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

Abstract

BackgroundTick-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.MethodsA 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.ResultsThe 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.ConclusionOur 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.

Tick-borne encephalitis virus in ticks detached from humans and follow-up of serological and clinical response

“…It is known from previous studies that the overall prevalence of TBEV in ticks usually is low (Table S1), and also that from Sweden and Norway the majority of TBEV infected ticks have a virus amount around the detection limit (Andreassen et al, 2012;Brinkley et al, 2008).…”

“…To confirm TBEV presence in ticks detected by real-time PCR, but with too few copies to sequence the E-gene as described above, we also performed pyrosequencing using the primers and probe TBE 320F, TBE 373R, TBE 339 (Table 1), targeting the E-gene (Andreassen et al, 2012).…”

Seroprevalence of antibodies to tick-borne encephalitis virus and Anaplasma phagocytophilum in healthy adults from western Norway

“…From 1998 onwards, human cases of tick-borne encephalitis (TBE) acquired along the southern coastline of Norway have been diagnosed [2,3], and tick-borne encephalitis virus (TBEV) has been detected in ticks in this area [4]. So far, no human cases acquired in western Norway have been described.…”

Prevalence of the infectious hypodermal and hematopoietic necrosis virus in shrimp (Penaeus vannamei) broodstock in northwestern Mexico

“…Prevalence screening of pooled samples is complicated since it is impossible to determine if a positive result is due to one or more infected organisms. Thus, several methods, as the Minimum Infection Rate and the Frequentist Method (Andreassen et al, 2012), have been developed. In the current study prevalence for pooled samples from different hatcheries was calculated by using the online pooled prevalence calculator of the Epitools site, which provides a measurement of uncertainty in the confidence interval (CI) associated with the apparent prevalence estimation.…”