Abnormalities of the Canine Estrous Cycle: A Review
(The following article, reprinted with permission, was taken from the 2008 Canine Breeder’s Symposium, sponsored by the Society for Theriogenology and the American Kennel Club Canine Health Foundation. The Society for Therinogenology will be holding another Breeder Seminar on August 29, 2009 in Albuquerque, New Mexico. It will be a one day seminar and you can find more information and registration requirements posted on their website at http://www.therio.org in the near future.)
By Scott D. Pretzer
Abilene Animal Hospital, P.A., 320 NE 14th Street, Abilene, KS 67410, USA
Telephone: +1 785 263 2301; Fax: +1 785 263 2925.
E-mail address: firstname.lastname@example.org
The estrous or reproductive cycle of the canine includes four stages: proestrus, estrus, diestrus, and anestrus . To truly recognize what is abnormal, one must have an understanding of what is considered normal for the canine estrous cycle. The bitch is classified as a monestrous animal, meaning that there is only one estrus period per estrous cycle . Other mammals such as cattle, horses, and swine return to estrus every three weeks during the breeding season unless pregnant, and when pregnancy occurs, this period is much longer than the normal diestrus period. In contrast, the canine differs from these animals in several aspects . First, each cycle is at least five months in duration. Second, the diestrus period is not changed significantly by pregnancy and is approximately the same duration. Third, a long period of relative ovarian inactivity occurs between cycles, called anestrus. The anestrus period occurs regardless of pregnancy status.
2. The normal canine estrous cycle
The normal estrous cycle of the canine can be described in several different ways, including behavioral aspects, clinical findings, hormonal values, and cytologic findings of the vaginal epithelium. The four stages of the canine estrous cycle will be reviewed according to these classifications. It is important to remember that the length of each stage of the estrous cycle in the bitch can vary from cycle to cycle. This means it is not dependable to expect that breeding on a given day of the cycle will be repeatable on any ensuing cycle.
The average duration of proestrus in mature bitches is nine days, with a normal range of 0 to 27 days . Proestrus is usually simply defined as that stage of the estrous cycle where noticeable signs of serosanguinous (thin, bloody) vaginal discharge begin. Varying degrees of bloody vaginal discharge are typically seen. The nature of the discharge is typically bright red and voluminous at the onset of proestrus and then becomes less voluminous towards the end. However, it should be noted there is great variation amongst bitches in regards to the amount and nature of their discharge. During proestrus, females are attractive to male dogs, but generally refuse matings. Pheromones, which are present in the female’s vaginal secretions, anal sac expressions, and urine are responsible for this attractiveness [5-8]. Sexual reflexes such as flagging of the tail (elevation of the tail away from the vulva and swaying of the hips from side to side) in response to touching the perineal region (area between and including the anus and vulva) begin in proestrus . The vulva slowly enlarges throughout proestrus due to edematous (turgid) swelling. As proestrus progresses, this swelling gradually subsides and the vulva is less turgid.
Hormonally, the proestrual bitch is under the influence of estrogen and this is the dominant hormone during this stage of the cycle. Estrogen is responsible for most of the clinical signs seen in bitches during proestrus, because this hormone stimulates growth and activity of the glandular epithelium of the uterus and promotes swelling and increased vascularity of the lining of the uterus (mucosa) . The serosanguinous vaginal discharge is a direct result of these effects because the junctions between the cells lining the capillaries leak and permit passage of red blood cells into the uterine lumen which is eventually discharged vaginally after passage through the cervix. Serum progesterone concentrations during proestrus are at basal levels (<0.5 ng/mL) and then start a gradual rise at the end of proestrus. Preovulatory follicular luteinization (transformation of the estrogen secreting follicle to a progesterone secreting structure), unique to the canine, is responsible for this increase in progesterone concentration [10,11].
The effects of estrogen dominance during proestrus are also reflected when vaginal cytology is evaluated. Exfoliated (sloughed) vaginal epithelial cell types vary during proestrus and are usually accompanied by numerous red blood cells. As proestrus progresses, the percentage of superficial and large intermediate cells increases, while numbers of parabasal and small intermediate cells decline.
Estrus in the canine is characterized by the bitches willingness to allow mounting and intromission. The estrus phase begins with this acceptance of the male and ends when the bitch no longer permits a mating. The average duration of estrus in the bitch has been reported to be nine days, with a range of 4 to 24 days .
Clinically the bitch attracts males during this phase and also exhibits flagging of the tail, similar to that in proestrus. However, in estrus the vulva becomes less turgid and more soft and flaccid. The character of the vaginal discharge is also different in estrus in most bitches and is classically strawcolored due to the diminishing presence of red blood cells. Some bitches, however, may continue to have a serosanguinous vaginal discharge through the estrus period.
Hormonally, the bitch appears to be receptive to the male in estrus due to declining estrogen levels and increasing progesterone levels [9,12,13]. Serum progesterone concentrations gradually and steadily rise during estrus. At the start of estrus, progesterone concentrations are typically near 1.0 ng/mL and reach levels near 2.0 ng/mL at the preovulatory LH (Luteinizing Hormone) surge. At the time of ovulation two days later, the serum progesterone concentration is typically in the range of 4.0 to 10 ng/mL . Progesterone concentrations continue to rise after ovulation and reach peak levels in diestrus. Cytologically, estrus can be defined by the presence of greater than 90 percent of the vaginal epithelial cells being superficial cells . Fewer red blood cells are also present during estrus and white blood cells are rare to absent.
Diestrus is the phase of the cycle that follows estrus and is characterized by progesterone dominance. The duration averages 56 to 58 days in pregnancy and 60 to 100 days in the nonpregnant bitch . Diestrus is generally considered to occur clinically when the estrus bitch first refuses a mating. Bitches are also less likely to be attractive to males at this time. Progesterone secretion is maximal approximately 2 to 3 weeks after the beginning of diestrus and reach peaks of 15 to 90 ng/mL at this time . After this peak, progesterone gradually declines over the remainder of diestrus. It has been suggested that the onset of diestrus be defined by vaginal cytology rather than mating behavior because events such as whelping can be more accurately timed . When defined by vaginal cytology, the onset of diestrus occurs when there is a sharp decrease in the percentage of superficial cells and an increase in the percentage of small intermediate and parabasal cells. Neutrophils, metestrual cells and foam cells are commonly seen as well during diestrus.
Anestrus is the quiescent phase of the canine reproductive cycle when defined by behavioral or clinical signs . Bitches in anestrus are not attractive to males and are not receptive to mating. The vulva is normally small and there is no discharge present. In the pregnant bitch, anestrus is the stage of the cycle when uterine involution occurs, beginning with whelping and ending with proestrus. In the nonpregnant bitch, the onset of anestrus is not readily discernable clinically from the end of diestrus.
Anestrus is often defined endocrinologically as the phase following diestrus when progesterone levels decline to less than 1.0 to 2.0 ng/mL. Predominant cell types present in vaginal cytology specimens in anestrus include parabasal and small intermediate cells.
Similar to other phases of the estrous cycle, there is normal variance in the duration of anestrus. This variation depends on breed, health, age, time of year, environment, and multiple other factors . The approximate duration of anestrus in the bitch is 4 to 4.5 months. Following pregnancy the length of anestrus may be extended by 1 to 2 months.
In the bitch, puberty is recognized when the first signs of proestrus occur. Onset of puberty in the bitch is breed-dependent, beginning between 6 and 10 months of age for bitches of many smaller breeds but may not begin for up to 24 months in bitches of some larger breeds [18,19]
4. Failure to exhibit estrus at the expected time
4.1 Primary anestrus
Primary anestrus is defined by the lack of estrous cycling in a bitch by 24 months of age . Some large-breed dogs may not experience their first estrus until approximately this time so investigation into failing to cycle before 24 months may not be necessary. Causes of primary anestrus in the bitch include previous ovariohysterectomy (spay, OHE), silent heat, abnormalities of sexual differentiation (chromosomal or genetic defects), drug-induced anestrus, congenital hypothyroidism, underlying systemic disease, and ovarian abnormalities 
. 4.1.1 Previous ovariohysterectomy
Although this differential diagnosis may seem simplistic and obvious, it is a real possibility due to the fact that many puppies may be neutered at an early age and owners may not be aware of this if it occurs prior to their ownership. The presence of a midline abdominal scar is suggestive but not definite confirmation of previous ovariohysterectomy (OHE), as surgeries for other reasons are achieved via a similar incision. Serum LH is elevated in ovariectomized bitches due to the lack of negative feedback from the nonexistent ovary, and can be used as a diagnostic aid to determine if a female has had a previous OHE [21,22] Elevations in LH are suggestive for previous OHE, but ovarian dysfunction or the preovulatory LH surge can also result in a similar LH elevation. Repeated elevations in LH (over a 2 to 3 week period) make the diagnosis of prior OHE more likely. Definitive diagnosis for previous OHE is via exploratory surgery.
4.1.2 Silent heat
Silent heat is defined as ovarian activity with no associated vulvar swelling, serosanguinous vaginal discharge, or attractiveness of male dogs . While ovarian activity is occurring normally, no outward signs of “heat” are observed by owners or by other dogs, including intact males. Serum progesterone concentrations can be measured on a monthly basis to determine if functional ovaries are present. Levels greater than 2.0 ng/mL indicate the presence of functional luteal tissue . Vaginal cytology can also be assessed on a regular basis to determine if percentages of superficial epithelial cells are increasing. Increasing percentages suggest influence of estrogen and functional ovarian tissue. Silent heats are not uncommon  and can make determining if a young bitch is truly anestrus or not difficult.
4.1.3 Abnormalities of sexual differentiation
Females can have normal appearing external genitalia but have abnormal gonads due to aberrant chromosome complements. One such example is male pseudohermaphroditism, where an animal has male gonads but normal external female genitalia. Diagnosis includes assessment of the karyotype and histopathology of the excised gonads .
4.1.4 Drug-induced anestrus
Drugs such as androgens and progestins prevent estrous cycles in bitches. Additionally, exogenous (administered by humans or ingested vs. being produced by the animal itself) glucocorticoids have also been shown to affect serum LH and normal cycling . Diagnosis is via thorough history taking when bitches present for anestrus, including any medications the owners or family members may take for their own health problems that may inadvertently affect the bitch through contact (i.e. hormone patches and lotions).
While clinical reproductive signs have been reported in approximately ten percent of hypothyroid bitches and primary anestrus has been a reported sign resulting from hypothyroidism there is now good evidence that there is little or no effect of hypothyroidism on reproductive function [1,23-25]. Congenital hypothyroidism may result in primary anestrus but is reversible with replacement of L-thyroxine.
4.1.6 Systemic disease
Any systemic disease may have a negative impact on reproductive function. Hyperadrenocorticism (elevated cortisol in the bloodstream) , renal (kidney) failure, and neoplasia (cancer) are some examples of systemic diseases that may impact normal cycling. Diagnosis is via thorough physical exam and screening labwork such as a complete blood count, serum chemistry profile, and urinalysis.
4.1.7 Ovarian abnormalities
Ovarian abnormalities that may cause primary anestrus include a progesterone-secreting ovarian cyst, ovarian aplasia (failure of the ovary to develop normally), and immune-mediated oophoritis (inflammation of the ovary) . Definitive diagnosis of these conditions includes histopathology of ovarian tissue.
4.2 Secondary anestrus
Secondary anestrus, or prolongation of the interestrous interval, is failure to cycle by 10 to 18 months of the previous cycle [27,28]. Knowledge of normal breed characteristics is necessary, as some dogs that may have apparent prolonged interestrus intervals may actually be normal for that breed (i.e. Basenji, Tibetan Mastiff) .
Secondary anestrus can be exhibited with concurrent hypothyroidism, hyperadrenocorticism, or potentially any other nonendocrine disease. Silent heats, although more common at the pubertal estrus, should also be included in the differential diagnosis.
5. Persistent estrus
Persistent estrus is defined as combined proestrus and estrus of greater than six weeks  or willingness to breed for longer than 21 to 28 consecutive days in any one ovarian cycle . Persistent estrus can also be defined if superficial vaginal epithelial cells predominate on vaginal cytology for more than 21 to 28 consecutive days. Persistent estrus may be a direct result of endogenous (produced by the animal itself) estrogen production or exogenous estrogen administration . Sources of external estrogen may include contact with human estrogen patches or lotions, ingestion of human estrogen supplements, ingestion of foods with high estrogen content (moldy foods), or during treatment for urinary incontinence or vaginitis with estrogens. Serum estrogen levels are frequently not elevated in bitches with persistent estrus , and serum progesterone values typically stay in the preovulatory range (<2.0 ng/mL) [31,32].
Bone marrow suppression is a concern in bitches with persistent estrus because of estrogen toxicity. The secondary signs of estrogen toxicity most commonly include a nonregenerative anemia (a low red blood cell count that does not correct itself) and thrombocytopenia (low platelet count). Causes of true persistent estrus include functional ovarian follicular cysts, granulosa cell tumors, or exogenous estrogen administration. Other reported causes include hepatic (liver) portosystemic shunts  and idiopathic lymphocytic oophoritis (immune-mediated ovarian inflammation) . One should also consider the ranges of normal for duration of proestrus and estrus in the bitch, as proestrus can last nearly four weeks and estrus as long as three weeks in some bitches.
Ultrasonography is a valuable diagnostic tool in determining the cause of persistent estrus in the bitch. Intervention has been recommended if 21 days of estrus have been documented to prevent bone marrow suppression and pyometra . Methods of intervention include hCG (human chorionic gonadotropin) or GnRH (gonadotropin releasing hormone) administration or OHE. Functional ovarian follicular cysts may respond to hCG but granulosa cell tumors will not. OHE is recommended if hCG administration fails, and subsequent histopathology of ovarian sections can then provide a definitive diagnosis.
6. Irregular estrus
6.1 Shortened interestrus interval
Endometrial involution and repair is a necessary phase after an estrous cycle and requires from 130 to 150 days in the bitch . Subfertility is generally expected when interestrous intervals are less than four months in duration [28,29]. Length of interestrous interval is influenced by breed, with shorter intervals being a common finding in German Shepherd dogs, Rottweilers, Basset hounds, Cocker Spaniels, and Labrador Retrievers [29,34,35]. In addition to incomplete uterine involution, shortened interestrous intervals may be seen in bitches with uterine disease [29,36,37]. One such common uterine disease is cystic endometrial hyperplasia (CEH). Treatment of bitches with short interestrous intervals due to uterine disease is induction of anestrus with a synthetic androgen such as mibolerone .
6.2 Split heat
Split heat has been defined as appearance of physical and behavioral changes characteristic of proestrus with no progression through ovulation and estrus. A short anestrus period follows which lasts days to weeks, and then this is followed by a normal, fertile estrous cycle . Similar to silent heats, they are more commonly seen in young pubertal bitches but can be exhibited by bitches of any age. Split heats may be caused by insufficient gonadotropin release or break-through bleeding at onset of folliculogenesis (follicle development) . Split heats can be confusing because the anestrus period between the two “heats” can be 2 to 10 weeks in duration and can appear similar to a shortened interestrous interval. Serum progesterone monitoring helps to differentiate the two as ovulation does not take place in the first “heat” of a split heat. Progesterone levels during the first “heat” of a split heat likely will not exceed 4 to 6 ng/mL because ovulation does not occur. No association exists between split heats and later infertility or between split heats and ovarian and uterine disorders .
Anovulation is simply defined as failure of ovulation, in which serum progesterone concentrations fail to exceed 4 to 8 ng/mL during cytologic estrus . Anovulation is related to a split heat because it can be the first stage of a split heat. Anovulation has been reported to be an uncommon clinical finding, with incidences of approximately one percent [39,40]
Suggested causes of anovulation include failure of the ovary to deliver a sufficient estrogen signal to cause an LH surge, failure of the hypothalamus to secrete sufficient GnRH or the pituitary to secrete sufficient LH, or failure of the ovary to respond to a normal LH surge .
Although reported therapies for anovulation include GnRH or hCG , treatment may not be appropriate because of the possibility that this is a heritable condition.
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