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.