Canine Reproduction : Vaginal Smears and Timing of Ovulation

When I am doing a consultation for timing of ovulation in canines, vaginal smear is a test I will always perform and recommend.

It is simple & quick… and it can bring you tons of valuable clinical information.

Vaginal Smear : what is a that ?

A vaginal smear is a test we use a lot in our canine reproduction consultations.

Using a vaginal swab (which honestly is basically an elongated cotton tip), we will collect vaginal cells, put them on a microscope slide and after staining the preparation, check it under the microscope.

The morphology of the vaginal cells is what we will be looking at.

Indeed, the vaginal cells are organized in what we call an epithelium, and their morphology will be influenced by the amount of keratine (=a protein) they contain.

And this amount of keratine will be influenced by sex hormones, especially estrogens.

Therefore, the morphology of the vaginal cells will vary throughout the bitch’s cycle.

What does it look like ?

When the bitch is in anestrus (=not in season), the vaginal cells are small and round, with big nuclei clearly visible. We call those parabasal cells and when they are the only ones visible, it tells you there is no estrogen (and therefore no ovarian activity at all).

When the bitch is at the beginning of the season (=pro-estrus) we will start to see a change. The vaginal cells become more angulated (because of the accumulation of this keratin protein we mention), they are getting bigger.

This cellular modification will get to its highest point when the bitch is getting close to ovulation (=estrus) : this is when we mostly only see cornflakes-shape like cells, with almost no nucleus. This is when the estrogen secretion is at its peak and this is reflected in the vaginal cells’ morphology.

5-6 days post ovulation, this will change again. The estrogen secretion is waning off, therefore those angulated cells progressively disappear and we are again seeing less angulated cells, very often organized in columns. This is what we call diestrus.

What does it tell us ?

We used to tell owners when to breed the bitch based on the morphology of the vaginal cells.

Basically we would see an estrus smear… and we’d say : “Start breeding her every day or every other day.”

Today, we know better.

Indeed, the best way to determine the day of ovulation in the bitch is to perform progesterone testing. This is and remains THE gold standard.

Vaginal smears are not accurate enough to help us target this 4 day fertility window we see in canines, as we can observe the estrus smear described earlier from 10 days before ovulation to sometimes 10 days after…

However, that does not mean vaginal smears are useless.

As I mentioned in the beginning of this post, I’d perform in every single timing of ovulation consultation. ALWAYS.

And here are a few reasons :

> Some bitches indeed ovulate very late in their cycle (I have seen bitches ovulated 30 days after they started their season)… Performing vaginal smears can therefore be a good way to initiate the timing of ovulation. When the bitch is in season, we start by doing vaginal smears every 2-3 days… and we will only start doing progesterone tests when we observe the estrus vaginal smear we described earlier.

> Vaginal smears can also tell us… when it is too late. It is indeed not uncommon to see a bitch in consultation and the owner tells us he is not sure when she actually started. If the vaginal smear is evocative of diestrus, it tells us that ovulation must have occurred at least 6-7 days before… No need to breed in these cases…

> On vaginal smears, there are other cellular elements we look for. And the ones I am particularly interested in are called polynuclears. Those are white blood cells and in adult bitches, they are an indicator that there is an inflammation. We can see a few of them during diestrus, but in estrus in particular seeing them is totally abnormal. Indeed, in estrus, the vaginal epithelium is at its most differentiated state. This is when it is the most resistant to infections (basically because it is at its thickest). This tells us that there is a potential vaginitis (=inflammation of the vagina, in adult bitches typically of bacterial origin) or worse, an infection of the uterus ; and obviously will definitely push us to investigate further (vaginoscopy, ultrasounds are the tests I’d usually recommend in those cases).

Vaginal smears are a great clinical tool, and now you better understand why in canine reproduction we perform them so often.

Remember that while it is definitely not an accurate technique to determine the day of ovulation, the information it provides can really help us better approach any individual case.

That is why, in our veterinary clinics, I’d definitely keep recommending using them as much as possible !