Estrus induction and synchronization in canids and felids

Abstract

Indications for estrus induction in the dog and cat include potential missed breeding opportunities or conception failure, the treatment of primary or secondary anestrus, out-of-season breeding (feline) and synchronization of ovulation for embryo transfer programs. Reported methods for estrus induction in bitches and queens include the use of synthetic estrogens (diethylstilbesterol), dopamine agonists (bromocriptine and cabergoline), GnRH agonists (lutrelin, buserelin, fertirelin, deslorelin, and leuprolide), exogenous gonadotropins (LH, FSH, hCG, PMSG, and human menopausal gonadotropin) and opiate antagonists (naloxone). These methods vary widely in efficacy of inducing estrus as well as in the fertility of the induced estrus. The applicability of some of these methods for clinical practice is questionable. This review will summarize published reports on estrus induction in canids and felids, both wild and domestic, and provide an update on research using a long-acting injectable deslorelin preparation in bitches.

Introduction

Domestic bitches (Canis familiaris) are non-seasonally monoestrus. As a result of this unique reproductive physiology, bitches spontaneously ovulate only once or twice per year and ovulation can occur at any time of the year. However, there are few exceptions, such as the Tibetan Mastiff and the Basenji [1]. The interestrus interval is the time from the onset of proestrus to the subsequent onset of proestrus, and includes proestrus, estrus, diestrus and obligate anestrus periods. Proestrus is diagnosed clinically by the onset of vulvar edema and/or serosanguinous discharge, whereas estrus is defined as the onset of either behavioral signs (willingness to allow mating) or vaginal epithelial exfoliative cytology (>90% cornification).

The interestrus interval averages 31 wk [2], [3], with a typical range of 16–56 wk [3]. In some bitches, estrus is much less frequent or outward (visible) signs of estrus are minimal to non-existent, giving the appearance of a prolonged interestrus interval (persistent anestrus). The variation in interestrous interval length owes itself to differences in the duration of anestrus. The duration of anestrus differs between and within dog breeds indicating a genetic basis for anestrus length [4]. Bitches with longer than average interestrus intervals have reduced opportunities of becoming pregnant [5]. It is important to mention that histological changes similar to involution in the bitch's endometrium are not complete until 135 d after the most recent estrus, regardless of whether the bitch was pregnant or not [6]. Induction of estrus before this time may result in reduced fertility.

There are many indications for estrus induction in the bitch. The development of a method to induce estrus effectively and predictably in bitches would be advantageous in the clinical management of prolonged anestrus or infertility. Estrus induction is also clinically useful in conjunction with routine breeding management when breeding opportunities are missed or following conception failure or if a particular mating must be timed around the availability of the stud dog. Estrus induction improves the management of breeding colonies (number of litters per year), especially in establishments where a continuous supply of pups is required (e.g. service dog industry). In the research laboratory, it is often desirable to control the timing of pregnancy and to breed a group of bitches to whelp at the same time. Induction of estrus in the bitch would also be beneficial to research of certain canine reproductive disorders and investigation of the physiology of the estrous cycle. In addition, reliable synchronous estrus induction is a necessity for canine embryo transfer programs. Estrus induction is also an effective tool for teaching canine reproduction.

In the bitch, progression from early to late anestrus is characterized by a higher amplitude and larger number of hypothalamic GnRH pulses [7], an increase in pituitary sensitivity to GnRH [8], and an increase in ovarian responsiveness to LH and FSH [8], [9]. Serum FSH concentrations are increased throughout much of canine anestrus, whereas LH concentrations are low except near the end of anestrus [10]. An increase in basal plasma FSH concentration is critical for initiation of folliculogenesis in dogs [11], [12]; FSH induces expression of LH receptors in the ovarian granulosa cells [4]. Following initial follicle recruitment, LH is progressively able to replace FSH in the support of follicular maturation [13]. In fact, supraphysiologic doses of LH alone administered to bitches in anestrus will induce follicle growth and proestrus [10], [14]. It is also important to note that factors causing a decrease in opiodergic activity promote LH release and the termination of anestrus [10]. There is convincing anecdotal evidence that a “dormitory effect” occurs in canids, such that co-housing bitches in mid to late anestrus with proestrous or estrous bitches will shorten the duration of anestrus by 30 d or more in anestrous bitches [10]. It is assumed that this phenomenon of “natural” estrus induction is mediated by pheromones, but the mechanism by which an increase in gonadotropin secretion is mediated is not known.

Domestic cats (Felis catus) are seasonally polyestrous. Queens are long-day breeders such that a prolonged anestrus occurs during short-day length (September to January in the Northern Hemisphere) [15], [16]. Adjusting the lighting time can control the onset of estrus; reducing the daily lighting time to 8 h can inhibit estrus [17]. Fourteen hours of light administered during the non-breeding season will result in the return to cyclicity in 15.6 ± 0.5 d [17]. It is important to note that estrus can be induced with as little as 12 h of light when social stimuli (a male stimulus or an estrual female) were introduced 3 wk after an increase to 12 h of light. In addition, estrus could be induced with 12 h of light when female cats were given a 1-h light exposure after a period (1–4 h) of darkness (“interrupted darkness”) [17].

Proestrus is not commonly observed in queens [18], [19]. Estrus is defined by estrous (receptive) behavior or changes in cornified vaginal cytology secondary to increased circulating estradiol 17-β concentrations [18], [20], [21], [22], [23], [24], [25], [26]. One of the many challenges facing conservationists is that many wild felid species never display overt signs of sexual receptivity (e.g. clouded leopard). Average length of the interestrous interval in non-bred domestic queens is 9.0 ± 7.6 d throughout the breeding season, unless ovulation occurs [18]. The interestrous interval also varies depending on whether the bred queen conceived. Reported lengths of the interestrous interval in bred queens that ovulated but did not conceive are 45.0 ± 10.3 d [18], 50.3 ± 2.7 d [26], and 61.5 ± 14.5 d [27].

Queens are induced ovulators, requiring external stimulation (such as natural breeding) to stimulate the release of pituitary LH and ovulation of mature follicles [24], [26], [28], [29], [30], [31], [32], [33]. However, queens are often sexually receptive prior to the time when ovulation can occur. Breeding too early in estrus (prior to the 3rd or 4th day of estrus) can result in an attenuated LH secretion and ovulatory failure [34]. Although the LH response sufficient to induce ovulation can occur after a single mating, repeated matings may be needed to produce a maximal rise in LH. In one report, 10 of 48 queens ovulated after a single mating, but 30 of 36 queens ovulated after multiple matings [33]. For optimal breeding management, allowing the pair to mate three times per day at 4-h intervals throughout estrus should be recommended [35].

While considered to be induced ovulators, as many as 60% of unpaired, unmated and unstimulated female domestic cats ovulate without external provocation [36], [37], [38]. This phenomenon is not restricted to domestic cats; 43% of clouded leopards (Neofelis nebulosa) ovulated spontaneously [39]. In the leopard (Panthera pardus) [40] and lion (Pantera leo) [41], spontaneous ovulations occurred primarily in females that are housed together. This would suggest that a female-to-female interaction may induce ovulation. However, in clouded leopards, individually housed females will still spontaneously ovulate [42].

Once follicles are mature, ovulation can be induced via exogenous administration of gonadotropins. Single or multiple injections of porcine LH [43], [44], a single injection of hCG [45], [46], [47], [48], [49], [50], [51], single or multiple injections of GnRH [28], [52] or a single injection of a GnRH analogue (leuprolide) [53] induced ovulation in queens. Use of hCG at the time of breeding queens reduces the number of matings necessary and results in predictable gestation lengths. Dosages of hCG for inducing ovulation in domestic cats range from 25 to 500 IU [45], [46], [47], [48], [49], [50], [51]. However, Wildt and Seager reported that administration of 250 or 500 IU of hCG for 2 d during estrus resulted in significantly higher ovulation rates than 50 IU hCG for 2 d [54]. Higher doses of hCG appeared to increase susceptibility to oocyte degeneration [55]. Administration can be made intramuscularly or intravenously. Tanaka et al. reported that ovulation could be reliably induced (100% of cats) with intravenous administration of two doses of hCG (100 IU) at 24 h intervals between the 2nd and 4th day of estrus [48]. The same authors reported an ovulation rate of 91.7% in domestic queens with a single injection of 250 IU hCG [48]. The time of ovulation following hCG administration in domestic cats is reported to be 25–27 h [56], [57]. However, Tsutsui et al. reported ovulation occurring in 40% of cats between 15 and 20 h after hCG treatment [58]. Wild felids ovulate consistently at 37–42 h after hCG administration [53].

Given alone to anestrual queens, hCG is highly folliculogenic [59] and has the side effect of causing secondary follicular growth when administered during estrus [51], [60], which can decrease pregnancy rates [61], [62]. Also, hCG is long-acting and known to stimulate the production of anti-gonadotropin antibodies [62]. Administration of hCG at intervals more frequent than every 6 mo should be avoided.

Gonadotropin-releasing hormone or its analogues have also been used for ovulation induction in queens. Although a single injection of 5–25 μg/cat of GnRH increased serum LH concentrations in estrual domestic queens, Chakraborty et al. reported that only the 25 μg/cat dosage consistently resulted in ovulation [28]. This is in contrast to Swanson et al. who found that domestic queens required two injections of GnRH 12 h apart on 4th day of estrus reliably induced ovulation [52]. In wild felids, a single injection of leuprolide (Lupron^®) has also been used to induce ovulation, with reported response rates of 25–86% in female clouded leopards [53].

The frequent natural estrous cycles in the non-bred queen make the utilization of estrus induction in clinical practice less likely. However, of all felid species, the domestic cat is the only one not classified as threatened with extinction [53]. Wild populations of some species have been reduced to a few hundred individuals, whereas in other species, wild populations no longer exist. The domestic cat is often used as an experimental model to develop assisted reproductive techniques for wild endangered felids. Assisted breeding is a valuable tool for genetic management of rare cat species, especially those in which natural breeding in captivity is not possible due to male aggression (e.g. clouded leopard).