Not-In-Pig (NIP)

Occurrence: Worldwide.
Age affected: Gilts, sows.
Causes: Undetected return to heat; viral infections; mismanagement.
Effects: Served females neither farrow nor return to heat.


Sows and gilts may be empty when expected to be in pig or reach and pass expected farrowing dates without result. The principal causes are poor management and infertility. Management results in empty sows and gilts when pigs are identified poorly, housed in groups running with the boar, where services are not supervised and where there is poor observation of animals post service for returns to oestrus.

If pregnancy testing is not carried out, then sows may reach term without being pregnant. Infertility may be solely responsible when management is good. Failure of fertilisation may occur and return to oestrus not be detected. When embryos are produced and die, sows return to oestrus at uneven intervals. Where disease or cold have reduced the ability of the animal to return to oestrus or where housing is such that oestrus cannot be observed, then the animals will remain undetected until a routine pregnancy check or the expected farrowing date, whichever is the soonest.

In some cases, animals will remain anoestrus after service. In these cases there may be undetected abortion, cystic ovaries or pyometra in which uterine infection maintains the corpus luteum (remnant of the follicle which later supports the pregnancy).

Mode of transmission

Where infectious disease is responsible, the mode of transmission is that of the disease concerned. In some cases, it may be venereal. In cases where management is responsible, the condition may continue until improvements are made. 

Clinical signs

‘Not-in-pig’ sows do not return to heat after service but also fail to develop the abdominal enlargement and teat development typical of pregnancy. The sows are found to be non-pregnant when examined by ultrasound or blood tests or when the uterus is examined at the abattoir, after culling. A history of vulval discharge may pinpoint the stage at which pregnancy ended.

Failure to farrow leads to a diagnosis of not-in-pig as can earlier pregnancy tests. Examination of the uterus by ultrasound or slaughter will confirm that it is empty. The reasons for the failure of pregnancy can be determined by examining the records. If recording is not carried out, services are not supervised, pregnancy checks not carried out and returns to oestrus not checked visually or by use of a boar, then management causes cannot be excluded.

Where animals are in poor condition, too cold or kept in light too poor for oestrus to be seen, they should be re-examined after these factors have been corrected. Where records confirm that service was carried out and pregnancy has been confirmed reliably, then abortion or resorption of the products of conception has occurred. Ultrasound examination may detect cystic ovaries or corpora lutea and may identify pyometra. Analysis of blood sample for hormones may not contribute to the investigation of the cause, but tests to identify antibody to common causes of abortion may be useful. 

Postmortem lesions

Post-mortem examination of the barren sows will confirm that the uterus is empty and will confirm the presence of reproductive tract abnormalities such as pyometra and cystic ovaries. This is usually carried out following slaughter.

Treatment and prevention

Before animals are treated as not-in-pig, a pregnancy test should be carried out, as the commonest cause of failure to return to service is pregnancy. Cystic ovaries and subdued oestrus can be treated using chorionic gonadotrophin. Pyometra may respond to antimicrobial treatment by injection and allow a return to oestrus.

Prevention is largely a matter of management, although vaccination against conditions such as parvoviurs, erysipelas, leptospirosis, PRRS, influenza and Aujeszky’s disease (where relevant) may reduce the loss of the products of conception. Sows must be identified individually, services must be attended and full records kept of the boar or semen used and the results of any pregnancy tests. Attention to service must include ensuring that mating is successful and occurs at the correct time, that semen quality is adequate and that no post-mating discharges occur.

Examination for returns to service should not be compromised by cold, dim light or understaffing and any deficiencies should be corrected. The ability of returning sows to express oestrus should be ensured by maintaining them in groups where oestrus behaviour will occur and by exposing them to a boar at regular intervals. Regular pregnancy testing should be carried out to ensure that not-in-pig does not occur unnoticed.