Prevalence of infections and co-infections with 6 pathogens in Dermacentor reticulatus ticks collected in eastern Poland

Occurrence of co-infections with various pathogens in ixodid ticks creates a risk of increased severity of tick-borne diseases in humans and animals exposed to bite of the ticks carrying multiple pathogens. Accordingly, co-infections in ticks were subject of numerous analyses, but almost exclusively with regard to Ixodes ricinus complex whereas potential tick vectors belonging to other genera were much less studied. Taking into consideration the role of Dermacentor reticulatus in the transmission of various pathogens, we carried out for the first time the comprehensive statistical analysis of co-infections occurring in this tick species. An attempt was made to determine the significance of the associations between 6 different pathogens occurring in D. reticulatus (Tick-borne encephalitis virus = TBEV, Anaplasma phagocytophilum, Rickettsia raoultii, Borrelia burgdorferi s.l., Babesia spp., Toxoplasma gondii), using 2 statistical methods: determination of Odds Ratios (ORs) and the Fisher's exact test. 634 questing Dermacentor reticulatus ticks (370 females and 264 males) were collected in 20112013 by flagging the lower vegetation in 3 localities in the area of Leczynsko-Wlodawskie Lakeland, situated in the Lublin region of eastern Poland. The presence of individual pathogens was detected by PCR. Ticks were infected most often with Rickettsia raoultii (43.8%), less with TBEV (8.5%), and much less with Babesia spp., Toxoplasma gondii, Borrelia burgdorferi s. l., and Anaplasma phagocytophilum (2.5%, 2.1%, 1.6% and 1.1%, respectively). The locality-dependent variability proved to be significant for TBEV (chi(2)=11.063; P=0.004) and Toxoplasma gondii (chi(2)=11.298; P=0.0035), but not for other pathogens. Two hundred seventy (42.6%) of the examined ticks were infected only with a single pathogen, and 54 (8.5%) showed the presence of dual co-infections, each with 2 pathogens. The most common were dual infections with participation of Rickettsia raoultii (7.41%); next, those with participation of the TBEV (5.21%), Toxoplasma gondii (1.58%), Borrelia burgdorferi s. l. (1.26%), Anaplasma phagocytophilum (0.95%), and Babesia spp. (0.63%). On the total number of 15 possible associations, in 9 cases co-infections occurred whereas in 6 cases they were not detected. The most noteworthy were positive co-infections with the participation of TBEV, which proved to be weakly significant (0.05 <0.1) in associations with Toxoplasma gondii and Anaplasma phagocytophilum, with Odds Ratios over 3.3 and 4.4, respectively. The values of Odds Ratios exceeded 3.0 also at the co-infections of Rickettsia raoultii with B. burgdorferi s. l., and T. gondii with Babesia spp., but these associations did not attain a significance level. The co-infections of Rickettsia raoultii with Babesia spp. appeared not to be significant (0.05 <0.1) with OR below 0.3. In conclusion, co-infections with various pathogens in D. reticulatus ticks seem to be relatively rare and mostly not significant.