Tick-Borne Encephalitis Virus: Seasonal and Annual Variation of Epidemiological Parameters Related to Nymph-to-Larva Transmission and Exposure of Small Mammals.

Laure Bournez,Franck Boué,Céline Richomme,Marie Moinet,Christophe Caillot,Jean-Marc Boucher,Jean-Michel Demerson,Elodie Devillers,Elodie Devillers,Elodie Devillers,Elodie Devillers,Sara Moutailler,Gerald Umhang,Yves Hansmann

doi:10.3390/pathogens9070518

Abstract

A greater knowledge of the ecology of the natural foci of tick-borne encephalitis virus (TBEV) is essential to better assess the temporal variations of the risk of tick-borne encephalitis for humans. To describe the seasonal and inter-annual variations of the TBEV-cycle and the epidemiological parameters related to TBEV nymph-to-larva transmission, exposure of small mammals to TBEV, and tick aggregation on small mammals, a longitudinal survey in ticks and small mammals was conducted over a 3-year period in a mountain forest in Alsace, eastern France. TBEV prevalence in questing nymphs was lower in 2013 than in 2012 and 2014, probably because small mammals (Myodes glareolus and Apodemus flavicollis) were more abundant in 2012, which reduced tick aggregation and co-feeding transmission between ticks. The prevalence of TBEV in questing nymphs was higher in autumn than spring. Despite these variations in prevalence, the density of infected questing nymphs was constant over time, leading to a constant risk for humans. The seroprevalence of small mammals was also constant over time, although the proportion of rodents infested with ticks varied between years and seasons. Our results draw attention to the importance of considering the complex relationship between small mammal densities, tick aggregation on small mammals, density of infected questing nymphs, and prevalence of infected nymphs in order to forecast the risk of TBEV for humans.

Highlights

Tick-borne encephalitis virus (TBEV) is responsible for the most frequent human viral tick-borne disease in Europe
The aims of the present study were to characterise the epidemiology of tick-borne encephalitis virus (TBEV) in Alsace and to describe both seasonal and inter-annual variations of the TBEV cycle’s epidemiological parameters: (i) the density of TBEV-infected questing nymphs (DIN) and the prevalence of TBEV in questing nymphs (NIP) related to TBEV nymph-to-larva transmission; (ii) the TBEV seroprevalence of small mammals related to their exposure to TBEV; and (iii) the prevalence of tick infestations of small mammals as a proxy for the intensity of aggregation on hosts, since these parameters are well correlated
Our results confirm a low circulation of TBEV in ticks and rodents on the study site

Summary

Introduction

Tick-borne encephalitis virus (TBEV) is responsible for the most frequent human viral tick-borne disease in Europe. This flavivirus affects the human central nervous system, causing meningitis or Pathogens 2020, 9, 518; doi:10.3390/pathogens9070518 www.mdpi.com/journal/pathogens. Pathogens 2020, 9, 518 meningoencephalitis with long-term sequelae, with the more severe forms progressing into a loss of consciousness, coma, and even death [1]. In western Europe, only the European subtype (TBEV-Eur), the least virulent subtype, is present [2]. TBEV-Eur is mainly transmitted to a host by a tick bite and is maintained in nature by a cycle involving ticks, mainly Ixodes ricinus, and small mammals, especially those belonging to the rodent genera Apodemus and Myodes [2]. I. ricinus requires three blood meals during their lifecycle. Each stage of the tick life cycle takes from several months to around one year to develop to the so the entire life cycle is generally completed in two or three years, this can vary from two to six years depending upon the geographical location [4,5]

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