School of Biological Sciences, University of Liverpool, Crown Street, Liverpool, UK.
Epidemics. 2009 Sep;1(3):162-7. doi: 10.1016/j.epidem.2009.05.002. Epub 2009 May 28.
In natural populations, infection and condition may act synergistically to trigger a vicious circle: poor condition predisposes to host infections, which further reduce condition, and so on. If this vicious circle originates from a reduced resistance to infection, it will not only result in greater proneness to becoming infected of those that are in poorer condition, but it may also cause infections of higher intensity. Here, we investigate the temporal relationship between host condition and intensity of infection by a specific pathogen using as a system the dynamics of the protozoan Trypanosoma microti in field vole (Microtus agrestis) populations.
With two years of longitudinal data from three monthly-sampled populations, we evaluated if individuals acquiring a high intensity of infection previously had lower haematological indicators of condition (red blood cells [RBCs] and lymphocyte counts) than those that acquired lower infection intensities. Also, the association of these indicators with past and present trypanosome blood levels was investigated. The individuals that developed high levels of parasitaemia were those that previously had low lymphocyte counts. Greater intensity of infection corresponded with lower RBCs only in low to moderate weight females, and no effect of intensity of infection on lymphocyte counts was observed. However, delayed effects of high trypanosome intensity were seen on both RBCs and lymphocytes.
The vicious circle may also result in high infection intensity: individuals in poor condition are not only more likely to become infected by one pathogen; they may also be the most important source of infection for that and for other pathogens, and thus key protagonists for parasite dynamics.
在自然种群中，感染和状况可能协同作用，引发恶性循环：较差的状况易使宿主感染，而感染进一步降低状况，如此循环往复。如果这种恶性循环源于对感染的抵抗力降低，那么不仅状况较差的个体更容易感染，而且感染的强度也可能更大。在这里，我们通过一个具体的病原体（即原生动物微小巴贝斯虫）在田鼠（草原田鼠）种群中的动态变化，研究了宿主状况和感染强度之间的时间关系。
利用来自三个每月采样的种群的两年纵向数据，我们评估了在获得高强度感染之前，个体的血液学状况指标（红细胞[RBC]和淋巴细胞计数）是否低于那些获得低强度感染的个体。此外，还研究了这些指标与过去和现在的锥虫血液水平的关联。发展为高寄生虫血症的个体是那些先前淋巴细胞计数较低的个体。只有在低至中度体重的雌性中，感染强度较大才与 RBC 减少相关，而感染强度对淋巴细胞计数没有影响。然而，高强度感染的延迟效应也出现在 RBC 和淋巴细胞上。
恶性循环也可能导致高感染强度：状况较差的个体不仅更容易被一种病原体感染，而且他们可能也是该病原体以及其他病原体的最重要传染源，因此是寄生虫动态的关键主角。
