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Population studies on the infective stage of some nematode parasites of sheep I. The frequency distribution of strongyloid infective larvae in random samples of pasture

Published online by Cambridge University Press:  06 April 2009

A. D. Donald
A. D. Donald
Affiliation:
Divison of Animal Health, C.S.I.R.O., McMaster Laboratory, Glebe, N.S.W., Australia
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Extract

Observations have been made on the frequency distribution of strongyloid infective larvae in random samples of pasture herbage collected on four occasions from a quarter-acre field grazed by five sheep carrying mixed strongyloid infections. The distribution of numbers of infective larvae of the genera Haemonchus, Trichostrongylus and Cooperia in each of the samples has been examined. The variance of each series was found to be substantially larger than its mean and, hence, the distributions were all highly overdispersed statistically. Of a number of frequency distribution models, the ‘truncated lognormal’ was found to provide the best empirical fit to the sample distributions.

The implications of these findings for determining the significance of the difference between two random sample estimates of the abundance of infective larvae in pasture, have been discussed. It has been shown that, because of the limit imposed by technical considerations on the maximum size of herbage samples, and because the distribution of infective larvae in pasture departs markedly from a random one, only quite large differences between the means of two random sample estimates are likely to be significant. However, because the numbers of infective larvae of the different strongyloid genera are highly correlated within sample units, considerably smaller differences between the means for different genera, within the same sample, can be detected as significant.

From a consideration of the complexity of the biological components generating populations of infective larvae on pasture, it has been concluded that simple frequency distribution models can contribute little towards an improved understanding of the population dynamics of strongyloid infective larvae in pastures grazed by sheep.

Type
Research Article
Information
Parasitology , Volume 57 , Issue 2 , May 1967 , pp. 263 - 279
Copyright
Copyright © Cambridge University Press 1967

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References

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