Age affected: Gilts, sows.
Causes: Weaning before 2 weeks; genetic or congenital defect; misuse of hormones; stress
Effects: Irregular heats or anoestrus, persistent heat (rare), infertility.
Cysts are commonly found in the ovaries of sows and single ones do not affect fertility. Multiple cysts occur in up to 10% of ovaries of anoestrus sows and are responsible for anoestrus, but the factors which cause them are not entirely clear. A number of suggestions have been made. One involves stress which gives rise to adrenocorticotrophic hormone (ACTH) production, acts on the adrenal cortex and produces corticosteriod which inhibits or reduces luteinising hormone (LH) levels reaching the developing follicles and results in their failure to release eggs from developing follicles leading to cyst formation. Other models involve progesterone or prostaglandin as the inhibitor of LH. Cystic ovaries may be congenital and sometimes hereditary. They may also arise from the misuse of hormones, for example, accidental feeding of medicated feed to breeding sows, or giving ‘cycle starter’ hormone injections to sows which are already cyclic. Weaning sows earlier than 3 weeks increases the likelihood that ovarian cysts will develop after weaning.
Mode of transmission
Cystic ovaries may be hereditary, and occur in families, but usually arise from husbandry or management practices or are of unknown cause. They are not transmissible.
The presence of multiple cysts in the ovaries of sows results in prolonged anoestrus and thus, in infertility. No other clinical signs result, and the presence of ovarian cysts does not cause specific changes in blood hormone levels and cannot be distinguished from those of the normal inter oestrus period. The presence of cysts can be detected by ultrasound and can therefore be determined in life.
When cystic ovaries are suspected as a cause of infertility, an individual infertile pig only has a 10% chance of having cystic ovaries. Cysts are commonly over-diagnosed, because normal developing corpora lutea are often cystic in the early stages; abnormal cysts are ovarian structures at least 25 mm in diameter of which more than half is occupied by a fluid-containing cavity. Confirmation of diagnosis cannot come from blood hormone levels. Diagnosis can be confirmed in life by rectal palpation in sows which have had a litter or ultrasound which can demonstrate the presence of multiple fluid filled cysts in both ovaries.
Cystic ovaries are most commonly seen at slaughter, when multiple, thin walled cysts 2-3 cm in diameter are seen in both ovaries. They are moderately thick walled and filled with clear fluid. The uterus is inactive and non-pregnant, although regressing corpora lutea may be present in the ovary to indicate that ovulation has taken place in the past, evidence for active ovulation is rare in cystic ovaries.
Treatment and prevention
Affected pigs are usually culled. Possible treatments are a course of synthetic progestagen (e.g. altrenogest) by mouth or gonadotrophic hormones by injection. In some cases, cysts recur after treatment. Attention to stress factors, or delaying weaning may reduce the chance of more sows being affected.
This condition does not render the carcase unsafe for human consumption, provided the withdrawal periods for treatments have been met.