Hantaviruses and their hosts in Europe: reservoirs here and there, but not everywhere?

Five hantaviruses are known to circulate among rodents in Europe, and at least two among insectivores. Four (Dobrava, Saaremaa, Seoul, and Puumala [PUUV] viruses) are clearly associated with hemorrhagic fever with renal syndrome (HFRS). PUUV, the most common etiological agent of HFRS in Europe, is carried by the bank vole (Myodes glareolus), one of the most widespread and abundant mammal species in Europe. This host-virus system is among hantaviruses also the most studied one in Europe. However, HFRS incidence varies throughout the continent. The spatial as well as temporal variation in the occurrence of HFRS is linked to geographic differences in the population dynamics of the reservoir rodents in different biomes of Europe. While rodent abundance may follow mast seeding events in many parts of temperate Europe, in northern (N) Europe multiannual cycles in population density exist as the result of the interaction between rodent populations and specialist predator populations in a delayed density-dependent manner. The spatial distribution of hantaviruses further depends on parameters such as forest patch size and connectivity of the most suitable rodent habitats, and the conditions for the survival of the virus outside the host, as well as historical distribution patterns (phylogeographies) of hosts and viruses. In multiannually fluctuating populations of rodents, with population increases of great amplitude, one should expect a simultaneous build-up of recently hantavirus-infected (shedding) rodents. The increasing number of infectious, virus-shedding rodents leads to a rapid transmission of hantavirus across the rodent population, and to humans. Our review discusses these aspects for PUUV, the only European hantavirus for which there is a reasonable, yet still far from complete, ecological continental-wide understanding. We discuss how this information could translate to other European hantavirus-host systems, and where the most important questions lie for further research.