When does a supershedder become a superspreader?: The impact of individual-level heterogeneities on population-level transmission and spread

个体对感染的易感性和将感染传染给他人的可能性差异很大。来自各种疾病暴发的证据表明，通常是少数个体（约20%）引起了大多数新病例（约80%）——这些人就是所谓的“超级传播者”。然而，目前还不清楚是什么特征使一个人成为超级传播者。正如我们从2019冠状病毒病大流行中清楚地认识到的那样，个人的感染风险取决于他们与受感染的人相处的距离和时间，因此一些受感染的人可能会接触到比例过高的易感个体，成为“超级接触者”。因此，超级接触者可能确实是超级传播者，并推动许多传染病的传播。然而，也有可能有些人比其他人释放出更多的感染阶段，充当“超级传播者”，这也可能导致更大的传播潜力。更复杂的是，这两个过程，超脱落和超接触，可以相互作用——要么加剧传播（通常，如果超级脱落者也往往是超级接触者），要么减少传播（如果超级脱落者往往很少接触，反之亦然）。这些过程，无论是由有大量接触者还是具有高度传染性的个体驱动的，然后扩大规模，以确定寄生虫在宿主群体中传播的速度，以及感染“热点”的空间分布。因此，了解决定超级脱模者和超级接触者的个体特征，以及它们如何耦合产生超级传播者，对于预测疾病传播和制定有效的控制措施至关重要。到目前为止，关于宿主特征构成超级传播者的研究几乎完全集中在人类的致病性病毒或细菌爆发上。虽然从公共卫生的角度来看显然是必要的，但这种对某些人类病原体的狭隘关注限制了我们明确测试超级传播机制的能力。此外，超接触和超脱落的概念如何适用于与引起人类流行病（或大流行）的病原体具有非常不同的传播模式和生物学的大量寄生虫，目前还远不清楚。特别是，寄生蠕虫（蠕虫）是所有自然生态系统中无处不在的组成部分，在构建生态群落方面发挥着至关重要的作用，并对野生动物、人类和家畜产生重大的健康和经济后果。这些蠕虫通常通过在环境中长期存在的感染阶段感染新宿主，从而模糊了“接触”的定义，并且几乎不可能确定每个最初被感染的个体产生的新感染数量。因此，有关超脱落、超接触和超传播的既定概念并不直接适用于这一重要的寄生虫群体。在这里，我们将通过一项全面的研究来推动这一领域的发展，该研究在一个高度易处理的自然宿主-寄生虫系统中使用新的种群水平的现场实验，木鼠和它们的寄生虫，我们可以特别减少超级脱落或超级接触个体的感染。我们将把这些实地实验与新的理论和强大的分析方法相结合，以深入了解传染性、运动模式和寄生虫传播模式的个体差异如何相互作用，从而推动寄生虫在自然群落中的传播。

Individuals vary greatly in both their susceptibility to infection and their likelihood of transmitting infections to others. Evidence from various disease outbreaks suggests that typically a minority of individuals (~20%) cause the majority of new cases (~80%) - these are the so-called "superspreaders". However, it is not clear what characteristics make someone a superspreader. As we are only too aware from the COVID-19 pandemic, an individual's infection risk depends on how close and for how long they spend near infected individuals, such that some infected individuals may contact a disproportionately high number of susceptible individuals, acting as 'supercontactors'. Hence, supercontactors may indeed be superspreaders and drive transmission for many infectious diseases. However, it is also possible that some individuals release many more infectious stages than others, acting as 'supershedders', which can also lead to disproportionately greater transmission potential. To complicate matters, these two processes, supershedding and supercontacting, can interact together - either exacerbating transmission (generally, if supershedders also tend to be supercontactors) or diminishing transmission (if supershedders tend to have few contacts, and vice versa). These processes, whether driven by individuals having large numbers of contacts or being highly infectious, then scale up to determine how fast the parasite spreads through the host population, and the spatial distribution of infection 'hotspots'. As such, understanding the individual characteristics that determine supershedders and supercontactors, and how they are coupled to give rise to superspreaders, are crucial to predicting disease spread and for devising effective control measures.So far, research on what host characteristics make a superspreader has focussed almost exclusively on pathogenic viral or bacterial outbreaks in humans. While clearly essential from a public health perspective, this narrow focus on certain human pathogens limits our ability to explicitly test the mechanisms underlying superspreading. Furthermore, it is far from clear how ideas of supercontacting and supershedding apply to the vast range of parasites with very different transmission modes and biologies from the pathogens that cause epidemics (or pandemics) in humans. In particular, parasitic worms (helminths) are a ubiquitous and integral component of all natural ecosystems, playing a vital role in structuring ecological communities and having significant health and economic consequences for wildlife, humans and domestic animals. These helminths typically infect new hosts through long-lived infective stages that reside in the environment, thereby blurring the definition of 'contacts', and making it virtually impossible to determine the number of new infections arising from each initially infected individual. Hence, established concepts relating supershedding, supercontacting, and superspreading do not apply directly to this important groups of parasites.Here we will move this field forward through a comprehensive study that uses novel population-level field experiments in a highly tractable yet natural host-parasite system, wood mice and their parasitic worms, in which we can specifically reduce infections in either supershedding or supercontacting individuals. We will pair these field experiments with new theory and robust analytical methods to develop an in-depth understanding of how between-individual variation in infectiousness, movement patterns, and parasite transmission mode interact to drive parasite spread in natural communities.