Abnormalities of the heat cycles can include silent heat, split heat, prolonged heat, absent heat, or irregular heat. Occasionally a bitch will skip an entire heat period. Young bitches frequently have irregular or silent heat periods. Generally, their cycles become regular by 2 to 3 years of age. Most bitches will follow the heat cycle pattern of their dams.
Silent heat is a fertile heat cycle that escapes detection because of minimal vulvar swelling and vaginal bleeding. A bitch with silent heat may show no interest in the stud, except during a short period around the time of ovulation. Small breeds who achieve early sexual maturity may have one or two silent heats before exhibiting an obvious cycle. When heat goes undetected, the bitch is often given the mistaken diagnosis of absent heat.
Vaginal bleeding may go unnoticed if the female is fastidious and licks herself clean. If you are not familiar with the size of the normal vulva, you may not notice the swelling of proestrus. However, you should be able to recognize the mild vulvar enlargement and slight bloody discharge of proestrus if you carefully inspect the vulva once or twice a week. You can also expose the bitch to a stud dog twice a week and observe the behavior of both animals.
Veterinary examination using vaginal cytology and progesterone measurements is an accurate method of determining whether the bitch is cycling.
In bitches with this abnormality, the heat appears to be split into two separate cycles. In the first, or false, cycle the bitch attracts the male and develops the vulvar swelling and vaginal bleeding typical of proestrus. Because she does not proceed to estrus, however, she goes out of heat without becoming receptive. A second heat cycle occurs 2 to 10 weeks later. This cycle often proceeds to standing heat.
Split heat typically occurs in young females. It is caused by lack of pituitary output of LH. Because the LH does not rise, the ovaries do not produce ovulatory follicles and serum progesterone remains low (see Hormonal Effects During Estrous). In most cases no treatment is required. The next heat cycle is usually normal.
This occurs when a bitch remains in heat for more than 21 days. During the prolonged heat she continues to display vaginal bleeding and remains attractive to males. Prolonged heat may occur in maiden bitches during the first heat cycle. It normalizes with maturity.
In all other cases, the hormonal basis is a persistent elevation of estrogen, caused by an estrogen-producing ovarian cyst or, occasionally, by a granulosa cell tumor of the ovary. Vaginal cytology and serum estrogen measurements will confirm the diagnosis of prolonged estrus. Ultrasonography may show an ovarian cyst or tumor. Ovarian cysts may regress. If they do not, surgery is required. Ovarian tumors must be surgically removed.
Heat can be terminated by giving an androgen preparation for three to four months. Bitches who respond to androgens usually begin a new heat cycle four to five months later and can often be bred at that time. As an alternative, an attempt can be made to breed the bitch on the same cycle by inducing ovulation with human chorionic gonadotropin (hCG) or gonadotrophin releasing hormone (Gn-RH). If further breeding is not desired, spaying is the treatment of choice.
Absent heat is due to a failure to cycle. Bitches younger than 2 years of age may not cycle because of sexual immaturity. Some large-breed dogs do not attain sexual maturity and experience their first heat until they are 2 years old. If a bitch does not go into heat by 24 to 30 months of age, have her examined by a veterinarian.
Heat will not occur if a female has been spayed. This becomes a consideration if the medical history is unknown. Inspect the lower abdomen to find the scar that indicates she was spayed. Females who have been treated with androgens or progesterones will not cycle during treatment and for some time thereafter. The same is true for bitches receiving corticosteroids.
Bitches who are malnourished or debilitated from recent illness often do not cycle until they are in a better state of health. Hypothyroidism is a common cause of absent heat. Other signs of hypothyroidism may or may not be present. The diagnosis is made by a thyroid blood test. Cushing’s syndrome is an uncommon cause. Most bitches with Cushing’s are over 8 years of age and are no longer in their reproductive years.
Ovarian hypoplasia is a disease in which the ovaries do not develop to sexual maturity and are incapable of producing adequate amounts of estrogen. The mammary glands and vulva remain small and underdeveloped. Ovarian hypoplasia may be the result of sex chromosome abnormalities. An immune-mediated inflammation of the ovaries may be responsible for some cases of absent heat. Tumors of the ovary have also been associated with absent heat.
The diagnosis of absent heat is confirmed by weekly vaginal cytology and progesterone assays that show no hormonal effects of estrous. An elevated LH is present in bitches with ovarian hypoplasia and those with absent ovaries. Ultrasonography may reveal an immature uterus or an ovarian tumor. Karyotyping is recommended for bitches who show no evidence of cycling after six months of study. If the chromosomes are normal and the bitch has not cycled by 30 months of age, estrus and ovulation often can be induced by FSH and hCG in a protocol determined by your veterinarian. A bitch with major infertility problems should not be bred, as these problems may be passed on genetically.
This is the interval between one heat cycle and the next. The average interval is five to nine months. Often a bitch’s interestrous interval is either longer or shorter than normal, or occurs at irregular intervals. An abnormal interestrous interval is one in which the interval between two heat cycles is either less than four months or greater than one year.
Some bitches come into heat every four months and others every 10 to 12 months. This may be genetically influenced. Basenjis and wolf-dog crosses, for example, come into heat once a year.
Prolonged Interestrous Interval
This condition, also referred to as prolonged anestrus, occurs in previously cycling bitches who do not come into heat after 16 or more months. A common cause is cessation of ovarian activity due to an ovarian cyst that produces progesterone. The administration of progesterone and androgenic drugs produces a similar effect. Hypothyroidism and Cushing’s syndrome also produce prolonged anestrus.
Hypothyroidism can be diagnosed by a thyroid blood test. Cushing’s syndrome generally occurs in older females and is not a common reproductive problem.
Bitches with normal ovaries who are slow to come into heat will often do so when they are kenneled with other cycling females and regularly exposed to a male. If this is not successful, and the bitch has a low serum progesterone level and no discernible medical cause for the prolonged anestrus, consider karyotyping to rule out intersex. If the karyotype is normal, an attempt can be made to induce estrus and ovulation, as described on page 450 for Absent Heat.
Shortened Interestrous Interval
In bitches with this condition, the interval between one cycle and the next is four months or less. A shortened interestrous interval should be distinguished from split heat, in which the first cycle does not progress beyond proestrus, and thus is incomplete.
The problem with a shortened interval is that the lining of the uterus has not had sufficient time to repair the progesterone-induced damage caused by the previous heat cycle. Thus, the endometrium is not hormonally receptive to embryo implantation and the bitch with a shortened interestrous interval cannot become pregnant.
Treatment may not be necessary. Most young females develop normal interestrous intervals as they mature. In a mature bitch with a shortened interestrous cycle, the process of uterine repair can be expedited by using an androgen to terminate heat before ovulation, thus paving the way for a normal endometrium on the next heat cycle.
Premature Ovarian Failure
Ovarian function begins to decline at about 6 years of age and ceases, on average, when a bitch is 10 years old. This usually is not a problem, because most bitches are not bred beyond 7 to 8 years of age. The ovaries of some bitches, however, may cease to function as early as 6 years of age, resulting in permanent anestrus. This can be confirmed by FSH and LH concentrations; both will be extremely high in premature ovarian failure. There is no treatment for premature aging of the ovaries.