Exploring the Reservoir Hosts of Tick-Borne Encephalitis Virus

Abstract

Tick-borne encephalitis virus (TBEV) is an important arbovirus, which is found across large parts of Eurasia and is considered to be a major health risk for humans. Like any other arbovirus, TBEV relies on complex interactions between vectors, reservoir hosts, and the environment for successful virus circulation. Hard ticks are the vectors for TBEV, transmitting the virus to a variety of animals. The importance of these animals in the lifecycle of TBEV is still up for debate. Large woodland animals seem to have a positive influence on virus circulation by providing a food source for adult ticks; birds are suspected to play a role in virus distribution. Bank voles and yellow-necked mice are often referred to as classical virus reservoirs, but this statement lacks strong evidence supporting their highlighted role. Other small mammals (e.g., insectivores) may also play a crucial role in virus transmission, not to mention the absence of any suspected reservoir host for non-European endemic regions. Theories highlighting the importance of the co-feeding transmission route go as far as naming ticks themselves as the true reservoir for TBEV, and mammalian hosts as a mere bridge for transmission. A deeper insight into the virus reservoir could lead to a better understanding of the development of endemic regions. The spatial distribution of TBEV is constricted to certain areas, forming natural foci that can be restricted to sizes of merely 500 square meters. The limiting factors for their occurrence are largely unknown, but a possible influence of reservoir hosts on the distribution pattern of TBE is discussed. This review aims to give an overview of the multiple factors influencing the TBEV transmission cycle, focusing on the role of virus reservoirs, and highlights the questions that are waiting to be further explored.

Tick-borne encephalitis virus (TBEV) prevalence in field-collected ticks (Ixodes ricinus) and phylogenetic, structural and virulence analysis in a TBE high-risk endemic area in southwestern Germany

“…However, one recent study from Germany where ticks were collected from the upper Rhine valley (sample size: 4064 ticks) and another crowdsourcing project from Finland from a nationwide tick collection (sample size: 20,000 ticks) calculated similar TBEV MIR rates in I. ricinus of 0.1% and 0.2%, respectively [36,48]. Since it has been suggested that the spatial distribution of TBEV is constricted to certain areas, forming natural foci that are restricted to very limited areas in size (c.500 m 2 ), our approach might have more likely detected the prevalence of those foci than the frequency of TBEV within a given focus [56]. Thus, the overall prevalence of TBEV within one single focus might be manifold higher.…”

Revealing new tick-borne encephalitis virus foci by screening antibodies in sheep milk

“…TBEV has a patchy geographical distribution and is restricted to certain areas, often small spots, so-called natural foci [35]. Furthermore, within a given focus the percentage of TBEV-infected ticks is usually as low as < 1% [36,37].…”

Baltic Group Tick-Borne Encephalitis Virus Phylogeography: Systemic Inconsistency Pattern between Genetic and Geographic Distances

“…A much more speculative assumption would be that replication is possible directly in an unknown flying animal. There are two groups of vertebrate animals that can fly with ticks: bats [ 47 , 48 , 49 ] and birds [ 50 , 51 , 52 , 53 , 54 , 55 ]. The role of bats in the distribution of TBEV is currently unknown.…”