Cyclic Irregularity
Claudia Klein
Cyclic irregularities become evident in the display of irregular estrous cycles (i.e., an abnormal interval from one estrus to the subsequent estrus). Irregular estrous cycles can be physiologic, such as during the transition period in seasonal breeders; can be pathologic such as a sequel to endometritis; or may be an artifact, such as failure in estrus detection (Box 12.4).
Cattle
Irregular estrous cycles in the cow may result from a number of factors (Box 12.5). Detection of heat in cattle is challenging given that the average duration of estrus is 18 hours and in many cows may be appreciably shorter. Failure to detect heat in intensively managed herds bred by artificial insemination is common and is one of the most important management factors contributing to a perceived abnormal estrus interval.22 Missed heats should be expected if the heat intervals approximate multiples of 21 days, the normal cycle length. Errors in estrus detection are further indicated when 1) more than 10% of the interestrus intervals are 3 to 17 days or 25 to 35 days long, 2) more than 5% of the cows have two artificial inseminations within 3 days, or 3) cows are calving early or are further in gestation than predicted from the last breeding when examined for pregnancy. Seventy percent of cows should have an interestrus interval of fewer than 30 days, and 60% should be in the range of 18 to 24 days. The ratio of 18- to 24-day cycles to 38- to 46-day cycles should be 5 : 1.23 Evaluation of heat detection methods is important in artificial insemination programs. Cows need to be observed at least twice daily for signs of estrous behavior. Heat detection aids such as tailhead chalking (will be rubbed off), adhesive heat pads (will change color), and pedometry (record an increase in activity) are indispensable tools for efficient estrus detection.
Especially pedometry can detect estrus accurately, is least labor intensive, and through a software program generates daily lists of cattle with increased activity.24Silent heat, which is the lack of cycling cows to express estrous behavior (silent ovulation), occurs most commonly in the immediate postpartum period. More than 50% of cattle display a silent heat on their first ovulation postpartum with the incidence declining thereafter.25 This may be related to insufficient progesterone priming of the behavior center for optimum expression of estrus. Lameness or general disease impairing an animal's ability to move will mask signs of heat
■ BOX 12.4
Causes of Cyclic Irregularity in Mares
Common Causes
Transition season
Erroneous heat detection
Diestrus ovulation
Intrauterine therapy
Diestral endometrial biopsy
Pneumovagina
Endometritis
Pubertal cycles
Early embryonic death
Spontaneous corpus luteum prolongation
Endophyte-infested fescue
Cervical dilation
Less Common Causes
Urovagina
Rectovaginal fistula
Ovarian tumors
Pyometra
Persistent follicles
Split heats
Endotoxemia
Hemorrhagic anovulatory follicles
Old age
Use of Deslorelin implant
Uncommon Causes
Contagious equine metritis
Corpus luteum inadequacy
Anabolic steroids
Progesterone therapy
Phosphorus deficiency
Sex chromosome anomalies
Administration of oxytocin during days 7 to 14 of the estrous cycle
■ BOX 12.5
Causes of Cyclic Irregularity in Ruminants
Common Causes
Erroneous heat detection
Endometritis Intrauterine therapy Cystic ovaries Heat stress Leptospirosis Infectious bovine rhinotracheitis (IBR) (B) Bovine virus diarrhea (BVD) (B) Campylobacteriosis Trichomoniasis
Embryonic death after maternal recognition of pregnancy
Uncommon Causes
IBR- or BVD-infected semen Bluetongue-infected semen Zearalenone toxicity Ovarian neoplasia Corpus luteum inadequacy Copper deficiency (O, C) Molybdenum deficiency (O, C) Iodine deficiency Phytoestrogen toxicity Segmental aplasia of the uterus
B, Bovine; C, caprine; O, ovine.
independently from its reproductive status. Silent heat can be diagnosed only through rectal palpation of the ovaries or the use of progesterone assay in milk or plasma.
While the cow cycles normally in the face of failure in heat detection or silent heat, true, irregular cyclicity does occur. Endometritis, through the release of prostaglandin, can result in premature lysis of the corpus luteum (CL) and therefore a shortened diestrus phase.26 Cystic ovarian follicles, either in the form of follicular or luteal cysts, interfere with regular cyclic ovarian activity. This condition is mainly observed in high-yielding dairy cows during the first months postpartum. Estrogen-producing follicular cysts may result in nymphomania, a state of persistent or frequent heats. Progesterone-producing luteal cysts result in absence of estrous behavior and prolonged interestrus interval.27 Early embryonic death occurring after the time period of maternal recognition of pregnancy will prevent lysis of the CL, and the cow will undergo a prolonged interestrus interval due to the action of progesterone.28 A less common cause for prolonged diestrus is segmental aplasia of the uterine tract.29 If ovulation occurs ipsilateral to a missing uterine horn, the CL will persist due to the lack of exposure to prostaglandin.
Mares
Irregular estrous cycles in the mare are most commonly encountered during the transitional period and are considered physiologic then. Follicles will initiate growth during the transition from winter anestrus to spring reproductive activity, resulting in estrous behavior. Due to inadequate gonadotropin, support follicles may fail to ovulate and eventually regress after a varying period of time leading to irregular estrus intervals.30 Mares that have been artificially induced to develop a follicle and ovulate during seasonal anestrus using gonadotropinreleasing hormone (GnRH) or recombinant follicle-stimulating (FSH) usually do not display regular estrous cyclicity following the artificially induced ovulation till the onset of breeding season.
Likewise, mares foaling during the nonbreeding season may display foal heat, which may be followed by a variable period of anestrus. Mares older than 20 years of age may experience irregular estrous cycles independent of the time of the year, and it is not unusual for mares older than 25 years of age to stop cycling altogether. During the regular breeding season, prolonged interestrus intervals may occur in association with diestrous ovulations.31 Follicles developing during diestrus may either regress or, less commonly, ovulate. If this ovulation occurs on or after day 10 of diestrus, the newly resulting CL will be refractory to the actions of prostaglandin released from the endometrium around day 14 of the cycle. This becomes evident in a prolonged interestrus interval. Prolonged diestrus may also be observed following early embryonic death occurring after the time of maternal recognition of pregnancy. If a pregnancy is lost following formation of endometrial cups, the mare may not display regular cyclic activity during the remainder of that breeding season. Ovulation failures such as hemorrhagic luteinized follicles or anovulatory persistent follicles interfere with regular cyclicity. Anovulatory hemorrhagic follicles that luteinize may persist for more than a month without treatment, which is reflected in a prolonged diestrus phase. Anovulatory persistent follicles may or may not interfere with cyclic activity. Some mares will develop a new dominant follicle despite the presence of this abnormal structure, whereas other mares will not show signs of heat as long as these structures are present.32 Persistent estrous or stallion-like behavior is commonly described as a sequel to the presence of a granulosa cell tumor.33 Following unilateral ovariectomy, mares usually return to regular cyclicity within 6 months. Endometrial irritation such as endometritis or uterine lavage can result in shortened intervals between heats due to the release of prostaglandin.34 A less commonly encountered cause of irregular cyclicity in mares is chromosomal abnormalities such as XO genotype.35 Iatrogenic causes of irregular cyclicity are the accidental administration of prostaglandin (shortened luteal phase) or the twice-daily administration of oxytocin during days 7 to 14 (prolonged luteal phase).36Sheep and Goat
The first ovulation at the beginning of the breeding season is often not accompanied by overt signs of estrus due to the absence of progesterone priming. The corpora lutea arising from these ovulations often regress prematurely, again because of the inadequate progesterone priming. A pathologic cause of irregular or prolonged estrous cycles is the consumption of phytoestrogens from plants such as clover and alfalfa, which may lead to signs of irregular and prolonged estrous cycles.