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Explaining variability in parasite aggregation levels among host samples

Published online by Cambridge University Press:  24 January 2013

ROBERT POULIN
ROBERT POULIN*
Affiliation:
Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand
*
*Corresponding author: Tel: +64 3 479 7983. Fax: +64 3 479 7584. E-mail: [email protected]
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Summary

Aggregated distributions among individual hosts are a defining feature of metazoan parasite populations. Heterogeneity among host individuals in exposure to parasites or in susceptibility to infection is thought to be the main factor generating aggregation, with properties of parasites themselves explaining some of the variability in aggregation levels observed among species. Here, using data from 410 samples of helminth parasites on fish hosts, I tested the contribution of (i) within-sample variation in host body size, taken as a proxy for variability in host susceptibility, and (ii) parasite taxon and developmental stage, to the aggregated distribution of parasites. Log-transformed variance in numbers of parasites per host was regressed against log mean number across all samples; the strong relationship (r2 = 0·88) indicated that aggregation levels are tightly constrained by mean infection levels, and that only a small proportion of the observed variability in parasite aggregation levels remains to be accounted for by other factors. Using the residuals of this regression as measures of ‘unexplained’ aggregation, a mixed effects model revealed no significant effect of within-sample variation in host body size or of parasite taxon or stage (i.e. juvenile versus adult) on parasite aggregation level within a sample. However, much of the remaining variability in parasite aggregation levels among samples was accounted for by the number of individual hosts examined per sample, and species-specific and study-specific effects reflecting idiosyncrasies of particular systems. This suggests that with most differences in aggregation among samples already explained, there may be little point in seeking universal causes for the remaining variation.

Keywords

host body size variationfishhelminthsvariance-to-mean ratio
Type
Research Article
Information
Parasitology , Volume 140 , Issue 4 , April 2013 , pp. 541 - 546
Copyright
Copyright © Cambridge University Press 2013

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