Shortening of Interestrous Intervals With Cabergoline in Bitches: A Clinical Trial
Two consecutive interestrous intervals (n=46) were recorded in 23 bitches of different breeds. At varying times after day 100 from the onset of the second proestrus, cabergoline (5 μg/kg per os q 24 hours) was administered from early (n=11), mid- (n=10), and late (n=2) anestrus until 2 days after the beginning of the following proestrus. Interestrous intervals (IEI) were significantly shorter in the cabergoline-treated time periods when compared to the nontreated IEI (184±4.5 days versus 239±4.5 days; P<0.01). The mean number of days of cabergoline treatment until the onset of proestrus was 21.4±2.9 (least square means and standard error of the mean [LSM±SEM]). Mean cabergoline treatment durations beginning in early, mid-, and late anestrus were 27.4±3.7, 17.6±3.8, and 5±3 days (LSM±SEM), respectively. A significant correlation was found between the stage of anestrus in which the treatments began and the duration of the treatments required to induce estrus (0.51, P=0.01).
Introduction
Compared to other domestic and laboratory species, the canine estrous cycle is truly biphasic (i.e., having follicular and luteal phases) and characterized by the slow progression of its phases.1 Specifically, an obligatory long anestrus results in cycle intervals that can be as long as 11 months in certain breeds.23 Therefore, shortening the interestrous interval (IEI) and inducing fertile estrus may be desirable in certain bitches intended for breeding. Indications for shortening the IEI may include missed breeding opportunities and failure to conceive. Some control over the cycle of bitches is also necessary when artificial reproductive technologies are used.
Reliable fertile estrus induction protocols have been difficult to devise in the dog, because the factors regulating termination of anestrus and the onset of a new estrous cycle are not completely understood.4 A major advance in this field has been the experimental finding that in dogs, certain dopaminergic agonists can regulate the IEI.4–6 Dopaminergic agonists (e.g., bromocriptine, cabergoline) are ergot derivatives that act directly upon dopamine pituitary receptors to modify the synthesis and release of prolactin by pituitary cells.7–9 Cabergoline has fewer side effects, has a longer duration of action, and is more potent than bromocriptine.9 The precise mechanism of action by which dopaminergic agonists are able to initiate an estrous cycle is not yet clear; however, theories include a direct stimulatory effect on the gonadal axis, an indirect effect by decreasing prolactin concentrations, and, less likely, a peripheral effect at the ovarian level.10–12
The finding that dopaminergic agonists could modify the IEI led to a number of experimental studies with either bromocriptine or cabergoline in laboratory beagles.813–17 In an initial study of 23 bitches experiencing prolonged anestrus and given the ergoline derivative FCE 21336, all the animals came into estrus within 12 days of treatment, and 12 of 14 that were bred became pregnant.13 In another study, cabergoline (5 μg/kg q 24 hours) was administered to a group of bitches for 14 days, 4.5 or 6 months following the previous estrus, and were compared to a placebo group. No significant differences were found in IEI between the two groups.8 More recently, however, cabergoline was used at the same dose from day 30 after the luteinizing hormone surge to 2 days after the onset of proestrus in five beagle bitches, with a reduction in the IEI from 216 to 66.5 days in four of the five bitches. Unfortunately, none of the bitches became pregnant after breeding.14 Conversely, in a recent placebo-controlled study, 15 beagle bitches were treated with the aforementioned cabergoline protocol beginning in early (day 93 to 108), mid- (day 123 to 156), and late (day 161 to 192) anestrus, resulting in a shorter and more synchronous IEI in 14 of the dogs when compared to the control group. Twelve of the 15 treated animals became pregnant following breeding, and they produced normal litters.15
At the present time, very little field data is available on the use of cabergoline in bitches other than beagles.1617 Therefore, it was the purpose of this study to provide information regarding the efficacy and safety of cabergoline in estrus induction and in the shortening of the IEI in bitches of different breeds under field-controlled conditions.
Materials and Methods
Study Population
Twenty-three, 2- to 5-year-old, privately owned and clinically healthy, purebred bitches (i.e., Labrador retriever [n=1], Dogue de Bordeaux [n=1], beagle [n=1], Saint Bernard [n=1], rottweiler [n=1], collie [n=1], cocker spaniel [n=2], basset hound [n=2], boxer [n=5], and Argentine boar hound [n=8]) that were determined to be in anestrus were included in this trial. Most of the dogs belonged to different breeding kennels, with the exception of the Argentine boar hounds (all of which were from the same kennel). Anestrus was established by typical findings on vaginal cytology and vaginoscopy, and it was confirmed by serum progesterone (P4) concentrations (<1.5 ng/mL).2,3 The bitches remained in their normal surroundings at home during the study period, except at the time of examinations and blood sampling. Written informed consent was obtained from all owners.
Study Protocol
Two consecutive IEIs (n=46), with IEI defined as the period between the first day of proestrus (i.e., vulvar swelling and bloody vaginal discharge) of one estrous cycle and the first day of proestrus of the following cycle,218 were recorded for all bitches. After day 100 from the beginning of the second proestrus, cabergolinea (5 μg/kg per os [PO] q 24 hours) was administered to all the bitches in their food from early (n=11), mid- (n=10), and late (n=2) anestrus until 2 days after the onset of the next proestrus. Ninety days was arbitrarily considered to be the duration of both the follicular and luteal phases in all the animals.23 Stage of anestrus was calculated for each particular bitch by subtracting the 90 days from the previous IEI and then dividing the remaining days into thirds, defining the first third as early anestrus, the second third as mid-anestrus, and the last third as late anestrus, respectively.
All the bitches were naturally bred during the subsequent (following cabergoline administration) induced estrous cycles based on estrous behavior. Any potential side effects to cabergoline administration were recorded during the study period.
Vaginal Cytology
Vaginal cytology was performed daily from the onset of proestrus until diestrus (both preceding and following cabergoline administration) to follow follicular development. The smears were stained using a modified Giemsa-Wright stain,b and each smear was interpreted by two of the authors.
Progesterone Assay
Blood samples for serum progesterone (P4) were taken from all the animals by peripheral venipuncture 3 weeks after estrus to confirm ovulation (P4, >5 ng/mL) in both the treated and control periods. Serum P4 was measured by radioimmunoassay using a solid-phase kit.c The sensitivity at 95% binding was 0.1 ng/mL, and the intra- and interassay coefficients of variation were 4.9% and 7.6%, respectively.
Statistical Analysis
Days of IEI of pretreated (n=23) and treated periods (n=23) were analyzed by least squares analysis of variance using the General Linear Model Procedure (PROC GLM).d,19 The mathematical model included the main effect of treatment on each study animal. Correlation analysis between the periods of anestrus (i.e., early, mid, late) in which the treatment was begun and the days of treatment required to induce proestrus was carried out (PROC CORR).d,19 The proportion of bitches that achieved normal cycles (i.e., normal follicular phase and ovulation), gestation, and side effects was also calculated. Data was expressed as least square means and standard error of the mean (LSM±SEM).19
Results
Interestrous intervals were significantly shorter during the cabergoline-treated IEIs when compared to the nontreatment IEIs (184±4.5 versus 239±4.5 days, respectively; P<0.01). The mean number of days from the initiation of cabergoline treatment until the onset of proestrus was 21.4±2.9 days (LSM±SEM; range, 2 to 48 days). Mean (LSM±SEM) cabergoline treatment durations beginning in early, mid-, and late anestrus were 27.4±3.7, 17.6±3.8, and 5±3 days, respectively. One cocker spaniel bitch, whose onset of therapy was during midanestrus, received 77 days of treatment prior to the development of proestrus; therefore, this dog was considered an outlier and was not included in these calculations [see Table]. A significant correlation was found between the stage of the anestrus during which the treatments were begun and the duration of the treatments (P<0.01). Cabergoline treatment begun earlier in anestrus required more days to induce proestrus than when treatment was begun in late anestrus.
All cabergoline-induced estrous cycles (23/23) were normal, based on vaginal cytology and evidence of ovulation. Duration and physical characteristics were similar to pre-treatment, spontaneous estrous cycles. Nineteen of the 23 postcabergoline estrous cycles (82.6%) were followed by pregnancy and whelping of normal litters [see Table]. Such successful cycles occurred in 9/11, 9/10, and 1/2 dogs that were induced in early, mid-, and late anestrus, respectively. No gastrointestinal upsets were observed during the study period, as have been previously reported in cabergoline-treated dogs.9
Discussion
Estrus induction and control are often useful for breeders for a variety of reasons. Because there is often a natural estrous cycle synchronization among dams within the same breeding facilities, puppies are often produced only during limited times of the year. By altering the IEIs, it might be possible to produce puppies at varying times. Another benefit for owners of bitches with normally long IEIs is to shorten the IEI so that the number of litters produced per year can be increased.
Most research on this topic has been published only in laboratory beagles, which may have reproductive physiological characteristics unique to their breed. Moreover, laboratory dogs usually live in large colonies, a type of life quite different from that of most household pets. Although the use of laboratory animals in a laboratory setting allows for the strict control of most variables, extrapolation of study results to other purebred pets or field conditions is not straightforward. Therefore, it was the purpose of this study to test the efficacy and safety of cabergoline in shortening the IEI in a controlled clinical field trial utilizing a variety of different breeds. As significant differences have been reported in the duration of the IEI among different breeds,31820 and even within the same breed,321 each bitch in this study was used as its own control to reduce variability between subjects.
The results of the study reported here were similar to previous laboratory studies, demonstrating that the use of cabergoline is an effective method of inducing estrus and thereby shortening the IEI in bitches of different breeds.15 The results of this study disagree with a previous one in which no shortening of the IEI was found.8 In that study, cabergoline treatment was fixed at 14 days, which may have precluded an appropriate response, especially for bitches in which treatment was begun early in the estrous cycle.8
In a previous trial, the same dose of cabergoline was used in three bitches of different breeds, with time to induce estrus being 12, 27, and 28 days.16 This compares favorably to the mean of 21.4 days found in the clinical trial reported here.
In comparison with a previous experimental study in beagles, duration of cabergoline treatment in the three different stages of anestrus was slightly longer in this study.15 In the earlier study, the process of inducing proestrus in early, mid-, and late anestrus took 20±5, 14±6, and 6±2 days compared to 27.4±3.7, 17.6±3.8, and 5±3 days, respectively, in the authors’ study.15 These differences could reflect breed variations in the response to treatment. In the previous report and this study, the later in anestrus that the cabergoline treatment was begun, the more quickly was the onset of proestrus.15 Therefore, when deciding the time to begin estrus induction, a quicker response should be weighed against a minimal shortening of the IEI.
All estrous cycles induced by cabergoline treatment were deemed to be normal, as shown by vaginal cytology and serum P4 ovulation concentrations. Also, gestation percentages were similar to those pregnancy rates obtained in previous studies, which ranged from 85.7% to 93%.81415 As in the present trial, breeding management was only based on behavioral estrus, and examination of males for breeding soundness was not performed; therefore, the failure of pregnancy in four bitches cannot be fully attributed to treatment failures. The lack of pregnancies found in an earlier study probably occurred from insufficient uterine involution during the previous cycle,14 as the treatment was initiated on day 30 of the luteinizing hormone surge.
In contrast to the mild gastrointestinal side effects previously reported for the same dose of cabergoline, in the present study no digestive side effects were noted.815 Administration of cabergoline with food may have mitigated any bowel upset and thereby accounted for this difference.
Conclusion
The administration of cabergoline (5 μg/kg PO q 24 hours) during anestrus is an efficient, safe, and easy method for induction of normal fertile estrous cycles and for decreasing IEI in this group of bitches of different breeds. If used ethically, cabergoline may provide a reliable method for improving canine reproductive performance in the bitch. Further study is warranted to examine any possible health effects from repeated use of cabergoline.
Galastop; CEVA-VETEM, France
Hemacolor; Merck, Darmstdt, Germany
Coat-A-Count, DPC; Los Angeles, CA
SAS, 1989; Cary, NC
Acknowledgments
The authors thank CEVA-VETEM (France) laboratory for providing the cabergoline.
