Churcher T S, Ferguson N M, Basáñez M G
Department of Infectious Disease Epidemiology, Faculty of Medicine, St Mary's Campus, Imperial College London, Norfolk Place, London W2 1PG, UK.
Parasitology. 2005 Jul;131(Pt 1):121-32. doi: 10.1017/s0031182005007341.
The influence of density-dependent processes on the transmission of parasitic helminths is determined by both the severity of the regulatory constraints and the degree of parasite overdispersion among the host population. We investigate how overdispersed parasite distributions among humans influence transmission levels in both directly- and indirectly-transmitted nematodes (Ascaris lumbricoides and Onchocerca volvulus). While past work has assumed, for simplicity, that density dependence acts on the average worm load, here we model density-dependence as acting on individual parasite burdens before averaging across hosts. A composite parameter, which we call the effective transmission contribution, is devised to measure the number of transmission stages contributed by a given worm burden after incorporating over-dispersion in adult worm mating probabilities and other density-dependent mechanisms. Results indicate that the more overdispersed the parasite population, the greater the effect of density dependence upon its transmission dynamics. Strong regulation and parasite overdispersion make the relationship between mean worm burden and its effective contribution to transmission highly non-linear. Consequently, lowering the intensity of infection in a host population using chemotherapy may produce only a small decline in transmission (relative to its initial endemic level). Our analysis indicates that when parasite burden is low, intermediate levels of parasite clustering maximize transmission. Implications are discussed in relation to existing control programmes and the spread of anthelmintic resistance.
密度依赖过程对寄生蠕虫传播的影响取决于调节限制的严重程度以及宿主群体中寄生虫的过度分散程度。我们研究了人类中寄生虫分布的过度分散如何影响直接传播和间接传播的线虫（蛔虫和盘尾丝虫）的传播水平。过去的研究为了简化，假设密度依赖作用于平均蠕虫负荷，而在这里，我们将密度依赖建模为在对宿主进行平均之前作用于个体寄生虫负担。我们设计了一个复合参数，称为有效传播贡献，以测量在纳入成虫交配概率的过度分散和其他密度依赖机制后，给定蠕虫负担所贡献的传播阶段数量。结果表明，寄生虫群体的过度分散程度越高，密度依赖对其传播动态的影响就越大。强烈的调节和寄生虫的过度分散使得平均蠕虫负担与其对传播的有效贡献之间的关系高度非线性。因此，使用化疗降低宿主群体中的感染强度可能只会使传播（相对于其初始流行水平）略有下降。我们的分析表明，当寄生虫负担较低时，中等程度的寄生虫聚集会使传播最大化。我们将结合现有的控制计划和驱虫抗性的传播来讨论其影响。
