Prolongation of Tick-Borne Encephalitis Cycles in Warmer Climatic Conditions

Abstract

Tick-borne encephalitis exhibits profound inter-annual fluctuations in incidence. Previous studies showed that three-fifths of the variation can be explained in terms of four superimposed oscillations: a quasi-biennial, triennial, pentennial, and a decadal cycle. This study was conducted to determine how these cycles could be influenced by climate change. Epidemiological data, spanning from the 1970s to the present, and originating from six regions/countries bridging Scandinavia and the Mediterranean, represented a temporal/latitudinal gradient. Spectral analysis of time series was used to determine variation in the cycles’ length/amplitude with respect to these gradients. The analysis showed that—whereas the lengths of the shorter cycles do not vary substantially—cycles in the decadal band tend to be longer southwards. When comparing the disease’s oscillations before- and after the mid-1990s, a shift towards longer oscillations was detected in the pentennial–decadal band, but not in the biennial– triennial band. Simultaneously, oscillations in the latter band increased in intensity whereas the decadal oscillations weakened. In summary, the rhythm of the cycles has been altered by climate change. Lengthened cycles may be explained by prolonged survival of some animal hosts, and consequently greater inertia in herd immunity changes, slowing down a feedback loop between the herd immunity and amount of virus circulating in nature.

The Emergence and Dynamics of Tick-Borne Encephalitis Virus in a New Endemic Region in Southern Germany

“…The establishment of new permanent foci may depend on complex environmental requirements [35]. Among them, climate and climate change have been mentioned as important factors for the expansion or emergence of new TBEV foci [36,37].…”

The Impacts of Climate Change on Ticks and Tick-Borne Disease Risk

“…For example, TBE virus exhibits large interannual variations in prevalence and incidence related to interactions among host population cycles, host immunity, and longevity. Zeman (89) demonstrated cyclical oscillations in TBE virus in large areas of Europe (Sweden, Germany, the Czech Republic, Slovenia, Austria, and Italy) and showed that climate change alters the length of these cycles: Warmer conditions lengthen the cycle frequency due to increased host longevity reducing the speed of variation in host immunity, which feeds back into the level of TBE virus circulating in the host (and tick) population.…”

Climate Changes Exacerbate the Spread of Ixodes ricinus and the Occurrence of Lyme Borreliosis and Tick-Borne Encephalitis in Europe—How Climate Models Are Used as a Risk Assessment Approach for Tick-Borne Diseases

“…The temperature rise constitutes a risk of increased TBE cases from a possible transmission, which occurs during winter, as it causes the extension of the tick’s activity period during the winter months and consequently the reduction of the tick’s inactivity period. Moreover, the higher temperatures that occur as a result of climate change influence the length of the round fluctuations in TBEV in numerous European countries such as Sweden, Germany, Czech Republic, Slovenia, Austria, and Italy [ 82 ]. Τhe simultaneous blood meal of I. ricinus uninfected larvae and infected nymphs from a host correlates with the occurrence of TBE cases and depends on the rhythm of the temperature reduction in the autumn [ 21 ].…”