Seasonal and inter-annual variation of TBEV prevalence in ticks
and rodents in 2012-2014
Although the variations were low, we detected an annual and seasonal
effect on NIP (the TBEV prevalence of questing nymphs) but neither on
DIN nor rodent seroprevalence. Our study site in 2012-2014 was
characterised by a high inter-annual and inter-seasonal variation in
small mammal and questing nymph densities. Annually, the peak of
activity for questing larvae and nymphs, of larvae feeding on small
mammals, and of small mammal density occurred during the same season,
i.e. in June-July. The density of bank voles and yellow-necked mice
started to decrease from September. This decrease was greater for
yellow-necked mice than for bank voles, only 13% of yellow-necked mice
being captured in September-October. This seasonality is similar to
field observations in TBEV-infected sites in neighbouring countries (C.
Burri et al., 2011; S. E. Randolph et al., 1999; Rosà et al., 2007), but
differs from findings in Brittany, western France, where the peak of
small mammal densities and of feeding larvae were found to occur in
autumn (Perez et al., 2017). Our observations on inter-annual variations
comply with previous observations (Kiffner et al., 2011; Krawczyk et
al., 2020; Ostfeld et al., 2018; Perez et al., 2017; Rosà et al., 2007):
we found that the density of questing nymphs was higher the year
following a year of high rodent density and that the intensity of tick
aggregation on small mammals was higher when the density of small
mammals was low. Indeed, the prevalence of infestation by both stages
— larvae and nymphs — and the mean infestation was higher in 2013 (a
low-density year for small mammals) than in 2012 (a high-density for
small mammals).
NIP was higher in both 2012 and 2014 compared than 2013; this is
contrary to the density of questing nymphs, for which we found opposite
variations. However, DIN was near-constant over those years. Therefore,
our results suggest that the proportion of infected larvae produced was
higher in 2013 when more larvae and nymphs fed on the same very few
hosts because there were fewer hosts available. This led to a higher
proportion of infected questing nymphs the year after. This is
consistent with the fact that the proportion of larvae feeding on
rodents infested by nymphs was higher in 2013 than 2012. Therefore, the
inter-annual difference in NIP can be explained by the variation in
aggregation intensity of ticks on rodents along with rodent abundance.
However, the overall number of fed larvae produced was probably lower in
2013, the year of a lower abundance of small mammals. Consequently, the
DIN stayed constant over the years. Our study was only conducted over a
3-year period and the results cannot be generalised. Further
investigations over a longer period would be needed to better understand
the relationship between DON, DIN, NIP and small mammal density for
TBEV. Few studies have investigated these relationships for other
tick-borne pathogens amplified by rodents. In North America, Ostfeld et
al. (Ostfeld et al., 2018) studied for 19 years the relationship between
the Ixodes scapularis tick, the Borrelia burgdorferi sl.
bacteria and the white-footed mouse (Peromyscus leucopus ). They
found no effect of white-footed mouse density on
NIPBorrelia burgdorferi sl. whereas DIN
NIPBorrelia burgdorferi sl was positively
but not linearly related to mouse density in the previous year. At low
mouse density values, DIN Borreliaburgdorferi sl. was almost constant. In a short-term study in
the Netherlands, Krawczyk et al. (Krawczyk et al., 2020) observed that
the relationship between NIP, DIN and the rodent density the previous
year depended on the transmission mode of the tick-borne pathogens.
Contrary to most tick-borne pathogens amplified by rodents, TBEV is
mainly transmitted by co-feeding and the nymph-to-larva transmission of
TBEV strongly depends on the intensity of tick aggregation on small
mammals. These elements probably induced a different relationship
between DIN, NIP and rodent density that it would be interesting to
investigate more.
Although the difference was small, TBEV prevalence in questing ticks was
higher in autumn than in spring. This finding had already been observed
in the previous study conducted in Alsace (Perez-Eid et al., 1992) and
in studies targeting other tick-and-rodent-borne pathogens in Germany
and Luxembourg (May, Jordan, Fingerle, & Strube, 2015; Reye, Hubschen,
Sausy, & Muller, 2010). There are no data on the timings of diapause
and moulting of I.ricinus stages in the Alsace region. However,
if larvae that feed early in the season (in April-May) become nymphs
that quest later the same year (in July-October) as observed in
Switzerland (Perret et al., 2004), then this could be a potential
mechanism to explain the higher prevalence in questing nymphs in autumn.
Indeed, TBEV transmission to larvae might be higher in June-early July
since this period coincides in our study area with the peak abundance of
questing larvae and nymphs, of larvae feeding on small mammals and of
small mammals themselves, especially Apodemus sp ., which has been
shown experimentally to be more efficient in fostering TBEV transmission
than M. glareolus (M. Labuda et al., 1993; Milan Labuda et al.,
1997). Moreover, the virus titre may drop over time after the moult and
in unfed ticks undergoing a winter diapause, as observed experimentally
(Mishaeva and Erofeeva (Mishaeva & Erofeeva, n.d.) cited by Perez-Eid
et al. (Perez-Eid, 1990)), which would also induce lower prevalence in
early spring.
Surprisingly, from 2012 to 2014 we did not detect any annual or seasonal
effects on rodents’ TBEV infection probability in keeping with the
variation in the probability of a rodent being infested by ticks,
whereas DIN values were constant. A higher seroprevalence could be
expected in those years or seasons of low rodent abundance along with
the higher prevalence of tick (nymph) infestation on rodents, as
observed in other studies (Caroline Burri et al., 2012) and for other
tick-borne pathogens amplified by rodents (Perez et al., 2017).
Similarly, there was no species effect on the infection probability
despite A. flavicollis being found to be more infested by ticks
in this study (especially in spring when A. flavicollis is
abundant). This apparent contrast may result from a difference in the
immune response between species with a lower TBEV antibody titre and
persistence in A. flavicollis compared with M. glareolus(Heigl & Von Zeipel, 1966; Knap et al., 2012; Milan Labuda et al.,
1996, 1997; Zeipel & Heigl, 1966). We could then have expected to see
an annual or seasonal difference in the infection probability per
species, but our sample size of rodents was probably too small to detect
a significant difference given the low seroprevalence of rodents, the
low persistence of the antibodies and the low DIN. Another explanation
for the absence of any annual effect on small mammal seroprevalence
could be a lower detection of seropositive small mammals the years when
they were scarce. Indeed, during those years, small mammals may acquire
the infection much earlier in the year given the high tick aggregation
level when fewer hosts are available. The exposure to this infection
would probably remain undetected since the anti-TBEV antibodies’
half-life is very short in small mammals.
In conclusion, this study shows that the virus was circulating at a very
low level in our study site. Despite this very low-level circulation, we
were able to observe a significant variation in the inter-annual and
inter-seasonal prevalence of TBEV in questing nymphs, indicating that
the nymph-to-larva transmission of TBEV varied over time. However, the
density of infected questing nymphs showed no detectable variations over
time, suggesting that the rate of exposure for humans was probably
relatively constant over the period of the study. The seroprevalence of
small mammals was constant over time although the prevalence of tick
infestation varied on an annual and seasonal basis. More studies are
needed to understand the relationship between the density of
TBEV-infected questing infected nymphs, the density of questing nymphs,
the prevalence of TBEV in questing nymphs, TBEV seroprevalence in small
mammals and small mammal density.