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

个体在对感染的易感性和将感染传播给他人的可能性方面差异很大。来自各种疾病爆发的证据表明，通常少数个体（约20%）导致大多数新病例（约80%）-这些是所谓的“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COVID-19大流行中意识到的那样，个人的感染风险取决于他们与受感染者的距离有多近以及接触的时间有多长，因此一些受感染者可能会接触不成比例的大量易感个体，充当“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，也有可能一些个体比其他个体释放更多的传染性阶段，充当“超级散毒者”，这也可能导致不成比例的更大传播潜力。更复杂的是，这两个过程，即超脱落和超接触，可以相互作用--要么加剧传播（一般来说，如果超脱落者也倾向于成为超接触者），要么减少传播（如果超脱落者倾向于有很少的接触，反之亦然）。这些过程，无论是由具有大量接触或高度传染性的个体驱动，然后按比例扩大，以确定寄生虫在宿主人群中传播的速度以及感染“热点”的空间分布。因此，了解决定超级传播者和超级接触者的个体特征，以及它们是如何结合起来产生超级传播者的，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，关于超级传播者的宿主特征的研究几乎完全集中在人类中的病原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不可少的，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，目前还不清楚超级接触和超级脱落的想法如何适用于传播模式和生物学与引起人类流行病（或大流行病）的病原体非常不同的大量寄生虫。特别是，寄生虫（蠕虫）是所有自然生态系统中普遍存在的组成部分，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，人类和家畜的健康和经济产生重大影响。这些蠕虫通常通过驻留在环境中的长寿命感染阶段感染新宿主，从而模糊了“接触者”的定义，并且几乎不可能确定每个最初感染的个体产生的新感染的数量。因此，已建立的概念，有关超脱落，超接触，和超传播不直接适用于这一重要群体的寄生虫。在这里，我们将通过一个全面的研究，使用新的人口水平的现场实验，在一个高度听话的，但自然的宿主-寄生虫系统，木老鼠和他们的寄生蠕虫，我们可以专门减少感染的超脱落或超接触的个人。我们将这些实地实验与新的理论和强大的分析方法相结合，以深入了解感染性，运动模式和寄生虫传播模式的个体间变化如何相互作用，以推动寄生虫在自然社区中的传播。

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.