Virginia Institute of Marine Science, William & Mary, Gloucester Point, Virginia, United States of America.
U.S. Geological Survey, Western Fisheries Research Center, Seattle, Washington, United States of America.
PLoS Pathog. 2021 May 10;17(5):e1009528. doi: 10.1371/journal.ppat.1009528. eCollection 2021 May.
Tradeoff theory, which postulates that virulence provides both transmission costs and benefits for pathogens, has become widely adopted by the scientific community. Although theoretical literature exploring virulence-tradeoffs is vast, empirical studies validating various assumptions still remain sparse. In particular, truncation of transmission duration as a cost of virulence has been difficult to quantify with robust controlled in vivo studies. We sought to fill this knowledge gap by investigating how transmission rate and duration were associated with virulence for infectious hematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss). Using host mortality to quantify virulence and viral shedding to quantify transmission, we found that IHNV did not conform to classical tradeoff theory. More virulent genotypes of the virus were found to have longer transmission durations due to lower recovery rates of infected hosts, but the relationship was not saturating as assumed by tradeoff theory. Furthermore, the impact of host mortality on limiting transmission duration was minimal and greatly outweighed by recovery. Transmission rate differences between high and low virulence genotypes were also small and inconsistent. Ultimately, more virulent genotypes were found to have the overall fitness advantage, and there was no apparent constraint on the evolution of increased virulence for IHNV. However, using a mathematical model parameterized with experimental data, it was found that host culling resurrected the virulence tradeoff and provided low virulence genotypes with the advantage. Human-induced or natural culling, as well as host population fragmentation, may be some of the mechanisms by which virulence diversity is maintained in nature. This work highlights the importance of considering non-classical virulence tradeoffs.
权衡理论假设，病原体的毒力既提供了传播成本，也提供了传播收益，该理论已被科学界广泛接受。尽管探索毒力权衡的理论文献非常丰富，但验证各种假设的实证研究仍然很少。特别是，作为毒力代价的传播持续时间的截断很难通过稳健的体内对照研究进行量化。我们试图通过研究传染性造血坏死病毒（IHNV）在虹鳟鱼（Oncorhynchus mykiss）中的传播率和持续时间与毒力的关系来填补这一知识空白。我们用宿主死亡率来量化毒力，用病毒脱落来量化传播，发现 IHNV 不符合经典的权衡理论。我们发现，由于感染宿主的恢复率较低，病毒的更毒力基因型具有更长的传播持续时间，但这种关系并不像权衡理论所假设的那样呈饱和状态。此外，宿主死亡率对限制传播持续时间的影响很小，远远超过了恢复的影响。高毒力和低毒力基因型之间的传播率差异也很小且不一致。最终，我们发现更毒力的基因型具有整体适应优势，而且 IHNV 的毒力增加并没有明显的进化限制。然而，通过使用用实验数据参数化的数学模型，我们发现宿主淘汰复活了毒力权衡，并为低毒力基因型提供了优势。人为或自然淘汰以及宿主种群的碎片化可能是维持自然界中病毒毒力多样性的一些机制。这项工作强调了考虑非经典毒力权衡的重要性。
