Ecology and evolution of shrew-borne orthohantaviruses in Finland

More than 60% of human emerging infectious diseases (EIDs) are zoonotic (Jones et al., 2008), and they all originate in wildlife. They include infectious diseases, such as Ebola and Middle East Respiratory Syndrome (MERS), and those caused by hantaviruses. Zoonoses are infectious diseases of animals (usually vertebrates) that can be transmitted to humans. Hantaviruses are emerging zoonotic pathogens that belong to the genus Orthohantavirus and family Hantaviridae in order Bunyavirales. Hantaviruses pose a serious threat to human health because their infection causes two highly fatal diseases: haemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). Rodents have been regarded as the main reservoir and evolutionary scene of hantaviruses. In the last three decades, our knowledge of hantaviruses has broadened significantly. In contrast to the initial assumption that hantaviruses are mainly carried by rodents, many novel hantaviruses have been detected in shrews, moles and bats during the last few years, and recently, they have even been detected in insects. These findings raise a number of significant questions about the evolutionary history of hantaviruses, their host association and adaptation, the role and frequency of spillover infections and host-switch events, and most importantly, their pathogenicity. In Finland, Puumala virus (PUUV) has been regarded as the only rodent-borne hantavirus present in the country. It causes a mild form of HFRS called nephropathia epidemica (NE) in humans. To search for novel hantaviruses other than PUUV, various novel hantaviruses were molecularly identified in different species of Soricomorpha ("shrewform"). Genetic analyses revealed that four soricomorph-borne hantaviruses circulate in Finland, including Boginia virus (BOGV) in Neomys fodiens and Asikkala virus (ASIV) in Sorex minutus. Common shrews (Sorex araneus) harboured two different hantaviruses: Seewis virus (SWSV) and an Altai-like virus, showing the first evidence of co-existence of two distinct hantavirus species circulating simultaneously in one host species population. This host sharing of two divergent hantaviruses in the European common shrews contradicts hantavirus-host specificity, further implying the complexity of hantavirus evolution. SWSV and its host S. araneus are widespread in Eurasia with a distribution ranging from Central Siberia to Western Europe. After screening hundreds of S. araneus trapped from all of Finland, we obtained a large data set of new SWSV sequences that enabled phylogeographic analyses of SWSV. The results demonstrated that this shrew-borne hantavirus is similar to rodent-borne hantaviruses, and the post-glacial spread of SWSV into Finland mirrors that of the host, S. araneus: these shrews colonized Finland from the east after the last ice age (12,000–8,000 years ago) and then subsequently spread along emerging land bridges towards the west or north. The phylogenetic analysis of partial S segment sequences suggested that all Finnish SWSV strains shared their most recent common ancestor with Eastern European strains, whereas the L segment suggested a separate introduction that was most likely via a more northern route. The difference between the L and S segment phylogenies implied that reassortment events played a role in the evolution of SWSV. From 2001 to 2015, common shrews were collected during several field studies in 45 locations/municipalities across Finland and screened for SWSV. SWSV RNA prevalence in the common shrew population had a spatial but not a temporal pattern with the eastern population having a higher infection rate than the others. SWSV could persistently infect common shrews with some pathological effects. SWSV RNA prevalence in adults or over-wintered common shrews was relatively higher than in summer-born shrews, suggesting that maternal antibodies can protect juvenile shrews from SWSV infection. The exposure risk to SWSV infection increases over the time. Most new hantaviruses discovered in soricomorph and bat hosts instead of rodents have raised questions as to whether any of them will emerge as human pathogens. Therefore, to predict human exposure risk, novel laboratory techniques for molecular and serological hantavirus detection were developed. No evidence of SWSV infection was found among a panel of 486 patient serum samples; however, we demonstrated a cross-reaction of anti-PUUV serum with shrew-borne hantavirus nucleocapsid (N) protein. This thesis focused on the diversity, host maintenance and crossspecies transmission dynamics of soricomorph-borne hantaviruses. The study presented innovative methods to investigate this pertinent topic at the interface of wildlife diseases and human health. The results provided new insights about the ecology, evolutionary origins and phylogeography, and most importantly, the potential pathogenicity of soricomorph-borne hantaviruses. This knowledge in combination with future studies will hopefully lead to a better understanding of host-parasite relationships.