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INTRODUCTION
Macroparasites are almost always aggregated across their host population, with a large number of hosts harbouring few parasites and few heavily infected hosts (Pacala & Dobson, 1988; Shaw & Dobson, 1995). The heterogeneity in parasite burdens is a consequence of biological processes and of their interactions: parasites may become aggregated on or in their host due to aggregated spatial distribution of free-living stages (Keymer & Anderson, 1979), differences in susceptibility of hosts or differences in host selection by parasite (Wilson et al . 2002) and environmental and demographic stochasticity (Anderson & Gordon, 1982). In trichostrongylid nematodes, the aggregated distribution among domestic ruminants (Barger, 1985) is usually thought to mainly reflect individual variability of ruminants in the acquisition and expression of the immune response against nematodes (Hoste, Chartier & Le Frileux, 2002). One consequence of aggregated distributions is that the small proportion of hosts in the tail of the parasite distribution is responsible for most parasite transmission and plays an important role in the persistence of the parasite (Anderson & May, 1985; Woolhouse et al . 1997). Moreover, as in macroparasites, host mortality and morbidity tend to be dose-dependent (Poulin, 1998), parasite infections are more harmful for heavily infected individuals and, in sheep, the extent of nematode-induced production loss is roughly proportional to worm burden (Steel, Symons & Jones, 1980). Because production loss and worm burden are positively correlated, the aggregative property of the parasite distribution has been widely used in genetic selection of sheep resistant to nematode infection (Gruner et al . 2004). The statistical distribution of worm burdens may also have implications for chemotherapy of nematode infections in domestic ruminants. Selective drenching of the most infected hosts has been suggested to slow down the rapid spread of treatment resistance in parasite populations (e.g. anthelmintic resistance in livestock (Hoste et al . 2002; Cornell et al . 2003). A suitable measurement of parasite aggregation and an adequate quantification of heavily infected individuals are thus necessary to set up targets for treatment. Knowing the distribution of parasites is also important in data analysis, mostly because most of the generally used statistical tests assumed that parasite distribution follows a normal distribution (Wilson & Grenfell, 1997).
Aggregation can be quantified by the variance to mean ratio. If...