Assessing systemic and non-systemic transmission risk of tick-borne encephalitis virus in Hungary.

Kyeongah Nah,Jianhong Wu,Xiaotian Wu,Felicia Maria G Magpantay,Attila János Trájer,Xue Zhang,Ákos Bede-Fazekas,Gergely Röst,Sam R Telford

doi:10.1371/journal.pone.0217206

Abstract

Estimating the tick-borne encephalitis (TBE) infection risk under substantial uncertainties of the vector abundance, environmental condition and human-tick interaction is important for evidence-informed public health intervention strategies. Estimating this risk is computationally challenging since the data we observe, i.e., the human incidence of TBE, is only the final outcome of the tick-host transmission and tick-human contact processes. The challenge also increases since the complex TBE virus (TBEV) transmission cycle involves the non-systemic route of transmission between co-feeding ticks. Here, we describe the hidden Markov transition process, using a novel TBEV transmission-human case reporting cascade model that couples the susceptible-infected compartmental model describing the TBEV transmission dynamics among ticks, animal hosts and humans, with the stochastic observation process of human TBE reporting given infection. By fitting human incidence data in Hungary to the transmission model, we estimate key parameters relevant to the tick-host interaction and tick-human transmission. We then use the parametrized cascade model to assess the transmission potential of TBEV in the enzootic cycle with respect to the climate change, and to evaluate the contribution of non-systemic transmission. We show that the TBEV transmission potential in the enzootic cycle has been increasing along with the increased temperature though the TBE human incidence has dropped since 1990s, emphasizing the importance of persistent public health interventions. By demonstrating that non-systemic transmission pathway is a significant factor in the transmission of TBEV in Hungary, we conclude that the risk of TBE infection will be highly underestimated if the non-systemic transmission route is neglected in the risk assessment.

Highlights

Tick-borne encephalitis (TBE), an arboviral infection of the central nervous system caused by bites from infected ticks, has been a major public health concern for much of Central Europe
We show that the TBE virus (TBEV) transmission potential in the enzootic cycle has been increasing along with the increased temperature though the tick-borne encephalitis (TBE) human incidence has dropped since 1990s, emphasizing the importance of persistent public health interventions
The risk of human infection depends on the prevalence of the virus spread in the tick-host ecological cycle, and on the tick-human contact that is effective for the disease transmission

Summary

Introduction

Tick-borne encephalitis (TBE), an arboviral infection of the central nervous system caused by bites from infected ticks, has been a major public health concern for much of Central Europe. Evaluating the human infection needs accurate estimation of the tick population dynamics which is highly regulated by the environmental and climatic conditions [3,4,5,6,7,8]; the effectiveness of pathogen transmission per tick-host contact, and the human case reporting given infection probability which is quite stochastic [9]. All of these involve substantial uncertainties in ecological, epidemiological and environmental parameters. The studies on the seropositivity of animals in 1960 to 1970 and 2005 support this argument [12, 13]

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