Epidemiology of vaginal prolapse in mixed-age ewes in New Zealand.

AIMS: Identify environmental, animal, and management factors associated with risk of vaginal prolapse in ewes, to enable farmers and advisors to make pragmatic decisions based on empirical observations for control of the condition.
METHODS: Two longitudinal studies conducted over 2 years to identify factors associated with incidence of prolapse in (i) cohorts of 200 individually identified mixed-age (MA) ewes, and (ii) all MA ewes, on voluntarily participating sheep-breeding farms in Hawkes Bay (HB) and Southland regions of New Zealand.
RESULTS: The overall annual incidences of prolapse on 113 farms in 2000 and 88 in 2001 were 1.21 and 0.82 per 100 MA ewes, respectively, and 1.05 for both years combined. A total of 406 prolapses were recorded among 36,695 individually identified cohort ewes. Individual farm incidences for both years varied from 0-5.9 (mean=1.56, median=1.39) on Southland and 0-3.9 (mean=0.75, median=0.54) per 100 ewes on HB farms. The crude relative risk of a prolapse occurring in a MA ewe was 5.31 times higher for ewes carrying twins and 11.3 times higher for ewes carrying triplets, than single lambs. Flocks made up of predominantly pure or crossbred Perendale ewes appeared to be at lower risk than flocks with other breeds. Shearing in the 3 months leading up to mating appeared to be protective, as was shearing in the second half of pregnancy. The risk was higher on farms with moderate to steep terrain than on farms with flat terrain. The identified risk factors in the individually identified cohorts were: access to salt and feeding of swedes in the latter part of pregnancy, moderate to steep lambing paddocks, multiple lambs detected at scanning, and weight gain between start of mating and scanning. The condition recurred in 2001 in six (35%) of 17 study ewes that had prolapsed during 2000. Culling policies for female offspring of affected ewes did not influence incidence at the farm level; nor did feeding hay or grain in late pregnancy. Furthermore, there was no association between incidence and body condition scores measured prior to and after mating, at scanning, or at time of set stocking. CLINICAL SIGNIFICANCE: Vaginal prolapse is an inevitable consequence of sheep reproduction and its incidence is expected to increase as reproductive rates increase. This study provides some firm leads as to the relative importance of risk factors and gives guidance for risk reduction, e.g. by identification and separate management of ewes carrying twins or triplets, using flat paddocks for lambing, and guarding against gain in weight between the start of mating and scanning.