Daniel et al.
Increased relative risk of tick-borne encephalitis in warmer weather
Front. Cell. Infect. Microbiol. 2018; 8:90, doi: 10.3389/fcimb.2018.00090

A study about seasonal tick activity has been carried out from 2001 to 2006 in a defined area near Prague (Czech Republic). Ticks were collected by flagging and were counted weekly from March to November. Meteorological data were compared with the activity of host-questing Ixodes ricinus ticks.

The seasonal incidence of TBE cases was characterized by a bimodal curve, and in 2001, the late summer peak was almost as high as the spring peak while in 2006, the late summer peak was even higher then the spring peak. The peak incidence for TBE was around week 29 and showed a slower decline compared to nymph activity. The highest number of questing ticks was in the spring-summer period around week 21 and in the summer-autumn period around week 40. There was a high degree of correlation in weekly count of TBE cases and weekly count of questing nymphs. The rate of TBE cases grew more rapidly in the summer-autumn period than in the spring-summer period. Temperature was the best predictor of nymph questing activity. In the categories 5.1 to 10 and 10.1 to 15 degrees, similar proportions and numbers of nymphs were questing in the two seasonal periods. However, the was a marked difference in the proportions and numbers of questing ticks in the higher temperature ranges from 15.1 to 20 and more than 20 degrees during the summer-autumn period compared to the spring-summer period. The relative proportions of questing nymphs and the number of weeks in which they were found were greater in the summer-autumn period compared to the spring-summer period at near-ground temperatures greater than 5 degrees and at standard and weekly average temperatures of greater than 15 degrees. The ratio of TBE cases to questing ticks was found to differ significantly between spring-summer and summer-autumn periods. The seasonal discrepancy in tick activity and TBE incidence could not be explained by human activity. Given that spring host questing nymphs are metamorphosed in the preceding year and overwintered as unengorged nymphs while summer-autumn host questing nymphs are predominantly nymphs metamorphosed in the same summer, a combination of physiological age and environmental temperature may influence TBE virus replication in ticks and account for higher relative numbers of TBE cases in the summer-autumn period. Higher temperatures may be associated with higher levels of TBE virus infection in ticks and may explain the discrepancies between tick activity and number of TBE cases in spring-summer compared to summer-autumn periods. Higher average temperatures during summer-autumn period may lead to higher levels in ticks and consequently an increased risk that humans develop TBE following an infected tick bite.

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