Density dependence in establishment, growth and worm fecundity in intestinal helminthiasis: the population biology ofTrichuris muris(Nematoda) infection in CBA/Ca mice

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Abstract
SUMMARY: The results are presented of an experimental study of the population biology of chronicTrichuris muris(Nematoda) infection in cortisone-treated CBA/Ca mice. Attention is focused upon both the validity of the common use of faecal egg counts to demonstrate density dependence in helminth fecundity, and the identification of other possible density-dependent mechanisms that may regulate worm numbers in chronic trichuriasis. The results show that faecal egg counts, although demonstrating high daily variation, are not an artefact of host faecal output but a significant density-dependent function of worm burden. This finding contrasts with the observations onHeligmosomoides polygyrusinfection in outbred MF1 mice, but accords with similar studies in a wide variety of host - helminth systems. Worm establishment in the murine host is found to be a density related function of infection dose. This is attributed to the probable existence of a physical gut-carrying capacity in the murine host forT. muris. Worm distribution in the gut is also shown to be density dependent, with worms being displaced from the caecum to the colon at increasing intensities of infection. The sex ratio of the adult parasites, however, is found to be both unitary and independent of worm burden. Evidence for a significant density-dependent decline in femaleT. murisgrowth or size is presented. The results also show a significant positive association between femaleT. murisweight andper capitafecundity. These findings indicate that the stunted growth of individual worms at high parasite densities may be a potential mechanism underlying density dependence in helminth fecundity.