Tick-borne encephalitis virus in an Alsatian forest, eastern France:
seasonal and annual variation of epidemiological parameters related to
nymph-to-larva transmission and exposure of small mammals
Abstract
Tick-borne encephalitis virus (TBEV) is responsible for the most serious
human viral tick-borne disease in Europe. A greater knowledge of the
natural foci of TBEV and their fluctuating characteristics is essential
to better assess the temporal variations of TBEV cycle’s epidemiological
risks. To describe the seasonal and inter-annual variations of the
TBEV-cycle epidemiological parameters related to TBEV nymph-to-larva
transmission, exposure of small mammals to TBEV and tick aggregation on
small mammals, a longitudinal survey was conducted over a 3-year period
to monitor a TBEV focus in a mountain forest in Alsace, eastern France.
We trapped small mammals and collected questing ticks five times per
year from 2012 to 2014. TBEV prevalence in questing nymphs was lower in
2013 than in 2012 and 2014, probably because small mammals (Myodes
glaerolus and Apodemus flavicollis) were more abundant in 2012, reducing
tick aggregation and co-feeding transmission between ticks. This
prevalence was higher in autumn than spring, which suggests that
nymph-to-larva transmission of TBEV mainly occurs in spring. Despite
these variations in prevalence, the density of infected questing nymphs
was constant over time, leading to a constant risk for humans. The
seroprevalence of small mammals was also constant over time although the
proportion of small mammals harbouring ticks varied between years and
seasons. Our results draw attention to the importance of considering the
complex relationship between small mammal densities, tick aggregation on
small mammals, density of infected questing nymphs and prevalence of
infected nymphs in order to forecast the risk of TBEV for humans.