The Seasonal Availability to Sheep of Infective Nematode Larvae on Pasture

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Abstract
The seasonal availability of sheep nematode infective larvae under normal farm conditions was investigated by exposing lambs reared worm free for two weekly periods in the fields. Data were collected in the form of populations found at post mortem examination.It was found that Nematodirus spp. were acquired in greatest numbers in the autumn after the date by which lambs normally have acquired resistance to superinfection. The conclusion was that self-augmentation of Nematodirus parasitism in lambs resulting from eggs the lambs conveyed to the ground was of minor significance and that eggs reaching the ground in one season are the source of parasitism in lambs in the following spring.It was found that greatest numbers of Haemonchus conlortus were acquired in summer. Heavy contamination of pasture with eggs of this species by ewes in the spring did not result in large populations in their lambs. It was concluded that the extent of parasitism in lambs in the late summer and autumn was contingent on there being favourable conditions at that time of the year for the development of infective larvae.
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