1. The investigation shows that the spring increase in worm egg output of Scottish hill sheep is correlated with an increase in worm burden.2. The increase is mainly due to members of the Trichostrongylidae, and of these the stomach worms, Ostertagia spp. and Trichostrongylus axei, are the most important.8. This increase in the worm burden in the spring is due to larvae picked up at that time, and there was no evidence that any were lying dormant throughout the winter in the mucous membrane of the abomasum and small intestine.

A survey of the helminths of Scottish hill sheep, made by collecting over seven hundred viscera from sheep which died from disease or accidents on over eighty farms, has shown that worms are the direct or indirect cause of the death of many sheep, and of loss in condition of many more.Infestations of H. contortus increase in incidence and severity in the late spring, but the majority of hill sheep are not infested with it and it is not a serious problem unless lack of herding, bad weather, etc. harm the sheep.Ostertagia spp. are often the most numerous species found in Scottish hill sheep; their numbers increase considerably for several weeks in the spring and to a less extent and for a shorter time in the autumn. Any prophylactic dosing programmes for lambs, hoggs or adult sheep must include Ostertagia spp. in the species against which they are directed.The numbers of T. axei increase in Scottish hill sheep during the winter and for some weeks from March onwards contribute to the pathogenic effect of clinical and subclinical helminthiasis of sheep.

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From the study of the worm egg output of nearly fifteen hundred sheep on many hill farms in Scotland it has been shown that:-(i) Worm egg output in ewes and gimmers is lowest in early winter and highest in spring and early summer; this is followed by a marked decrease towards the winter months.(ii) Generally in ewe hoggs the worm egg output is the same as that in older sheep, but shows greater variability; on occasion the spring peak may be absent.(iii) The level of worm egg output of hoggs is generally higher than that of older sheep, and in this respect gimmers tend to fall between hoggs and ewes.(iv) Weather conditions may have an influence on the height and duration of the spring peak of worm egg production.

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