Laboratoire Biogéosciences, CNRS UMR5561, Université de Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France.
Int J Parasitol. 2010 Oct;40(12):1447-53. doi: 10.1016/j.ijpara.2010.04.014. Epub 2010 May 23.
Parasites are known to exert strong selection pressures on their hosts and, as such, favour the evolution of defence mechanisms. The negative impact of parasites on their host can have substantial consequences in terms of population persistence and the epidemiology of the infection. In natural populations, however, it is difficult to assess the cost of infection while controlling for other potentially confounding factors. For instance, individuals are repeatedly exposed to a variety of parasite strains, some of which can elicit immunological memory, further protecting the host from subsequent infections. Cost of infection is, therefore, expected to be particularly strong for primary infections and to decrease for individuals surviving the first infectious episode that are re-exposed to the pathogen. We tested this hypothesis experimentally using avian malaria parasites (Plasmodium relictum-lineage SGS1) and domestic canaries (Serinus canaria) as a model. Hosts were infected with a controlled dose of P. relictum as a primary infection and control birds were injected with non-infected blood. The changes in haematocrit and body mass were monitored during a 20 day period. A protein of the acute phase response (haptoglobin) was assessed as a marker of the inflammatory response mounted in response to the infection. Parasite intensity was also monitored. Surviving birds were then re-infected 37 days post primary infection. In agreement with the predictions, we found that primary infected birds paid a substantially higher cost in terms of infection-induced reduction in haematocrit compared with re-exposed birds. After the secondary infection, re-exposed hosts were also able to clear the infection at a faster rate than after the primary infection. These results have potential consequences for the epidemiology of avian malaria, since birds re-exposed to the pathogen can maintain parasitemia with low fitness costs, allowing the persistence of the pathogen within the host population.
寄生虫对其宿主施加强大的选择压力，因此有利于防御机制的进化。寄生虫对宿主的负面影响在种群持续存在和感染的流行病学方面可能产生重大后果。然而，在自然种群中，在控制其他潜在混杂因素的情况下，评估感染的成本是困难的。例如，个体反复暴露于各种寄生虫株，其中一些可以引发免疫记忆，进一步保护宿主免受随后的感染。因此，感染的成本预计对初次感染特别强烈，并随着首次感染后幸存的个体再次暴露于病原体而降低。我们使用鸟类疟疾寄生虫（Plasmodium relictum 谱系 SGS1）和家雀（Serinus canaria）作为模型，通过实验检验了这一假说。宿主感染了受控剂量的 P. relictum 作为初次感染，对照鸟类注射了未感染的血液。在 20 天的时间内监测了血细胞比容和体重的变化。急性期反应蛋白（触珠蛋白）被评估为感染后炎症反应的标志物。还监测了寄生虫的强度。初次感染后 37 天，幸存的鸟类再次感染。与预测一致，我们发现初次感染的鸟类在感染引起的血细胞比容降低方面付出了更高的代价，而再次暴露的鸟类则不然。在二次感染后，再次暴露的宿主也能够比初次感染后更快地清除感染。这些结果对鸟类疟疾的流行病学有潜在影响，因为再次暴露于病原体的鸟类可以以低的适应度代价维持寄生虫血症，从而使病原体在宿主种群中持续存在。
