**1. Introduction**

Sexual-cycle abnormalities in bitches may present as anestrous, shorter, or longer cycles, as well as prolonged proestrus, prolonged estrus, split estrus, or anovulatory cycles. These cycle disorders may result from abnormal ovarian functions and are a cause of infertility [1].

#### **1.1. Anestrous**

Anestrous cycles in bitches may be either primary or secondary. If a bitch does not show signs of estrus despite having reached age of puberty, anestrous cycles are primary. The age of puberty in bitches is 6–14 months. In general, the "primary anestrus" diagnosis may be used if estrus has not occurred and the cycle has not started by 24 months of age. Although some small breeds experience first estrus at 6 months of age, cycles accompanied by estrus signs may be delayed, since the first cycles may be silent. Therefore, cycle problems are not usually investigated until a bitch reaches 2 years of age. A diagnosis of "secondary estrus" is used if estrus has not occurred for 10–18 months, although first estrus had occurred. In other words, secondary estrus is defined as the presence of a period longer than 10–18 months between

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

estruses. Normally, bitches experience estrus at 4–10 month (mean 6–7 months) intervals. Cycles shorter than 4 months or longer than 10 months are abnormal and can cause infertility [2–6].

Factors that lead to primary anestrus include ovariectomy or ovariohysterectomy at early age, silent heat (subestrus), abnormalities in sexual differentiation (chromosomal and genetic disorders), use of progesterone or glucocorticoids, congenital hypothyroidism, certain systemic diseases, ovarian anomalies or ovarian aplasia, progesterone-releasing ovarian cysts, and autoimmune oophoritis [2, 7].

Secondary anestrus may result from dysfunction of the thyroid gland or adrenal cortex, as well as nonendocrinological disorders, cachexia, obesity, and use of cycle-inhibiting drugs. Silent heat, luteal cysts, and some ovarian tumors may also lead to secondary anestrus as well as to primary anestrus [4, 5].
