Age affected: Gilts, sows
Causes: Physical or hormonal abnormality; malnutrition; mismanagement: pregnancy
Effects: Failure to demonstrate heat in gilts and failure to return to heat or no heat by 10 days after weaning.
Oestrus behaviour accompanies maturation of ovarian follicles, the release of eggs and signifies readiness for mating and acceptance of the boar. In sexually-mature pigs it occurs every 21 days unless interrupted by pregnancy, farrowing and suckling, and then occurs within 4-10 days after weaning. Pregnancy is therefore a cause of anoestrus. Anoestrus in gilts may be primary (the animal may be genetically or physically incapable of producing eggs) or secondary, caused by physical or behavioural factors during rearing which suppress normal oestrus behaviour. Anoestrus in sows may result from complete failure to produce eggs or to release them or from failure to produce sufficient hormone to trigger detectable behavioural effects. The most common cause is poor nutrition and loss of condition following pregnancy and suckling. The corpora lutea (the bodies remaining in the ovary after a follicle has shed an egg) which produce progesterone may persist and suppress the maturation of eggs, particularly when uterine infection (pyometra) is present. Early weaning may give rise to cysts which also prevent ovulation. Extremes of cold and lack of boar stimulation may suppress oestrus behaviour and poor lighting may lead to poor oestrus detection. Declining day length may lead to a seasonal anoestrus.
Anoestrus can result after infections of the reproductive tract and these are most common in sows. Most infections of the reproductive tract are the result of individual consequences of damage (such as pyometra following damage at farrowing) or anoestrus may follow loss of condition during transmissible disease such as influenza. Anoestrus is usually non-infectious, but can occur in groups as single incidents or continuously where the same management factor persists.
Oestrus in the sexually-mature pig can be detected by the behaviour of the animal. It allows other females to mount it and is attracted to boars in the vicinity. Foot marks or scratches may be seen on the back as a result of mounting and the vulva is usually swollen and reddish or pink. When animals in oestrus are introduced to the boar it allows mounting and penetration. Gilts or sows in oestrus respond to back pressure by remaining still, but do not do so if anoestrus. In anoestrus gilts and sows, none of the behavioural features of oestrus occurs. As animals not showing these signs may be cycling (producing eggs) normally, they should be identified and monitored for signs of oestrus for 21 days or for at least 10 days post weaning and then after a further 21 days. Anoestrus animals may be pregnant even though mating was not recorded and pregnancy testing will confirm this. When there is a continuing problem, anoestrus can be investigated by examining the ovaries and uterus by ultrasound for evidence of inactive ovaries, cysts, retained corpora lutea or uterine pus. Where gilts and sows are thin, very fat, are kept in conditions of poor lighting, cold, in the absence of boars, or singly, cycling may occur but not be detected and true anoestrus may not be present. Levels of oestrogen (produced by the developing follicle) exceed those of progesterone (the hormone which maintains pregnancy) during oestrus, so blood tests can confirm whether or not cycling is occurring. In cases where uterine infections are responsible, vulval discharge may be present. Where there is a herd problem, the reproductive tracts of culled sows can be examined by the veterinarian to confirm that anoestrus is occurring and to determine its nature.
Postmortem examination is usually carried out where there is a herd problem, most conveniently at slaughter. Bodily condition can be confirmed, and the reproductive tracts of culled sows can be examined by the veterinarian to confirm that anoestrus is occurring and to determine its nature. The presence of smooth, non-cycling ovaries, absence of follicles, the presence of cystic corpora lutea or regressing follicles all suggest the presence of anoestrus and can aid in treatment and control. Any abnormalities of the reproductive tract or local infections can be identified.
Anoestrus sows and gilts can be treated by injection with gonadotrophic hormones which can initiate or enhance oestrus but will not work if the sow has ovulated within the last 14 days. Pre-pubertal gilts should be treated at 170 days of age and 100 kg bodyweight, but not mated until the second or third oestrus. Weaned sows should be injected 24 hours after weaning to reduce the effects of seasonal anoestrus. Anoestrus sows should be injected, tested daily of oestrus for 7 days and then reinjected at 12 days and tested. Anoestrus can also be treated by correcting any management factors involved. Lighting and temperature should be optimum for the expression of oestrus behaviour and for mating. Animals should be kept in small groups of 4-6 so as to express oestrus behaviour and should be able to smell, see and touch a boar. Oestrus can sometimes be stimulated by changing the identity of the animals in a group or moving them from pen to pen. Where boars are not available or only young animals are present, boar taint aerosols may be used. Anoestrus in gilts may be associated with prepubertal management, and a period of rearing in the absence of boars may be beneficial before the first mating. The body condition of anoestrus animals should reach and be maintained at condition score 2.5-3.0 to improve oestrus expression.
Anoestrus alone should not prevent the submission of affected animals for slaughter.