**2. The canine estrous cycle**

Dogs are spontaneous ovulators, monoestrous, and nonseasonal species [6, 7], despite that there are some reports on an increased incidence of estrus during winter and late spring [8]. However, the photoperiodic control of estrus has only been clearly shown in the Basenji [9].

In dogs, each cycle is separated from the following by an obligatory anestrus stage of variable duration [6]. The period between two consecutive proestruses is often named as interestrous interval. The number of cycles per year varies greatly among bitches, accounting the high variability recorded for the interestrous interval among this species. However, this variation is independent of the animal size. Although the pattern and regularity of the estrus activity may vary between breeds or even genetic lines, it is regular for each female. Some females show only one cycle per year, such as the Basenji and the Tibetan Mastiff, others show two to three cycles yearly. Thus, the physiological interestrous interval may vary between 4 and 12 months [6, 10]. For example, some lines of Rottweilers, German Sheepdogs, and Bernese Mountain dogs may show a 4-month interestrous interval, while some lines of Collies, Labradors or Teckels may present an interval of 7–8 months. Usually, the reported average is 6–7 months [6, 7]. Therefore, it is important to collect the information on the regularity of the estrous cycle in a particular bitch when designing a contraceptive medical protocol, as this may require the adjustment of the administration schedule.

**1. Introduction**

140 Canine Medicine - Recent Topics and Advanced Research

The control of the reproductive activity in dogs is a main issue in today's societies. Overpopulation of stray and feral dogs is an universal problem and raises public health concerns related to the increased risk of zoonotic diseases and compromise of the environmental health that may foster the permanence of diseases in other species or the degradation of ecosystems [1],

At the start of this chapter, the authors would also strengthen that the best measure for a safe and definitive measure for the suppression of reproductive cycles in animals not intended for future breeding is ovariectomy (in prepubertal or young postpubertal female dogs) or ovariohysterectomy [2–4]. These procedures also have a protective effect on the incidence of uterine and mammary diseases, if performed early in the animal life. However, spaying of young animals, whether male or female dogs, is not an universal option; in some countries, it is described as an unnecessary surgical insult to the dog, and therefore must be performed only under medical indication. In other, the surgery is rejected for economic reasons, and medical contraception is regarded as a more economical alternative to surgery. The therapeutic options for mastering the companion animal reproduction substantially improved in recent decades. Also, the nonsurgical contraceptive treatments drew the attention of the industry and researchers, allowing the recent introduction of new methods in the clinical practice targeting safer long- and short-term suspension of gonadal activity in dogs. The information on the available methods alternative to progestins needs to be discussed and their advantages and disadvantages have been reviewed to disseminate basic information of their use and ease the

The suppression of canine reproductive activity includes: (1) the sterilization of both male and female, as a form to control either the ownerless or free-roaming or community-owned dogs as well as privately owned, confined or roaming dogs; and (2) the canine contraception for the periodic suspension of ovarian activity, which can also be applied on the male counterpart. This chapter focuses on the medical contraception in dogs. Moreover, as the manipulation of the ovarian activity in dogs, even if directed to its suspension, presumes the basic knowledge of the normal endocrinology of the estrous cycle of bitch, it will be reviewed, before discussing

Dogs are spontaneous ovulators, monoestrous, and nonseasonal species [6, 7], despite that there are some reports on an increased incidence of estrus during winter and late spring [8]. However, the photoperiodic control of estrus has only been clearly shown in the Basenji [9].

In dogs, each cycle is separated from the following by an obligatory anestrus stage of variable duration [6]. The period between two consecutive proestruses is often named as interestrous interval. The number of cycles per year varies greatly among bitches, accounting the high vari-

as well as to the social and urban problems associated with animal-human conflicts.

introduction of safer products in canine medicine [5].

the nonsurgical contraceptive treatments available in dogs.

**2. The canine estrous cycle**

Age at puberty in dogs is mostly affected by size, although nutritional or social cues may also modulate it. In female dogs, puberty occurs when the animal reaches 70–80% of its mature body weight [11].

Classically, the canine estrous cycle is divided into four stages (**Figure 1**) that recur at regular intervals [6–8]. The length of each stage of the canine estrous cycle varies individually, with exception of the diestrus that is fairly constant whether pregnancy occurs or not.

**Figure 1.** Schematic representation of the standard canine estrous cycle. Adapted with permission from [12].

Proestrus represents the first signs of reproductive activity; following the rapid follicular development of follicles in the ovary, which determines a rapid increase in the suprabasal estrogen levels, the female presents the external clinical signs associated with heat, such as swollen vulva, serous-hemorrhagic vulvar discharge, and increased restlessness and attraction of male [6–8]. The average length of proestrus is 9 days, but in fact it may range between 3 and 21 days [7]. The transition from proestrus to estrus is feebly detected on the basis of external or behavioral features, and so the two stages are usually grouped under the designation of "heat". Estrus is a transitional stage. Owing to the preovulatory luteinization of the granulosa cells in the growing follicles (progesterone levels increase above 2 ng/mL from LH peak onwards), early in this stage occurs a shift in the steroid environment, which changes from peaking estrogens to the progesterone dominance that will be maintained throughout diestrus. These changes induce a decrease in the amount of vaginal discharge, which also becomes more mucous and less hemorrhagic. In this stage, the female search more actively and the contact with the male and allows mating. Estrus lasts in average 9 days, but individual variations account for a range of 3–21 days. The LH surge occurs usually 24 h after the onset of estrus, but ovulation will take another 1.5–2.5 days to occur. The ovulation product is an immature oocyte that needs an additional period of 2–3 days for tubal maturation before fertilization [6].

Diestrus represents a prolonged luteal stage, similar in length whether or not pregnancy occurs. The decrease of progesterone levels below 1–2 ng/mL is often used to delimit the end of diestrus, as externally no clinical signs allow establishing the limits between diestrus and anestrus. Still, some bitches may present a residual mucous vaginal discharge or mammary development during diestrus that are absent in anestrus. The mean length of nongestational diestrus is 60 ± 15 days, while the gestational diestrus lasts for 63 ± 1 days. The levels of progesterone are already high at the diestrus onset; the peak is maintained for almost half the stage and gradually decreases by the end of this stage; the progesterone decline is more abrupt in gestational cycles than in nonpregnant ones [6, 8].

In anestrus, the sex steroids are maintained in basal values, except in the last third of the stage, when the initial development of a wave of follicles in the ovaries occurs, thereby inducing a small increase in estrogens [13, 14]. Although this stage is often considered a time of reproductive quiescence, in fact in the uterus an important remodeling and repairing of the endometrium occurs, which is of upmost importance to the bitch fertility [13]. The length of this stage is the most variable in dogs, despite the reported average length of 18–20 weeks. The minimum length of anestrus is 7 weeks after the progesterone drop, but it can reach up to 10 months [6, 7]. It is important to remember, however, that the duration of the anestrus may be modulated by external environmental factors. An anestrous bitch can be stimulated to resume proestrus when in close proximity to a bitch in estrus [7]. This in fact contributes to the synchronization of estrous cycles often observed when bitches are housed together.
