Anestrus

Anestrus is a term used in veterinary and wildlife biology to describe a period during which a female does not exhibit estrous cycles or ovarian activity. It can be a normal physiological phase or a result of underlying pathology. In many species, anestrus is defined by the absence of behavioral estrus and absence of ovulation over a defined time frame.

Physiological anestrus occurs naturally in several contexts. In prepubertal animals, the reproductive system has not yet reached maturity. In postpartum females, particularly those with substantial lactation, the return to normal cycling is delayed. Some species also show seasonal anestrus, related to photoperiod and hormonal regulation of the hypothalamic-pituitary-ovarian axis. Upon recovery, cycles resume with puberty, weaning, or the appropriate seasonal cue.

Pathological or non-physiological anestrus results from disorders that disrupt the normal reproductive axis or ovarian function. Causes include malnutrition or malcondition, chronic systemic illness, stress, excessive exercise, or hormonal or ovarian disorders such as hypothalamic or pituitary dysfunction, ovarian simple inactivity, or persistent luteal activity suppressing new follicular development.

Diagnosis relies on history, physical examination, and reproductive tract assessment. Hormonal testing (for example, progesterone to assess luteal involvement) and imaging such as ultrasound can help distinguish anestrus from pregnancy or silent estrus. Management targets the underlying cause. Nutrition optimization, reducing stress, and adjusting environmental cues can restore cycling in seasonal or lactational anestrus. Pharmacologic options in breeding programs may include hormones to regress the corpus luteum or to stimulate ovulation, used selectively and under veterinary guidance.

Prognosis varies with cause; many cases improve when the underlying factor is addressed, whereas persistent or complex cases require specialized veterinary evaluation.