Low circulation of TBEV in the studied site
TBEV was detected in nymph ticks from 2012 to 2014 with a very low prevalence varying annually from 0.03% [95% CI: 0 – 0.15]) to 0.24% [95% CI : 0.07 – 0.62]. Although adult ticks are generally more frequently infected than nymphs (Carpi, Bertolotti, Rosati, & Rizzoli, 2009; Gaumann, Muhlemann, Strasser, & Beuret, 2010; Perez-Eid et al., 1992; Rieille et al., 2014), our study did not detect the virus in adult ticks. This could nonetheless be explained by our small sample size. The method of collecting ticks by blanket dragging is more suitable for collecting nymph ticks than for adult ticks of I. ricinus . Considering the number of adults tested for TBEV, its prevalence in adult ticks was estimated to be lower than 0.7%. These estimations fit within the lower range values found in other studies in Europe, ranging from <0.1 to 5% (Süss, 2011), and are similar to those observed by Perez-Eid et al. (Perez-Eid et al., 1992) in the Neuhof forest in Alsace between 1970 to 1974 with a maximum prevalence of 0.12% [95% CI : 0.04 – 0.44] in nymphs and 0.77% [95% CI : 0.37 – 1.42] in adults.
In rodents, anti-TBEV antibodies were detected from 2012 to 2014 in 0% - 5.2% of individuals per season. It is worth noting that TBEV antibodies can only be detected during a short period between the 5th and the 13th day up to the 42nd day post-infection (Achazi et al., 2011; Milan Labuda et al., 1997). This is consistent with the observed disappearance of antibodies on seropositive animals captured a second time in our study and the finding of seronegative animals carrying infected larvae. These animals may have been recently infected and not yet seroconverted. Therefore, TBEV seroprevalence in small mammals is a good proxy for the occurrence of new infections in a month, although it is impossible to distinguish antibodies due to maternal transfer from those developed after a recent infection. In small mammals, maternal antibodies generally last from 6 to 10 weeks (Gomez-Chamorro et al., 2019; Grindstaff, Brodie, & Ketterson, 2003; Kallio et al., 2006) but this duration is unknown for TBEV. We captured two juveniles aged 3-4 weeks according to their weight that were seropositive. Considering the time needed for seroconversion after an infection, these antibodies were probably maternal antibodies. Although serological tests and sampling design vary according to studies, the seroprevalence we observed in 2012-2015 is similar to that found by Perez-Eid et al. in Alsace (Perez-Eid et al., 1992) (2.4%) and also fits within the lower values found in western and central Europe (prevalence ranging from 1.6% to 23% (Achazi et al., 2011; Bakhvalova, Potapova, Panov, & Morozova, 2009; Caroline Burri et al., 2012; Cagnacci et al., 2012; Knap et al., 2012; Kozuch, Gresíková, Nosek, Lichard, & Sekeyová, 1967; Tonteri et al., 2011)). In 2012 and 2013, we also observed a small proportion of rodents from which feeding ticks acquired the virus (2%) associated with a low prevalence in feeding ticks (1.6%).
Our results in rodents and questing ticks therefore suggest a very low circulation of TBEV in our site. This strengthens the hypothesis that the virus circulates at a very low level in the natural foci of the Alsace region, located at the western boundary of TBEV distribution. Randolph et al. (S. E. Randolph et al., 1999) theorised that the strength of TBEV enzootic cycles was influenced by the number of larvae co-feeding with nymphs on small mammals. Like the results of Perez-Eid et al. (Perez-Eid, 1990) in Alsace in the 1970s, we observed a very low number of co-feeding ticks in June-July during the period of highest tick activity, especially in 2012. We only captured 4.5% of rodents infested by nymphs in 2012 when rodent abundance was high, whereas 17% to 33% of rodents were reported to be infested by nymphs in TBEV-infected sites in Switzerland and Slovakia (C. Burri et al., 2011; S. E. Randolph et al., 1999). In addition, the mean infestation of larvae per rodent-session infested by ticks (2.2) and per rodent-session infested by nymphs (2) in 2012 was also lower than that observed in other TBEV-infected sites (from 10 to 80 larvae per infested rodent and from 5 to 65 larvae per host infested by nymphs) (C. Burri et al., 2011; S. E. Randolph et al., 1999; Rosà et al., 2007). However, the apparent difference in the number of feeding ticks in our study compared to other field observations might be slightly lower considering that our method for counting feeding ticks is not directly comparable with the methods employed in other field studies and may have increased the underestimation of the number of feeding ticks.