Leon A E, Fleming-Davies A E, Adelman J S, Hawley D M
Department of Biological Sciences, Virginia Tech, Blacksburg, Virginia, USA.
Department of Biology, University of San Diego, San Diego, California, USA.
mSphere. 2025 Apr 29;10(4):e0088624. doi: 10.1128/msphere.00886-24. Epub 2025 Mar 10.
Pathogen reinfections occur widely, but the extent to which reinfected hosts contribute to ongoing transmission is often unknown despite its implications for host-pathogen dynamics. House finches () acquire partial protection from initial exposure to the bacterial pathogen (MG), with hosts readily reinfected with homologous or heterologous strains on short timescales. However, the extent to which reinfected hosts contribute to MG transmission has not been tested. We used three pathogen priming treatments-none, intermediate (repeated low-dose priming), or high (single high-dose priming)-to test how prior pathogen priming alters the likelihood of transmission to a cagemate during index bird reinfection with a homologous or heterologous MG strain. Relative to unprimed control hosts, the highest priming level strongly reduced maximum pathogen loads and transmission success of index birds during reinfections. Reinfections with the heterologous strain, previously shown to be more virulent and transmissible than the homologous strain used, resulted in higher pathogen loads within high-primed index birds and showed higher overall transmission success regardless of host priming treatment. This suggests that inherent differences in strain transmissibility are maintained in primed hosts, leading to the potential for ongoing transmission during reinfections. Finally, among individuals, transmission was most likely from hosts harboring higher within-host pathogen loads. However, associations between disease severity and transmission probability were dependent on a given bird's priming treatment. Overall, our results indicate that reinfections can result in ongoing transmission, particularly where reinfections result from a highly transmissible strain, with potential implications for virulence evolution.IMPORTANCEAs COVID-19 dramatically illustrated, humans and other animals can become infected with the same pathogen multiple times. Because individuals already have defenses against pathogens that their immune systems encountered before, reinfections are likely less contagious to others, but this is rarely directly tested. We used a songbird species and two strains of its common bacterial pathogen to study how contagious hosts are when their immune systems have some degree of prior experience with a pathogen. We found that reinfected hosts are not as contagious as initially infected ones. However, the more transmissible of the two strains, which also causes more harm to its hosts, was able to multiply more readily than the other strain within reinfected hosts and was more contagious in both reinfected and first-infected hosts. This suggests that reinfections might favor more harmful pathogen strains that are better able to overcome immune defenses.
病原体再次感染广泛存在，但尽管再次感染的宿主对持续传播的影响关系到宿主-病原体动态变化，但其影响程度往往未知。家朱雀（Haemorhous mexicanus）初次接触细菌性病原体鸟疫衣原体（Mycoplasma gallisepticum，MG）后会获得部分保护，宿主在短时间内很容易被同源或异源菌株再次感染。然而，再次感染的宿主对MG传播的影响程度尚未得到验证。我们采用了三种病原体预处理方式——无预处理、中等程度（重复低剂量预处理）或高强度（单次高剂量预处理）——来测试先前的病原体预处理如何改变在指示鸟被同源或异源MG菌株再次感染期间传染给同笼伙伴的可能性。相对于未预处理的对照宿主，最高预处理水平在再次感染期间显著降低了指示鸟的最大病原体载量和传播成功率。用先前证明比所用同源菌株更具毒性和传播性的异源菌株进行再次感染，导致高预处理水平的指示鸟体内病原体载量更高，并且无论宿主预处理方式如何，总体传播成功率更高。这表明在预处理宿主中菌株传播性的内在差异得以维持，导致再次感染期间有持续传播的可能性。最后，在个体中，传播最有可能发生在宿主体内病原体载量较高的宿主身上。然而，疾病严重程度与传播概率之间的关联取决于特定鸟类的预处理方式。总体而言，我们的结果表明再次感染可导致持续传播，特别是在再次感染由高传播性菌株引起的情况下，这对毒力进化可能具有影响。
重要性
正如新冠疫情所显著表明的，人类和其他动物可能多次感染同一种病原体。由于个体已经对其免疫系统之前遇到过的病原体有防御能力，再次感染对他人的传染性可能较低，但这很少得到直接验证。我们利用一种鸣禽及其常见细菌性病原体的两种菌株来研究当宿主免疫系统对病原体有一定程度的先前接触经验时，宿主的传染性如何。我们发现再次感染的宿主不如初次感染的宿主传染性强。然而，两种菌株中传播性更强的那种，对宿主造成的危害也更大，它在再次感染的宿主中比另一种菌株更容易繁殖，并且在再次感染和初次感染的宿主中都更具传染性。这表明再次感染可能有利于更具危害性、更能克服免疫防御的病原体菌株。
