Signs of Estrus

How can I tell if my female dog is in season?

There is a period of time in the life of an intact female dog that is called being in season, being in heat, or her estrus cycle. These periods have distinct physical, hormonal and behavioral signs. Many of the characteristics of the estrus cycle, such as frequency, length of time and severity, will depend on your bitch’s age and breed.

A heat cycle, or season, is the fertile period of a bitch’s cycle when she can get pregnant. Females usually have their first heat at around six months of age, but some bitches do not begin cycling until 12 to 18 months of age. On average, the heat cycles recur every six to eight months, approximately twice a year, and last for approximately 16 to 21 days.

Unlike human females, who go through menopause at some point in their lives, dogs continue to come into heat throughout their whole lives unless they are spayed. Although some people think that a dog’s bleeding during her season is a dog’s “period,” it is actually a sign that she is at her most fertile.

What physical signs should I look for?

Bitches in season tend to have an enlarged, red and swollen vulva. During a portion of the heat cycle, there will be bleeding from the vulva. This discharge changes to a brownish color toward the end of the season.

Generally, larger-dogs bleed more than smaller dogs, but it varies between dogs. Some dogs bleed very little and the seasons may go unnoticed, especially in heavily coated breeds.

Many bitches lick their vulva vigorously and urinate more frequently. Females that groom themselves regularly may show very little blood spotting.

Are there behavioral changes as well?

You may see some changes in your bitch’s behavior. She may be overly friendly with other dogs, especially male dogs. To avoid an unwanted pregnancy, you will need to supervise her for her entire season and keep her separate from any intact male dogs.

She could attempt to mount and hump other dogs, both male and female. When ready to be bred, she may exhibit a behavior known as “flagging,” in which she stands and moves her tail to the side when touched in the tail area. This is a signal to male dogs that she is ready to be bred.

Some female dogs will become anxious and show nesting behavior. Even though she will have a bloody discharge during her heat cycle, the bitch is not in pain. If her symptoms of estrus seem to be causing her pain, you should consult your veterinarian.

What are the stages of the estrus cycle?

The estrus cycle consists of four periods of variable length. The age of the first estrus, the length of estrus, and the amount of time between cycles will vary with the breed of dog and individual dogs. The first season usually occurs when the bitch reaches the expected adult height and weight for her breed. This is generally before the bitch is two years of age for all breeds.

Estrus cycles usually occur every four and a half to eight months, with the exception of some Afro-Asian breeds, such as the Basenji and Tibetan Mastiff, and the sled-dog breeds, which may cycle once yearly in the spring.

Proestrus is the beginning of the heat cycle and lasts between seven and 10 days. During this time, the vulva begins to swell and the bitch begins to have bloody vaginal discharge. She will start attracting male dogs, but she is not ready to mate yet.

Estrus is the mating period of the cycle. It lasts five to 10 days. The vaginal bleeding may change to a lighter color or stop. The bitch will be ready to be bred during this phase.

Diestrus is the period of the cycle that follows estrus. This period lasts anywhere from 10 to 140 days. Your bitch is either pregnant during this time or she is in a period of rest.

Anestrus is the period of downtime before the next heat cycle. It lasts about six months.

Are there tests that can be done to determine if my bitch is in season?

If you are still unsure whether your bitch is in season, you can take her to your veterinarian for testing. One of the quickest and easiest ways to determine estrus is a vaginal smear. A moistened swab is inserted into the vulva and rubbed against the side of the vagina to collect cells. The swab is then rolled across a microscope slide. The slide is stained with dyes and examined under the microscope.

The cells lining the surface of the vaginal are round and smooth during diestrus and anestrus. As the bitch enters proestrus, the wall of the vagina thickens in preparation for breeding. The cells on the inner surface of the vaginal wall become dried out, or “cornified.” They are irregular in shape, with pointed edges, similar to the appearance of potato chips. The percentage of cornified cells increases as the bitch moves forward with her cycle.

The presence of cornified cells is a positive indication that the bitch is in season. Bitches are close or ready to be bred when 100 percent of the cells on the vaginal smear are cornified.

Progesterone is a hormone produced by the ovaries and placenta that helps to maintain pregnancy. During anestrus, progesterone levels will be very low. Progesterone levels start to rise during proestrus, continue to rise during estrus and ovulation, and stay elevated during diestrus for approximately nine weeks, whether or not the bitch is bred or pregnant.

There are many forms of progesterone tests available nowadays. Measuring the level of progesterone present in the blood can indicate whether the bitch is in season and give you an indication of where she is in her heat cycle.

Are there drugs that can be used to bring a bitch into season?

Using drugs to bring bitches into season is problematic. Many methods have been proposed, but the success rate and the fertility of the induced estrus cycle is low.

The dopamine agonists cabergoline given orally daily until two days after the onset of proestrus, metergoline injected intramuscularly every third day until proestrus, and bromocriptine given in increasing doses until proestrus are reported to induce fertile estrus in most bitches. The average length of treatment was 16 to 19 days.

The most widely studied gonadotropin for inducing estrus in bitches is equine chorionic gonadotropin (eCG), which is available in the United States only in a swine product. A single injection of this product was highly effective for inducing proestrus in 90 percent of treated bitches, but the ovulation rate was poor.

Are there drugs that can be given to prevent a bitch from coming into season?

In bitches, estrus suppression, for no more than 24 months, can be accomplished by giving the drug milbolerone orally once a day. This drug is an androgen, or male hormone, similar to testosterone. To be effective, treatment must be started at least 30 days before estrus. Estrus typically recurs soon after stopping the milbolerone. Fertility should be normal after the second season following treatment.

If the bitch has already entered proestrus, megestrol acetate, a synthetic progestogen, may be used to stop the cycle. It is given orally daily for eight days. This drug must be given within the first three days of proestrus. Vaginal bleeding and vulvar swelling usually disappear in three to eight days. Most bitches return to estrus in four to six months.

To postpone the estrus cycle, a lower dose of megestrol acetate can be started a few weeks before estrus is expected. After treatment, estrus is seen in two to nine months, and fertility is not affected. This protocol has been up to 95 percent effective.

When using megestrol acetate, there is a risk of pyometra developing, and two consecutive treatments are not recommended. Long-term treatment may result in obesity, diabetes mellitus, and cancer of the uterus and mammary glands.