Canine Heat Cycle

Estrus, or heat cycles, in the female dog are a fascinating process controlled by multiple hormones from the brain and abdominal organs.  Pet owners also use the term “in season” to refer to the heat cycle.

Please note that does not condone the thoughtless breeding of dogs and cats. The decision to breed one’s pets must be made with great care and sense of restraint. Literally millions of dogs and cats are euthanized, put to sleep, each year in this country because our pet overpopulation problem is so great. Those who do choose to breed their pets must limit their efforts to only the males and females with the best physical and emotional characteristics.

Sally Ann, spayed long ago, plays with one of her 2010 Christmas toys.
Sally Ann, spayed long ago, plays with one of her 2010 Christmas toys.

Let us begin in the period between heats called anestrus. The Latin prefix “an” means “without” and the root “estrus” comes from the Latin oestrus, meaning “frenzy.” If you’ve ever seen a pack of male dogs chasing a single female dog all over the neighborhood you know exactly where the term came from.

Anestrus lasts for approximately five months, at which point a preparatory phase of the uterus and vagina begins with rising levels of estrogen. A gland in the brain releases a hormone which stimulates the quiescent ovaries to increase production of estrogen. This stage is called proestrus, with the Latin prefix “pro” meaning “before.” Little can be detected by a pet owner at this stage, although some dogs may exhibit slight vulval swelling. Indeed, most veterinarians performing cytology of vaginal discharge would find it difficult to identify this stage by the microscope until the third or fourth day.

Proestrus may range from five to ten days, but is typically referred to as a seven-day episode. While a female dog may attract male dogs at this stage, she will not be receptive to mating with them until the next phase, estrus. Estrus in the dog is typified by a bloody vaginal discharge, extreme swelling of the external genitalia and active searching for a mate. The female dog will do anything necessary to satisfy her desire to reproduce including jumping over, digging under or backing up to a fence.

Most experts in reproduction, called theriogenologists, consider the second day of estrus to be the best day to begin mating. The schedule is usually for the female dog, called a dam or bitch, to be bred on the second and fourth or third and fifth days of estrus.

Therefore, if you want to prevent your female dog from becoming pregnant, but she has not yet had spay surgery, you must control her every movement during the entire time she is bleeding, plus a little longer.

Please note that ovaries and uteri don’t read textbooks. Individual variations in the length of phases, duration of receptivity and time of fertility can be quite broad.

It is in the stage of estrus that mammary tissue is first “primed” by estrogen to later respond to stimulation to cause mammary (breast) cancer. Pets who are spayed before the heat cycle have a nearly zero risk of mammary cancer later in life.

Estrus ends when metestrus begins. Like proestrus, metestrus is a transitional phase. It moves the uterus and ovaries into readiness for pregnancy, with the uterus beginning to produce more progesterone as estrogen production falls. If the uterine wall detects the presence of fertilized eggs attempting to attach, this process will continue.

If no embryos successfully attach to the uterus, the period of diestrus proceeds. Another transitional time, the uterus and ovaries begin to “wind down” as they prepare for another quiet anestrus.

While this describes a typical canine heat cycle, things can and do go wrong.

Some dogs may experience a “silent” estrus, with little bleeding, little swelling, little interest in mating and minimal attraction of male dogs. The female may still become pregnant, though the likelihood of attracting a mate is decreased.

Many dogs suffer from “false pregnancy” or “pseudopregnancy.” This mistake will occur if they uterus “thinks” it is pregnant and begins to collect fluid in preparation. The bitch’s abdomen may swell, she may begin to have mammary swelling and may even produce milk. In the worst cases the uterus may become infected and develop a condition known as pyometra.

Next Monday: the feline heat cycle.

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