The spring-rise phenomenon: The relationship between the time of lambing and the commencement of the rise in faecal worm-egg counts

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Abstract
Extract The greatest weight of evidence to date supports the view that the spring-rise in faecal worm-egg counts of sheep is due to the simultaneous maturation of latent over-wintering larvae, initiated by the depression of the immune status of the host (Spedding and Brown, 1956 Spedding, C. R. W. and Brown, T. H. 1956. The “spring rise” in the nematode egg-count of sheep. J. Helminth., 29: 171–178. [Crossref] [Google Scholar] ; Field et al., 1960 Field, A. C , Brambell, M. R. and Campbell, J. A. 1960. Spring rise in faecal worm-egg counts of housed sheep and its importance in nutritional experiments. Parasit., 50: 387–399. [Google Scholar] ; Soulsby, 1962 Soulsby, E. J. L. 1962. “Immunity to helminths and its effect on helminth infections”. In Animal Health and Production, Edited by: Grunsell, C. S. and Wright, A. I. 165–188. London: Butterworths. In [Google Scholar] ; Gibbs, 1964 Gibbs, H. C. 1964. Observations on an outbreak of clinical parasitism in ewes during the winter months. Canad. vet. J., 5: 8–11. [PubMed] [Google Scholar] ). The phenomenon may be enhanced by stress factors such as parturition ( Crofton, 1954 Crofton, H. D. 1954. Nematode parasite populationsin sheep on lowland farms I. Worm egg counts in ewes. Parasit., 44: 465–477. [Google Scholar] ) and severe climate and malnutrition (Paver et al., ( 1955 Paver, H. , Parnell, I. W. and Morgan, D. O. 1955. Some factors influencing the seasonal variation in worm egg counts in Scottish hill sheep. J. comp. Path., 65: 220–235. [Google Scholar] ). Crofton ( 1954 Crofton, H. D. 1954. Nematode parasite populationsin sheep on lowland farms I. Worm egg counts in ewes. Parasit., 44: 465–477. [Google Scholar] ) concluded that the rise in egg counts occurring in ewe flocks in spring was related to the time of lambing. He found that the rise in individuals occurred about six to eight weeks after parturition and was normally of short duration. While the mean time of the increased egg count of each individual was about two weeks, the increased egg count in the flock was spread over a period equivalent to the length of the lambing period. In a later paper, Crofton ( 1958 Crofton, H. D. 1958. Nematode parasite populations in sheep on lowland farms. V. Further observations on the post-parturient rise and a discussion of its significance. Parasit., 48: 243–250. [Google Scholar] ) described a similar post-parturient rise in autumn-lambing ewes.