Litter Size Response to Oestrous Induction with Deslorelin (Ovuplant®) in Dogs
Author's address (for correspondence): H Meyer, 102 Withycombe Hall, Department of Animal Sciences, Oregon State University, Corvallis, OR 97331, USA. E-mail: firstname.lastname@example.org
Four German Shorthair Pointer bitches each produced from two to five (total of 14) purebred litters in response to natural matings to either natural oestrus (n = 8) or oestrus induced by Ovuplant®, a sustained-release implant containing 2.1 mg deslorelin (n = 6). All bitches initially produced litters from natural oestrous matings prior to producing litters from induced oestrus, and two of the bitches also produced natural litters subsequent to their Ovuplant® litters. Mean litter size was lower for Ovuplant® litters (5.4 vs 8.6 puppies; p < 0.001) and within each bitch every induced litter was smaller than any of her natural litters.
Clinical applications of deslorelin in canids were first introduced during an investigation for a novel contraceptive (Suprelorin™ Peptech, Macquarie Park, Australia), which is now commercially available in Europe, Australia and New Zealand (Trigg et al. 2001). Preliminary investigations with this product demonstrated that it initially induced oestrus in all anoestrous bitches treated, followed by prolonged oestrous suppression. In addition, Bertschinger et al. (2001) reported that Suprelorin™ (containing 6 mg of deslorelin) induced oestrus in an African wild dog, resulting in a normal pregnancy and whelping. Ovuplant® (Ayerst Laboratories, Guelph, ON, USA) is a biodegradable, sustained-release, subdermal implant containing 2.1 mg of deslorelin, licensed for use in horses. Previous studies in anoestrous domestic dogs with Ovuplant® demonstrated its reliability for inducing a rapid and synchronous oestrus (Kutzler et al. 2009). Ovuplant® has also been used in grey wolves to induce oestrus and resultant offspring (Asa et al. 2006).
Canine oestrous induction rates, ovulation rates and pregnancy and whelping rates have been compared between various oestrous induction protocols (Kutzler 2007) but the effect of oestrous induction on litter size has not been investigated. In addition, there is a clinical impression that litter size following Ovuplant® oestrous induction is lower compared with spontaneous oestrous cycles (M. Kutzler, unpublished observations). The objective of this study was to compare litter size in bitches conceiving to spontaneous oestrous cycles and cycles induced with Ovuplant® in the same bitches.
Materials and Methods
Four German Shorthair Pointer bitches produced a total of 14 litters following multiple natural service matings during either spontaneous (n = 8) or induced (n = 6) oestrous cycles. All bitches were housed and maintained at the same facility, received the same diet and were mated to the same males; bitches were unrelated to males. Oestrous cycles were induced by implanting one 2.1 mg deslorelin implant beneath the vestibular mucosa during anoestrus. Anoestrus was confirmed by serum progesterone concentrations <0.6 ng/ml. All bitches had produced at least one litter from a natural oestrus before receiving Ovuplant® and two of the bitches each produced a litter from a natural oestrus subsequently. Bitches were typically mated once daily during their duration of receptivity to males. Residual implanted material was surgically removed under local anaesthesia at the end of mating. Litter size was determined at attended whelping. Litter size was analysed using Least Squares Analysis of Variance (Statistix 9.0 software, Analytical Software; Tallahassee, FL, USA).
Bitches and sires ranged from 2–7 years old at the time of mating. The mean age at mating from natural oestrus was 3.6 years (range 2–6) compared with mean age of 4.5 years (range 3–7) at mating for bitches receiving Ovuplant® (Table 1). All bitches treated with Ovuplant® showed first signs of oestrus within 5 days of implantation and were receptive to mating within 9 days. Their length of mating receptivity ranged from 4–7 days compared with a range of 8–14 days for bitches in spontaneous oestrus. Mean size of litters produced following oestrous induction with Ovuplant® was significantly lower than for litters produced from spontaneous oestrus (5.4 vs 8.6 pups; p < 0.001). All puppies were born alive and physically normal. Comparison of litters conceived during winter (November–January, n = 8) with litters conceived in summer (May–September; n = 6) revealed no seasonal effect on litter size.
Table 1. Results from 14 oestrous cycles in four bitches. Litter size was reduced in Ovuplant® induced oestrous cycles (in italics; p < 0.001)
| 7 || May || Ovuplant ® || 6 |
|4|| December || Ovuplant ® || 8 |
|5|| January || Ovuplant ® || 6 |
|Piper||3|| December || Ovuplant ® || 4 |
|4|| December || Ovuplant ® || 4 |
| 4 || December || Ovuplant ® || 4 |
The German Shorthair Pointer is a relatively prolific breed and the average litter size we observed for naturally occurring litters (8.6) was not atypical for the breed. It is relevant to note that for every bitch reported in this study, the smallest litter size(s) resulted following Ovuplant® treatment, and no natural litter was as small as her largest Ovuplant® litter. It is unknown if the observed negative effect of Ovuplant® on litter size would be exhibited in other breeds or under other conditions characterized by reduced litter size (e.g. male subfertility). Three additional litters produced by these same bitches were not included in the report owing to diagnosis of subfertility in the stud following an Ovuplant® litter of just two puppies from Delta; the two litters sired by this stud to natural oestrus in the interim (Gracie – 9 puppies; Piper – 5 puppies) were both larger than any of the Ovuplant® litters from these bitches. Inclusion of these three litters in the analysis would have made the estimated negative effect of Ovuplant® treatment even larger.
Oestrous induction with Ovuplant® proved very reliable for timing breeding and parturition; however, dog breeders must consider the potential resulting reduction in litter size. The basis for this reduction remains unknown but may have been owing to lower ovulation rates or higher early embryonic loss. Recently, Walter et al. (2011) reported that Beagle bitches with deslorelin (4.7 mg; Suprelorin™) induced oestrous cycles had significantly fewer oocytes or embryos (3.4 vs 5.0) in uterine horn flushings 9–19 days after ovulation compared with spontaneously cycling bitches. Conversely, in two female Mexican grey wolves stimulated with Ovuplant®, more oocytes were retrieved following ovarian aspiration compared with those who had not being stimulated (Boutelle et al. 2011). Combined, these other studies suggest that the cause for the reduction in litter size is not from a reduction in ovulation rate but rather a reduction in conception rate or higher early embryonic loss rate.
In contrast to other deslorelin formulations, Ovuplant® is in a biodegradable pellet form, and the composition and amount of residue removed following mating varied greatly. It is not known if this variation in apparent rate of release/absorption or if the amount of residue retained may have influenced resulting litter size.
While this study was confined to a single breed and small number of bitches, the trial design and analysis allowed for comparison of treatment effects within bitches, that is, each bitch served as her own control, thereby accounting for inter-bitch variation. As breeds clearly differ in litter size, we strongly advise that reproductive studies involving more than one breed attempt to account for interbreed variation in analysis of results.
Optimal time of puppy production varies considerably among breeds, seasons and breeders. The ability to manipulate oestrus so as to produce puppies on a predetermined schedule would be a tremendous asset for breeders provided concomitant costs were not excessive. Unfortunately, a penalty of more than two puppies per litter as found in this study would prove prohibitive for breeders producing a limited number of highly valuable dogs.
We thank Vicki Meyer for her assistance with animal care.
Conflicts of interest
The authors have declared no conflicts of interest.
Wolf monitored bitches and puppies, created tables, edited manuscript; Meyer conceptualized and funded project, provided animals, performed statistical analyses, co-wrote manuscript; Kutzler co-wrote manuscript.
This research was funded by the Department of Animal Sciences at Oregon State University and Chippewa Kennel, LLC.