The peri-parturient rise in sheep. Faecal worm egg counts in normal and late lambing ewes.

The trichostrongylid and strongylid egg output in a group of ewes lambing in March, the middle of the usual lambing time in the Netherlands, was compared with counts in groups of older ewes and yearling ewes lambing in May-June and with counts in a group of barren ewes. The groups lambing in March showed only a rise related to parturition date as normally seen in the Netherlands. A few of the late lambing ewes showed a slight rise in spring, but all of them showed a peri-parturient rise related to parturition date. In the late lambing ewes and in the barren ewes egg counts due to reinfection were seen in summer. It is concluded that under the present circumstances a true spring rise, the calendar phenomenon, occurs very irregularly, while a peri-parturient rise related to the parturition date is a common phenomenon. The occurence of high egg counts in summer, excluding consideration on the barren ewes, in the late lambing ewes only, suggests that changing the traditional husbandry system may influence the epidemiological pattern of the trichostrongylid infections.