Epidemiological Aspects of Crimean-Congo Hemorrhagic Fever in Western Europe: What about the Future?

Crimean-Congo hemorrhagic fever virus (CCHFV) is an arthropod-borne virus (arbovirus), mainly transmitted by ticks, belonging to the genus Orthonairovirus (family Nairoviridae, order Bunyavirales). CCHFV causes a potentially severe, or even fatal, human disease, and it is widely distributed in Africa, Asia, eastern Europe and, more recently, in South-western Europe. Until a few years ago, no cases of Crimean-Congo hemorrhagic fever (CCHF) had been reported in western Europe, with the exception of several travel-associated cases. In 2010, the CCHFV was reported for the first time in South-western Europe when viral RNA was obtained from Hyalomma lusitanicum ticks collected from deer in Caceres (Spain). Migratory birds from Africa harboring CCHFV-infected ticks and flying to Spain appear to have contributed to the establishment of the virus (genotype III, Africa-3) in this country. In addition, the recent findings in a patient and in ticks from deer and wild boar of viral sequences similar to those from eastern Europe (genotype V, Europe-1), raise the possibility of the introduction of CCHFV into Spain through the animal trade, although the arrival by bird routes cannot be ruled out (Africa-4 has been also recently detected). The seropositive rates of animals detected in regions of South-western Spain suggest an established cycle of tick-host-tick in certain areas, and the segment reassortment detected in the sequenced virus from one patient evidences a high ability to adaptation of the virus. Different ixodid tick genera can be vectors and reservoirs of the virus, although Hyalomma spp. are particularly relevant for its maintenance. This tick genus is common in Mediterranean region but it is currently spreading to new areas, partly due to the climate change and movement of livestock or wild animals. Although to a lesser extent, travels with our pets (and their ticks) may be also a factor to be considered. As a consequence, the virus is expanding from the Balkan region to Central Europe and, more recently, to Western Europe where different genotypes are circulating. Thus, seven human cases confirmed by molecular methods have been reported in Spain from 2016 to August 2020, three of them with a fatal outcome. A One Health approach is essential for the surveillance of fauna and vector populations to assess the risk for humans and animals. We discuss the risk of CCHFV causing epidemic outbreaks in Western Europe.