Density-dependence in Macroparasite Life Cycles
In macroparasite life cycles, density-dependent processes can influence parasite fecundity, survival, and establishment. Density-dependent processes can act across multiple points of the macroparasite life cycle. For filarial worms, density-dependent processes can act at the host/vector interface or within the host/vector life-cycle stages. At the host/vector interface, density-dependence may influence the input of L3 larvae into the host’s skin and the ingestion of microfilariae by the vector. Within the life-cycle stages taking place in the vector, density-dependence may influence the development of L3 larvae in vectors and vector life expectancy. Within the life-cycle stages taking place in the host, density-dependence may influence the development of microfilariae and host life expectancy.
In reality, combinations of negative (restriction) and positive (facilitation) density-dependent processes occur in the life cycles of parasites. However, the extent to which one process predominates over the other vary widely according to the parasite, vector, and host involved. This is illustrated by the W. bancrofti life cycle. In Culex mosquitoes, which lack a well-developed cibarial armature, restriction processes predominate. Thus, the number of L3 larvae per mosquito declines as the number of ingested microfilariae increases. Conversely, in Aedes and Anopheles mosquitoes, which have well-developed cibarial armatures, facilitation processes predominate. Consequently, the number of L3 larvae per mosquito increases as the number of ingested microfilariae increases.
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