epithelial cells
progresses from 0% to 100% during proestrus, remains at 100% during estrus or standing heat, and abruptly falls to 0% with
the onset of diestrus.
Veterinarians cannot identify an ovulation date or a bitch's fertility from vaginal cytology. All that can be identified is whether the bitch has been exposed to estradiol and at which stage of the cycle she is in at
the time of sample collection. The average bitch ovulates two days after the onset of cytologic estrus, defined as complete
cornification with more than 50% of the cells appearing anuclear. However, many normal bitches are not average, which makes
vaginal cytology results too inaccurate to consistently determine the ovulation date. The ovulation date can be most accurately
identified prospectively by measuring luteinizing hormone or progesterone concentrations in blood.
Margaret V. Root Kustritz, DVM, PhD, DACT
Department of Veterinary Clinical Sciences
College of Veterinary Medicine
University of Minnesota
St. Paul, MN 55108
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