Density-dependence and persistence of Morogoro arenavirus transmission in a fluctuating population of its reservoir host

Background A key aim in wildlife disease ecology is to understand how host and parasite characteristics influence parasite transmission and persistence. Variation in host population density can have strong impacts on transmission and outbreaks, and theory predicts particular transmission-density patterns depending on how parasites are transmitted between individuals. Here, we present the results of a study on the dynamics of Morogoro arenavirus in a population of multimammate mice (Mastomys natalensis). This widespread African rodent, which is also the reservoir host of Lassa arenavirus in West Africa, is known for its strong seasonal density fluctuations driven by food availability. Goal We investigated to what degree virus transmission changes with host population density and how the virus might be able to persist during periods of low host density. Methods A seven-year capture-mark-recapture study was conducted in Tanzania where rodents were trapped monthly and screened for the presence of antibodies against Morogoro virus. Observed seasonal seroprevalence patterns were compared with those generated by mathematical transmission models to test different hypotheses regarding the degree of density-dependence and the role of chronically infected individuals. Results We observed that Morogoro virus seroprevalence correlates positively with host density with a lag of one to four months. Model results suggest that the observed seasonal seroprevalence dynamics can be best explained by a combination of vertical and horizontal transmission, and that a small number of animals needs to be infected chronically to ensure viral persistence. Broad context Transmission dynamics and viral persistence were best explained by the existence of both acutely and chronically infected individuals, and by seasonally changing transmission rates. Due to the presence of chronically infected rodents, rodent control is unlikely to be a feasible approach for eliminating arenaviruses such as Lassa virus from Mastomys populations.

**背景**：野生动物疾病生态学的核心目标之一，是阐明宿主与寄生虫的特性如何影响寄生虫的传播与持续存在。宿主种群密度的波动可对疾病传播与暴发产生显著影响，而理论预测，根据寄生虫在个体间的传播方式不同，会呈现出特定的传播-密度关联模式。本研究针对多乳鼠（multimammate mouse，学名*Mastomys natalensis*）种群中的莫罗罗沙粒病毒（Morogoro arenavirus）动态开展了相关分析。这种广泛分布于非洲的啮齿动物同时也是西非地区拉沙沙粒病毒（Lassa arenavirus）的储存宿主，其种群密度会因食物可获得性呈现出强烈的季节波动特征。

**研究目标**：本研究旨在探究病毒传播随宿主种群密度变化的程度，以及病毒在宿主密度较低时期如何维持自身持续存在。

**研究方法**：研究团队在坦桑尼亚开展了为期7年的标记重捕法（capture-mark-recapture）研究，每月对啮齿动物进行诱捕，并检测其体内是否存在针对莫罗罗沙粒病毒的抗体。将观测到的季节型血清阳性率（seroprevalence）模式与数学传播模型生成的模式进行对比，以检验关于密度依赖（density-dependence）程度以及慢性感染个体（chronically infected individuals）作用的多种假说。

**研究结果**：我们观测到，莫罗罗沙粒病毒的血清阳性率与宿主密度呈正相关，且存在1至4个月的滞后效应。模型结果显示，观测到的季节型血清阳性率动态变化，可通过垂直传播（vertical transmission）与水平传播（horizontal transmission）的组合得到最佳解释，且仅需少量个体发生慢性感染即可维持病毒的持续存在。

**研究拓展**：传播动态与病毒持续存在的最佳解释为，种群中同时存在急性感染个体（acutely infected individuals）与慢性感染个体，且传播率随季节发生变化。由于慢性感染啮齿动物的存在，通过啮齿动物防控来清除莫斯托鼠属（Mastomys）种群中的拉沙沙粒病毒等沙粒病毒，并非可行的手段。