Mapping geographical areas at risk for tick-borne encephalitis (TBE) by analysing bulk tank milk from Swedish dairy cattle herds for the presence of TBE virus\u2013specific antibodies

Cécile Beck,Gunilla Blomqvist,Linda Svensson,Jean Francois Valarcher,Katarina Näslund

doi:10.1186/s13028-021-00580-4

Abstract

BackgroundThe vector-borne human viral zoonosis tick-borne encephalitis (TBE) is of growing concern in Sweden. The area where TBE is considered endemic has expanded, with an increasing geographical distribution of Ixodes ricinus as the tick vector and a rising number of reported TBE cases in humans. Efforts to map TBE risk areas have been carried out by sentinel monitoring, mainly based on individual sampling and analysis of wild and domestic animals, as well as ticks, for tick-borne encephalitis virus (TBEV). However, the interpretation of the geographical distribution has been hampered by the patchy and focal nature of TBEV occurrence. This study presents TBEV surveillance data based on antibody analysis of bulk tank milk collected from dairy herds located throughout Sweden before (May) and after (November) the vector season. A commercial TBEV antibody ELISA was modified and evaluated for use in this study.ResultsThe initial comparative TBEV antibody analysis revealed a good correlation between milk and serum antibody levels from individually sampled cows. Also, the TBEV-antibody levels for the mean-herd serum showed good comparability with TBEV antibody levels from bulk tank milk, thus indicating good predictability of seroprevalence when analysing bulk tank milk from a herd.Analyses of bulk tank milk samples collected from 616 herds in May and 560 herds in November showed a geographical distribution of TBEV seropositive herds that was largely consistent with reported human TBE cases. A few TBEV-reactive herds were also found outside known locations of human TBE cases.ConclusionSerological examination of bulk tank milk from dairy cattle herds may be a useful sentinel surveillance method to identify geographical presence of TBEV. In contrast to individual sampling this method allows a large number of animals to be monitored. TBEV seropositive herds were mainly found in coastal areas of southern Sweden similar to human TBE cases. However, some antibody-reactive herds were found outside known TBE areas at the time of the study.

Highlights

The vector-borne human viral zoonosis tick-borne encephalitis (TBE) is of growing concern in Sweden
tick-borne encephalitis virus (TBEV) seropositive herds were mainly found in coastal areas of southern Sweden similar to human TBE cases
Comparative analysis of serum TBEV‐antibody levels using microneutralization test (MNT), commercial indirect enzyme-linked immunosorbent assay (ELISA) and modified indirect ELISA Nine bovine sera with varying MNT titres were analysed with TBEV cE and TBEV mE

Summary

Introduction

The vector-borne human viral zoonosis tick-borne encephalitis (TBE) is of growing concern in Sweden. The area where TBE is considered endemic has expanded, with an increasing geographical distribution of Ixodes ricinus as the tick vector and a rising number of reported TBE cases in humans. Efforts to map TBE risk areas have been carried out by sentinel monitoring, mainly based on individual sampling and analysis of wild and domestic animals, as well as ticks, for tick-borne encephalitis virus (TBEV). Tick-borne encephalitis (TBE) is considered the most important viral tick-borne human zoonosis in Eurasia [1], and since 2012 TBE has been included on the list of notifiable human diseases in the European Union [2]. TBEV, like louping ill virus, Powassan virus, and deer tick virus, belongs to the group of tick-borne flaviviruses known to cause central nervous system disorders in humans. Single cases of TBE have been reported to occur in a sheep [10], a goat [11] and a mouflon [12]

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