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Prevalence of tick borne encephalitis virus in tick nymphs in relation to climatic factors on the southern coast of Norway

Ashild Andreassen1*, Solveig Jore3, Piotr Cuber4, Susanne Dudman1, Torstein Tengs3, Ketil Isaksen5, Hans Olav Hygen5, Hildegunn Viljugrein36, Gabriel Ånestad1, Preben Ottesen2 and Kirsti Vainio1

Author Affiliations

1 Division of Infectious Disease Control, Department of Virology, Norwegian Institute of Public Health, P. O. Box 4404, Nydalen, 0403, Oslo, Norway

2 Department of Pest Control, Norwegian Institute of Public Health, P. O. Box 4404, Nydalen, 0403, Oslo, Norway

3 Norwegian Veterinary Institute, Ullevålsveien 68, P.O.Box 750, centrum, 0106, Oslo, Norway

4 School of Pharmacy, Department of Parasitology, Medical University of Silesia in Katowice, 41-218, Sosnowiec, Ul. Jedności 8, Poland

5 The Norwegian Meteorological Institute, Oslo, Norway

6 Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, Oslo, Norway

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Parasites & Vectors 2012, 5:177  doi:10.1186/1756-3305-5-177

Published: 22 August 2012

Abstract

Background

Tick-borne encephalitis (TBE) is among the most important vector borne diseases of humans in Europe and is currently identified as a major health problem in many countries. TBE endemic zones have expanded over the past two decades, as well as the number of reported cases within endemic areas. Multiple factors are ascribed for the increased incidence of TBE, including climatic change. The number of TBE cases has also increased in Norway over the past decade, and the human cases cluster along the southern coast of Norway. In Norway the distribution and prevalence of TBE virus (TBEV) in tick populations is largely unknown. The objectives of this study were to estimate the TBEV prevalence in Ixodes ricinus from seven locations and to assess the relationship between the TBEV prevalence and site-specific climatic variables.

Methods

A total of 5630 questing nymphs were collected and analyzed in pools of ten. All pools were screened with an in-house real-time RT-PCR, and the positive pools were pyrosequenced. Two methods, minimum infection rate (MIR) and a frequentist method (EPP) for pooled prevalence estimations were calculated and compared. Climatic data were descriptively compared to the corresponding EPP of each location in order to explain variations in TBEV prevalence.

Results

The seven foci of TBEV had an estimated overall prevalence (EPP) in pools of nymphs combined, of 0.53% with 95% CI (0.35–0.75), with point prevalence ranging between 0.11%–1.22%. The sites with the highest point prevalences were within the municipalities which had the highest numbers of registered TBE cases. The results indicate that the location with highest point prevalence had the highest relative mean humidity and lowest mean saturation deficit and vice versa for the lowest EPP.

Conclusion

Our study confirms the existence of TBEV endemic foci in Norway. These results are of importance to increase the awareness of TBEV infections in Norway and could be used for public information and recommendations of TBE vaccination. EPP is the method of choice for pooled prevalence calculations, since it provides estimated prevalences with confidence intervals. Our findings emphasise the possible importance of microclimatic conditions regarding the TBEV prevalence in ticks.

Keywords:
Prevalence; TBEV; Estimated pooled prevalence; MIR; Climate; Pooled sampling; Ixodes ricinus