Different tick-borne encephalitis virus (TBEV) prevalences in unfed versus partially engorged ixodid ticks--evidence of virus replication and changes in tick behavior

Abstract

There is some evidence that tick-borne encephalitis virus (TBEV) prevalence in ticks, removed from humans, is higher than that in field-collected ticks from the same area. There are two possible explanations: (i) Infected ticks are more active and aggressive and can be found on humans more often. (ii) Some questing ticks are infected with TBEV in a low, undetectable concentration; during tick feeding, virus replicates and reaches the titers that can be detected. The aim of our work was to evaluate both hypotheses. Using unfed adult Ixodes ricinus, we compared three methods of tick infection with TBEV: (i) injection of the virus under the tick's 4th coxa (percoxal method), (ii) injection through anus (rectal method), and (iii) immersion of ticks in virus-containing medium. The percoxal method showed the best results and was used in further experiments. We compared the dynamics of virus reproduction in ticks that remain unfed after inoculation and in partially engorged ticks fed on mice. When ticks fed for 15 h, the titer of the virus increased in 3 log(10)PFU/tick since inoculation, while in unfed ticks it did not change. We also studied the reaction on the repellent DEET of uninfected versus TBEV-infected Ixodes ricinus ticks of the physiological age levels III and IV. We investigated ticks movements upwards in the direction of the bait on the cotton tape, impregnated with an increasing concentration of DEET. Obtained data showed that infected ticks were more active and tolerant to DEET. About 70% of the non-infected ticks and only 13% of the infected ticks did not get over the lowest concentration of the repellent (0.1%). Only infected ticks (5.6%) got over 1% concentration of DEET. Ticks of the physiological age level IV from both infected and uninfected groups were the most active and tolerant to the repellent. Both above-mentioned hypotheses were approved and can be used to explain higher virus prevalences in partially engorged ticks than in field-collected ticks.