Page 977 - Small Animal Internal Medicine, 6th Edition

CHAPTER 54 The Practice of Theriogenology 949

Estrus is the period of sexual receptivity and lasts an Three alternatives following estrus are possible in the
average of 7 days (3-16) and then subsides for an average queen: (1) ovulation does not occur, and an average 9-day
VetBooks.ir of 9 days (3-14). The external genitalia have no conspicu- (4-22) interestrus occurs before the next proestrus; (2) ovu-
lation occurs without fertilization, resulting in a 35- to
ous changes in appearance, but behavioral changes are pro-
nounced during estrus. Because of the relatively short length
(3) pregnancy. Pseudopregnancy occurs when ovulation and
of proestrus, the ability to inadvertently induce ovulation 40-day pseudopregnancy and a 1- to 10-day interestrus; or
by mechanical vaginal stimulation, and the less dramatic corpora lutea formation occur but not conception. The
cellular changes observed in the queen, vaginal cytology is corpora lutea produce progesterone, with levels rising rapidly
not used as commonly in the queen as it is in the bitch. from basal concentrations to a peak of 16 to 17 ng/mL 18 to
Behavioral observations should dictate the best time to place 25 days postovulation. After the peak, progesterone concen-
the queen with the tom. Estrus duration is unaffected by trations decline to basal at approximately 40 days postovula-
breeding or ovulation. If vaginal cytology is desirable, flush- tion. The normal pseudopregnancy duration lasts 35 to 40
ing a small amount (<1.0 mL) of saline in and out of the days. The corpora lutea appear to have a preprogrammed
vagina provides better results than using a swab. Interestrus finite life span in that they are not subject to regression from
is the period between successive estrus periods if ovula- uterine sources of prostaglandins. Lactation (pseudogenetra)
tion does not occur. If the queen is not bred or otherwise at the end of pseudopregnancy is less common in the queen.
stimulated to ovulate, estrus will occur every 2 to 3 weeks Induction of pseudopregnancy eliminates the so-called con-
during the appropriate photoperiod. If the queen ovulates, stant estrus seen in nonovulating queens; the duration of that
corpora lutea are formed and secrete progesterone. If the estrus is not changed. After pseudopregnancy, the queen will
queen is not pregnant, diestrus (pseudopregnancy) results go through a 2- to 4-week anestrus and either return to
and lasts 35 to 40 days. Anestrus is the seasonal period estrus if she is cycling or transition into a prolonged seasonal
when the cat does not cycle; artificial lighting can alter anestrus.
its occurrence. Gestation in the cat is 63 to 66 days when measured from
Queens are induced ovulators and ovulate in response to a fertile mating. Because kittens born before 60 days of gesta-
vaginal stimulation; however spontaneous ovulation can tion have little chance of survival, it is important that gesta-
occur in the queen. Ovulation is most commonly triggered tion progress to 63 days for maximum kitten viability.
by copulation or mechanical stimulation of the vagina, which Because queens are induced ovulators, clients should be
causes a reflex stimulation of the hypothalamus via pathways counseled to recognize the signs of estrus and place queens
in the spinal cord. The hypothalamus releases GnRH, which with tomcats for the minimal time to accomplish a fertile
then causes a release of LH from the anterior pituitary. LH mating (usually 2-3 days), making evaluation of the length
then stimulates ovulation and the development of corpora of gestation more predictable. Unlike in the pseudopregnant
lutea. Other forms of stimulation can occasionally be respon- queen, corpora lutea do not regress in the pregnant queen;
sible for ovulation; active corpora lutea have been found in despite placental progesterone production, the ovaries are
cats unexposed to males or artificial coital stimuli. Ovulation needed to maintain pregnancy past day 50. As in the bitch,
depends on adequate LH release, with both a peak concen- progesterone concentrations vary over time. Prolactin from
tration and duration of elevation being important. The LH the anterior pituitary also appears to be required for preg-
release occurs within minutes of coitus and peaks about 1 to nancy maintenance.
2 hours later. Release of LH is partially dependent upon the
duration of prior exposure to estrogen (time in estrus), so
the LH response varies depending on the day of estrus at OBSTETRICS
which coitus occurs. Multiple copulations result in higher
concentrations of plasma LH and are more likely to result in PREGNANCY DIAGNOSIS
ovulation than a single mating. The duration of elevated LH Early pregnancy evaluation permits optimal obstetrics (Table
in plasma also determines whether ovulation occurs, with 54.2). Pregnancy detection by abdominal palpation (best at
LH decreasing to baseline values within 12 to 24 hours after
a single mating or after multiple matings at less than 2-hour
intervals. However, LH remains elevated up to 38 hours after TABLE 54.2
multiple mating intervals every 3 hours. To achieve sufficient
LH release, repeated breeding at a reasonable interval should Methods for Pregnancy Diagnosis
be encouraged. The LH response to a single mating can vary GESTATIONAL AGE
substantially, and neither single nor multiple copulations can (FROM LH SURGE) METHOD
ensure ovulation. To increase the likelihood of ovulation,
breeders should try to maximize the number of matings and 25+ days Abdominal ultrasound, abdominal
breed on successive days of estrus. Ovulation occurs 24 to palpation
60 hours postcoitus and may vary depending on the mating >20-31 days Witness Relaxin (Pfizer/Zoetis)
pattern. Ideally, if the onset of estrus has been detected, >50 days Radiography
breeding should occur on days 2 to 3.