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Effects of infective stage density, host density and surface area available for transmission on the rate of transmission of the nematode Heligmosomoides polygyrus were investigated in a controlled environment where naive CD1 outbred mice were naturally infected. Within a range of densities of infective larvae (500-6000/cage), randomly spread on damp peat moss, a linear increase was observed in the mean number of fourth stage larvae embedded in the intestinal mucosa per host. As host density increased (1-6 mice/cage), but exposure dose held constant (8000L$ sb3$), the mean number of L$ sb4$ recovered per mouse decreased at a faster rate than expected assuming a constant per capita rate of transmission. In an attempt to interpret the experimental data, mathematical models were generated. Although the inclusion of a death rate of infective larvae dependent on mouse density did improve the fit of the mathematical model based on a constant per capita rate of transmission, the best fit was provided by a model assuming a per capita rate of transmission dependent on mouse density. An independent experiment exploring the impact of surface area available for transmission on the relationship between host density and the rate of infection confirmed the adequacy of the mouse density-dependent per capita rate of transmission hypothesis.

The importance of host heterogeneity in generating aggregation was investigated using Heligmosomoides polygyrus (Nematoda) in laboratory mice. The protocol compared parameters of infection between inbred and outbred mice, primary and challenge infections, and gavage and natural exposure, to investigate the relative effects of innate resistance, acquired resistance and behaviour, respectively. The results show that heterogeneity in acquired resistance is the most consistent factor contributing to the variability and aggregation of H. polygyrus. When groups of mice did not acquire resistance, either in certain inbred strains or when mice were immunosuppressed, variability after the challenge was comparable with that after a primary infection. Heterogeneity in host behaviour, particularly in behaviours enhancing skin contact with larvae, frequently increased aggregation above that detected in both primary and challenge infections. Surprisingly, inconsistent results were obtained when inbred and outbred groups of mice were compared. The results suggest that the relative contribution of these three factors is likely to vary spatially and temporally.

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Subclinical infections with the nematode Heligmosomoides polygyrus affect the behaviour and spatial learning of CD-1 mice in a Morris water maze. In early trials testing their ability to find and land on a hidden, submerged platform, uninfected mice displayed normal stress-related behaviours, such as avoiding the central areas of the circular maze and overswimming the platform. Infected mice manifested few such behaviours. Although both acquired the task, there were substantial differences between infected and uninfected mice in their patterns of task acquisition and execution. Although infected mice eventually reached comparable levels of competency, higher cognitive behaviours which suggest familiarity with the location of the platform, such as straight-line swimming, appeared earlier among uninfected mice. Other orientation behaviours, such as paddling and floating, occurred at higher levels among uninfected mice. These behavioural differences are discussed in terms of their adaptive significance to the survival of the host and the transmission of the parasite.

A stock "parent" (S) strain of the mouse parasite Heligmosomoides polygyrus was exposed to increasing levels of ivermectin at the L4 stage for 15 generations. A Passage line was also developed from the parent strain parallel with the ivermectin selected line to control for the effects of rapid passage of the parasite from host to host during drug selection. A dose titration trial indicated 1.5 fold resistance had developed in the ivermectin selected strain at the 8th generation (IVM-8) both at the L4 and adult stage. A higher dose of drug was required to kill the L4 stage compared to the adults at generation 8. Additional selection pressure for 7 generations (IVM-15) did not change the resistance status of adult worms. The Passage strains (P-8 and P-15) remained susceptible to drug. The life history traits of the parent strain (S), the ivermectin selected (IVM-8 and IVM-15) and the Passage (P-8 and P-15) strains were then compared. Eight generations of selection with ivermectin (IVM-8) resulted in an increase in establishment 8 days post-infection (pi) but decreased egg output and worm burden over 4 months compared with strain S. However these effects were not seen after 15 generations of drug selection. The ivermectin selected strain (IVM-15) had similar establishment, egg production and worm burden as the parent strain (S). Establishment in strain P-8 was intermediate and not different from S or IVM-8 however 15 generations of passage (strain P-15) resulted in higher establishment and more rapid development to adult. This was also reflected in the net egg output and worm burden during the first month of infection. There were no differences in per capita fecundity among the five strains. Environmental pressure exerted by passage of H. polygyrus from host to host rather than ivermectin selection caused shifts in some life history traits of this nematode.

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