Population studies on the infective stage of some nematode parasites of sheep

Abstract

Studies have been undertaken to examine the alternative hypotheses of a Poisson or a negative binomial distribution of the numbers of strongyloid nematode eggs in deposits of faeces produced by flocks of grazing sheep, which were proposed in certain mathematical models for the distribution of strongyloid infective larvae on pastures.Observations were made on the distribution of the weights of faecal deposits produced by flocks of ewes and lambs. In all cases the distribution was found to be significantly positively skewed. A possible fundamental basis for this skewness has been suggested.The distribution of strongyloid egg concentration in random samples of faecal deposits collected from pasture, and the distribution of egg concentration for the single species N. battus and N. filicollis in rectal samples of faeces collected from a lamb flock, have both shown highly significant departures from a Poisson distribution. This, coupled with the fact that the weights of individual deposits vary widely, provide strong empirical grounds for rejecting the Poisson distribution as a model for the distribution of egg numbers per deposit.An examination of the distribution of egg counts for N. battus and N. filicollis in samples of faeces collected from a lamb flock at weekly intervals through the spring and summer has suggested that the distribution tends to become more overdispersed with the passage of time and as the mean egg count is falling.Fundamental grounds for preferring a more general model, such as the negative binomial, to describe the distribution of strongyloid egg output in flocks of sheep have been discussed.These studies were carried out under the supervision of Dr H. D. Crofton, to whom I am deeply grateful for his stimulating encouragement and constructive criticism. My thanks go also to Professor J. E. Harris, C.B.E., F.R.S., for the provision of facilities in the Department of Zoology, and to Professor T. K. Ewer and Mr M. R. McCrea for the provision and handling of pastures and sheep at the School of Veterinary Science, Langford. This work was undertaken during tenure of a C.S.I.R.O. Overseas Research Studentship.