Ratcheting up wild virulence Partially protective vaccination can sometimes select for increasingly virulent pathogens. Fleming-Davies et al. asked what happens in a natural system. In the United States, the house finch population is suffering an increasingly virulent epidemic caused by Mycoplasma gallisepticum. The pathogen induces incomplete immunity that clears less virulent pathogens and offers partial protection against strains of greater virulence. In the birds, the partial immune response does away with competition from the less virulent pathogens. The partial immunity of the host also hinders replication of the more virulent pathogens enough to allow some birds to survive. This allows increasingly virulent forms of the pathogen to be transmitted. Science, this issue p. 1030 A study of house finches and their bacterial pathogen Mycoplasma gallisepticum elucidates how virulence is enhanced. Immune memory evolved to protect hosts from reinfection, but incomplete responses that allow future reinfection may inadvertently select for more-harmful pathogens. We present empirical and modeling evidence that incomplete immunity promotes the evolution of higher virulence in a natural host-pathogen system. We performed sequential infections of house finches with Mycoplasma gallisepticum strains of various levels of virulence. Virulent bacterial strains generated stronger host protection against reinfection than less virulent strains and thus excluded less virulent strains from infecting previously exposed hosts. In a two-strain model, the resulting fitness advantage selected for an almost twofold increase in pathogen virulence. Thus, the same immune systems that protect hosts from infection can concomitantly drive the evolution of more-harmful pathogens in nature.

中文翻译：

---

不完全的宿主免疫有利于新出现的病原体毒力的进化

增强野生毒力 部分保护性疫苗接种有时可以选择毒力越来越强的病原体。弗莱明-戴维斯等人。问在自然系统中会发生什么。在美国，家雀种群正在遭受由鸡毒支原体引起的日益严重的流行病。病原体诱导不完全免疫，清除毒力较小的病原体，并提供部分保护，防止毒力较大的菌株。在鸟类中，部分免疫反应消除了来自毒性较小的病原体的竞争。宿主的部分免疫也阻碍了足以让一些鸟类生存的毒性更强的病原体的复制。这允许传播越来越毒的病原体。科学，这个问题 p。1030 对家雀及其细菌病原体鸡毒支原体的研究阐明了毒力是如何增强的。免疫记忆进化来保护宿主免受再次感染，但允许未来再次感染的不完整反应可能会无意中选择更有害的病原体。我们提出了经验和建模证据，表明不完全免疫促进了自然宿主-病原体系统中更高毒力的进化。我们用不同毒力水平的鸡毒支原体菌株对家雀进行了连续感染。与毒性较低的菌株相比，毒性较低的菌株对宿主产生更强的再感染保护，从而排除了毒性较低的菌株感染先前暴露的宿主。在双应变模型中，由此产生的适应性优势被选择为病原体毒力增加了近两倍。因此，保护​​宿主免受感染的相同免疫系统可以同时推动自然界中更有害的病原体的进化。