Molecular Ecology and Evolution Lab, Department of Biology, Ecology Building, SE-223 62 Lund, Sweden.
Molecular Ecology and Evolution Lab, Department of Biology, Ecology Building, SE-223 62 Lund, Sweden.
Int J Parasitol. 2020 Jun;50(6-7):523-532. doi: 10.1016/j.ijpara.2020.03.008. Epub 2020 May 16.
Infectious diseases often vary seasonally in a predictable manner, and seasonality may be responsible for geographical differences in prevalence. In temperate regions, vector-borne parasites such as malaria are expected to evolve lower virulence and a time-varying strategy to invest more in transmission when vectors are available. A previous model of seasonal variation of avian malaria described a double peak in prevalence of Plasmodium parasites in multiple hosts resulting from spring relapses and transmission to susceptible individuals in summer. However, this model was rejected by a study describing different patterns of seasonal variation of two Plasmodium spp. at the same site, with the double peak only apparent when these species were combined. Here, we assessed the seasonal variation in prevalence of haemosporidian parasites (Plasmodium, Haemoproteus and Leucocytozoon) in house sparrows (Passer domesticus) sampled across 1 year at four temperate European sites spanning a latitudinal range of 17°. We showed that parasite prevalence and diversity decreased with increasing latitude, but the parasite communities differed between sites, with only one Plasmodium lineage (P_SGS1) occurring at all sites. Moreover, the nested PCR method commonly used to detect and identify haemosporidian parasites strongly underestimated co-infections of Haemoproteus and Plasmodium, significantly biasing the pattern of seasonal variation, so additional molecular methods were used. Finally, we showed that: (i) seasonal variation in prevalence of haemosporidian parasites varied between study sites and parasite lineages/species/genera, describing further cases where the double peak model is not met; (ii) the seasonal dynamics of single lineages (P_SGS1) varied between sites; and (iii) unexpectedly, seasonality was greatest at the southernmost site, a pattern that was mostly driven by lineage H_PADOM05. Limitations of the genotyping methods and consequences of pooling (parasite lineages, sites and years) in studies of haemosporidian parasites are discussed and recommendations proposed, since these actions may obscure the patterns of prevalence and limit ecological inferences.
传染病通常呈可预测的季节性变化，季节性变化可能是导致流行率在地理上存在差异的原因。在温带地区，疟疾等虫媒寄生虫预计会降低毒力，并随着寄生虫的出现，采用随时间变化的策略来更多地投资于传播。先前关于鸟类疟疾季节性变化的模型描述了多个宿主中疟原虫寄生虫流行率的双峰现象，这是由于春季复发和夏季易感个体传播所致。然而，当这些物种结合在一起时，一项描述同一地点两种疟原虫季节性变化不同模式的研究否定了这一模型。在这里，我们评估了在 1 年内，从欧洲四个温带地区的四个地点采集的麻雀（Passer domesticus）中血孢子虫（疟原虫、血变原虫和白细胞原虫）流行率的季节性变化。我们表明，寄生虫的流行率和多样性随着纬度的增加而降低，但寄生虫群落在不同的地点有所不同，只有一种疟原虫谱系（P_SGS1）存在于所有地点。此外，通常用于检测和鉴定血孢子虫的巢式 PCR 方法严重低估了血变原虫和疟原虫的混合感染，显著偏倚了季节性变化的模式，因此使用了额外的分子方法。最后，我们表明：（i）不同研究地点和寄生虫谱系/物种/属的血孢子虫流行率的季节性变化不同，进一步描述了不符合双峰模型的情况；（ii）单一线粒体（P_SGS1）的季节性动态在不同地点之间有所不同；（iii）出乎意料的是，最南端的地点季节性最强，这种模式主要由谱系 H_PADOM05 驱动。讨论了在血孢子虫研究中对基因分型方法的限制以及混合（寄生虫谱系、地点和年份）的后果，并提出了建议，因为这些操作可能会掩盖流行率的模式并限制生态推断。
