Pelosse Perrine, Kribs-Zaleta Christopher M
Mathematics Department, University of Texas at Arlington, Box 19408, Arlington, TX 76019-0408, USA.
Mathematics Department, University of Texas at Arlington, Box 19408, Arlington, TX 76019-0408, USA.
J Theor Biol. 2012 Nov 7;312:133-42. doi: 10.1016/j.jtbi.2012.07.028. Epub 2012 Aug 7.
Pathogens may use different routes of transmission to maximize their spread among host populations. Theoretical and empirical work conducted on directly transmitted diseases suggest that horizontal (i.e., through host contacts) and vertical (i.e., from mother to offspring) transmission modes trade off, on the ground that highly virulent pathogens, which produce larger parasite loads, are more efficiently transmitted horizontally, and that less virulent pathogens, which impair host fitness less significantly, are better transmitted vertically. Other factors than virulence such as host density could also select for different transmission modes, but they have barely been studied. In vector-borne diseases, pathogen transmission rate is strongly affected by host-vector relative densities and by processes of saturation in contacts between hosts and vectors. The parasite Trypanosoma cruzi which is transmitted by triatomine bugs to several vertebrate hosts is responsible for Chagas' disease in Latin America. It is also widespread in sylvatic cycles in the southeastern U.S. in which it typically induces no mortality costs to its customary hosts. Besides classical transmission via vector bites, alternative ways to generate infections in hosts such as vertical and oral transmission (via the consumption of vectors by hosts) have been reported in these cycles. The two major T. cruzi strains occurring in the U.S. seem to exhibit differential efficiencies at vertical and classical horizontal transmissions. We investigated whether the vector-host ratio affects the outcome of the competition between the two parasite strains using an epidemiological two-strain model considering all possible transmission routes for sylvatic T. cruzi. We were able to show that the vector-host ratio influences the evolution of transmission modes providing that oral transmission is included in the model as a possible transmission mode, that oral and classical transmissions saturate at different vector-host ratios and that the vector-host ratio is between the two saturation thresholds. Even if data on parasite strategies and demography of hosts and vectors in the field are crucially lacking to test to what extent the conditions needed for the vector-host ratio to influence evolution of transmission modes are plausible, our results open new perspectives for understanding the specialization of the two major T. cruzi strains occurring in the U.S. Our work also provides an original theoretical framework to investigate the evolution of alternative transmission modes in vector-borne diseases.
病原体可能会采用不同的传播途径，以在宿主群体中实现最大程度的传播。针对直接传播疾病开展的理论和实证研究表明，水平传播（即通过宿主间接触）和垂直传播（即从母体到后代）模式存在权衡取舍，原因在于，产生较大寄生虫负荷的高毒力病原体更有效地通过水平传播，而对宿主适应性损害较小的低毒力病原体则更适合垂直传播。除毒力外，宿主密度等其他因素也可能选择不同的传播模式，但相关研究几乎没有。在媒介传播疾病中，病原体传播率受宿主与媒介相对密度以及宿主与媒介接触饱和过程的强烈影响。由锥蝽传播给多种脊椎动物宿主的寄生虫克氏锥虫，是拉丁美洲恰加斯病的病原体。它在美国东南部的野生动物传播循环中也很普遍，在这个循环中，它通常不会给其常见宿主带来致死代价。除了通过媒介叮咬进行的经典传播外，在这些循环中还报告了在宿主中引发感染的其他方式，如垂直传播和经口传播（即宿主摄食媒介）。在美国出现的两种主要克氏锥虫菌株，似乎在垂直传播和经典水平传播方面表现出不同的效率。我们使用一个流行病学双菌株模型，考虑野生动物克氏锥虫的所有可能传播途径，研究了媒介与宿主的比例是否会影响这两种寄生虫菌株之间的竞争结果。我们能够证明，只要在模型中纳入经口传播作为一种可能的传播模式，且经口传播和经典传播在不同的媒介与宿主比例下达到饱和，并且媒介与宿主比例处于两个饱和阈值之间，那么媒介与宿主比例就会影响传播模式的演变。即使在野外缺乏寄生虫策略以及宿主和媒介种群统计学数据，无法检验媒介与宿主比例影响传播模式演变所需条件的合理程度，但我们的研究结果为理解美国出现的两种主要克氏锥虫菌株的特化情况开辟了新视角。我们的工作还提供了一个原创的理论框架，用于研究媒介传播疾病中替代传播模式的演变。
