Anestrus

Claudia Klein

Anestrus is the absence of estrous behavior and can be a physi­ologic state during pregnancy, lactation, or seasonal anestrus, or it may be a pathologic condition (Boxes 12.6 and 12.7).

■ BOX 12.6

Causes of Anestrus in Mares

Common Causes

Season (fall, winter)

Poor heat detection

Corpus luteum persistence

Diestrus ovulation

Pregnancy

Early embryonic death after recognition of pregnancy

Fetal death after endometrial cup formation Psychological impediments

Maternal behavior

Less Common Causes

Ovarian tumors

Pituitary tumors

Pyometra

Weight loss

Chronic disease

Lactation

Old age

Uncommon Causes

Gonadal dysgenesis

Intersex conditions

Progesterone therapy

Nonsteroidal antiinflammatory drugs (NSAIDs)

Phosphorus deficiency

Ovarian hypoplasia

Anabolic steroids

Zearalenone toxicity

Chromosomal abnormalities

Persistent endometrial cups

GnRH immunization

Deslorelin implants

■ BOX 12.7

Causes of Anestrus in Ruminants

Common Cause

Season (C, O)

Pregnancy Poor heat detection Luteal cysts (B, C) Pyometra Poor nutrition, energy Heat stress Foot and leg problems Poor footing (B) Nursing beef cows and ewes (B, O) Lactation (O)

Freemartinism (B, C)

Intersex conditions (C)

Postpartum period

Heavy lactation

Primiparity

Periparturient disease

Less Common Causes

Mucometra

Hydrometra Macerated fetus

Mummified fetus

Trichomoniasis pyometra

Anaplasmosis Johne's disease

Caprine arthritis-encephalitis (C) Pseudopregnancy (C)

Insufficient number of cycling herdmates

Uncommon Causes

Ovarian tumor

Segmental aplasia

Uterine foreign body

Ovarian hypoplasia

Zearalenone toxicity

Phytoestrogenism

Phosphorus deficiency

Copper deficiency

Cobalt deficiency

Manganese deficiency

Molybdenum toxicity

Progesterone implants Schistosomiasis (exotic) Lumpy skin disease (exotic)

B, Bovine; C, caprine; O, ovine.

breeders, although breed variations occur in small ruminants.³⁷ Certain breeds of sheep are less affected by the season and can have longer breeding seasons or practically cycle year- round. Some sheep and goats near the equator have cycles throughout the year presumably because of the consistency of the photoperiod. Prepubertal animals naturally do not show signs of estrus, as do some dams during lactation. The most common physiologic cause for the lack of estrous behavior is pregnancy; mares, however, are an exception because some of them may display signs of heat despite being pregnant. In intensively managed cattle herds bred by artificial insemination, failure in heat detection is a major contributor to perceived cases of anestrus.³⁸ A plethora of pathologic conditions can lead to anestrus, and a thorough examination of the history and evaluation of the reproductive tract is required for a definite identification of the underlying cause. Congenital condi­tions such as ovarian aplasia, chromosomal abnormalities, or freemartinism in cattle; intersex conditions that occur in polled goats; and gonadal dysgenesis in the mare are accompanied by the lack of estrous behavior. These animals frequently have rudimentary or hypoplastic ovaries.³⁹ Uterus unicornis will lead to persistence of the ipsilateral CL and subsequently interfere with regular estrous behavior.⁴⁰ Nutritional anestrus due to a negative energy balance during times of high milk produc­tion can lead to anestrus in cattle.⁴¹ In mares, anestrus due to nutritional deficiencies is less commonly observed than in cattle. In beef cattle nursing their offspring, a delayed resumption in ovarian activity is frequently observed.⁴² This is related to both the physical act of the calf nursing and olfactory and auditory cues received by the dam from the calf. Anestrus related to lactation is not observed in mares. Mares foaling during the short day-length period that have not been kept under lights for 2 months preceding parturition may show a first ovulation within the first 2 weeks postpartum (i.e., foal heat) but then revert to an anestrous state with cyclic activity not resuming until increasing day length.

A variety of conditions can lead to hypothalamic or pituitary suppression, preventing the release of GnRH from the hypo­thalamus, in turn resulting in a lack of FSH and luteinizing hormone (LH) released by the pituitary. Consequent ovarian inactivity results in the absence of behavioral estrus. Most common underlying conditions are poor nutrition, heavy lacta­tion, periparturient disease, weight loss, or idiopathic condition.

Any condition discussed in the Cyclic Irregularity section that results in prolonged luteal function will result in absence of overt estrous signs. Differential diagnoses further include the presence of cystic ovarian disease in cattle and anovulatory follicles in mares. Uterine abnormalities such as mucometra, hydrometra, and pyometra prevent the release of PGF_2α in ruminants.⁴⁴ Thus the CL persists and perceived anestrus results. Pyometra in the mare may result in anestrus if the endometrial damage is severe enough to prevent production of PGF_2α. The presence of a mummified or macerated fetus in the ruminant uterus prevents release of PGF_2α and results in a persistent CL and perceived anestrus.⁴⁵ Ovarian atrophy leading to persistent anestrus can be seen in high-producing dairy cattle as a sequel to nutritional problems or can be occasionally encountered as an idiopathic condition in mares.⁴¹ If no overt reason for ovarian atrophy can be identified, dysfunction at the hypothalamic-pituitary-ovarian axis has to be considered as a potential underlying cause.

Knowledge of the reproductive physiology of the species involved is essential to understanding the cause of anestrus. Determining the cause of anestrus relies on an in-depth general and reproductive history, a thorough physical examination, and evaluation of estrus detection programs involved. Pregnancy must be ruled out when dealing with any case of anestrus.