[Prevalence and Diversity of Tick-Borne Pathogens from Central and Eastern Europe well as West Africa]. (Doctoral thesis)
- Clinical and Applied Virology
Ticks are important vectors of human and animal pathogens and are endemic in tropical, subtropical and temperate regions throughout the world. After mosquitoes, ticks are the second most important vector of human diseases worldwide, responsible for more than 100,000 cases of tick-borne diseases annually (42). Furthermore, ticks are the main arthropod vector of disease in wild and domestic animals throughout the world with estimated minimal economic losses of US$ 7 billion annually (160, 226).
In Europe the most important tick-borne diseases in humans are Lyme Borreliosis and Tick-Borne Encephalitis (TBE), both causing tens of thousands of severe infections in humans annually (46, 94, 127). In Africa, the most important health threat caused by tick-borne pathogens is to livestock animals and major economical losses have been associated with the four tick-borne diseases anaplasmosis, heartwater, babesiosis and theileriosis (10).
In the framework of the present dissertation we analysed the prevalence of tick-borne pathogens in questing and feeding ticks from Western Europe (Luxembourg), Eastern Europe (Belarus, Bulgaria and the Kaliningrad enclave) and sub-Saharan Africa (Nigeria).
In Luxembourg, 8,104 larval, nymphal and adult Ixodes ricinus ticks were collected from the vegetation during the activity period of ticks in the years 2007, 2008 and 2009. A total of 33 collection sites were visited on a monthly basis from May to October. Significant variations of density of immature tick instars were observed in the three consecutive years, whereas the density of adult ticks remained similar. We hypothesized that the interplay of abiotic and biotic parameters is responsible for the observed population dynamics of Ixodes ricinus. Altogether 5,638 nymphal and adult ticks were analysed for the presence of all human tick-borne pathogens relevant in Europe using specific detection PCRs. The mean infection rate of ticks were 16.3% for Borrelia burgdorferi sensu lato, 6.7% for Spotted Fever Group (SFG) Rickettsia species, 1.8% for Babesia species, 0.9% for Anaplasma phagocytophilum and 0.1% for Bartonella species. We also found the non-endemic dog pathogen Hepatozoon canis in a female Ixodes ricinus tick, which possibly was introduced accidentally by the import of an infected dog from Southern Europe (6). Neither Tick-Borne Encephalitis (TBE) virus, nor Coxiella burnetii nor Francisella tularensis subspecies were detected in Luxembourgish ticks. Mixed infections were detected in 3.3% of all ticks and mainly involved the predominant Borrelia and Rickettsia species. Seasonal variations of tick infection rates were observed for the different Borrelia species each year, albeit most clearly in 2007. Our hypothesis that the observed decrease of the tick infection rate during summer months reflects a behavioural adaptation strategy of infected questing ticks seems to be supported by a recent study on the influence of Borrelia infection on the survival rate of ticks (89). Furthermore, we observed a positive correlation between the grade of urbanization and the Borrelia infection rate of ticks, suggesting an established urban zoonotic cycle. The significant interannual, seasonal and regional variations in the density of ticks and the prevalence of Borrelia burgdorferi s.l. and the other tick-borne pathogens entail that likewise significant changes in the risk of tick bites and infection are to be expected. The observed variations seem to be linked to abiotic and biotic factors, but further studies on the dynamics of ticks and tick-borne pathogens are warranted.
A seroprevalence study was conducted on samples from 280 forestry workers from Luxembourg and 35.4% displayed specific anti-Borrelia burgdorferi s.l. IgG antibodies. Evaluation of questionnaires revealed that age, hours spent outdoors, the number of tick encounters and the number of tick bites per year were important risk factors for exposure to spirochetes. The use of preventive measures like tick repellents, frequent body inspection and early removal of ticks did not seem to reduce the risk of exposure, whereas protective clothing seemed to have a slight beneficial effect. The regional variations in the seroprevalence rate of forestry workers matched to a certain extent with the Borrelia infection rate of questing ticks. However, comparison of the two distributions is difficult, as IgG antibodies can persist for years and infection was not necessarily obtained during working hours. According to our findings, Lyme Borreliosis is a major health concern in professional risk groups in Luxembourg.
In Nigeria, 836 ticks were collected from the vegetation and cattle. Four tick species were found to infest cattle (Rhipicephalus (Boophilus) annulatus, Amblyomma variegatum, Hyalomma impeltatum, Rhipicephalus evertsi), whereas only the latter species was collected from the vegetation. Predominant pathogens were the Rickettsia species closely related to the human pathogenic Rickettsia africae and the cattle pathogen Anaplasma marginale. Theileria mutans and Coxiella burnetii, which both are known to infect ruminants, were detected only in feeding ticks, whereas a potentially new member of the Borrelia burgdorferi s.l. group was detected only in questing ticks. Based on our finding that the diversity of pathogens was significantly higher in feeding than in questing ticks we hypothesized that Nigerian cattle may serve as an important reservoir for at least some of the detected pathogens. The impact of tick-borne infections on human and animal health and the resulting economic losses need to be further assessed.
In Belarus, 553 Ixodes ricinus and Dermacentor reticulatus ticks were collected from the vegetation and cattle and the pathogen diversity was found to be higher in questing than in feeding ticks. Rickettsia species were detected in 24.4%, predominantly a species related to the Rickettsia rickettsii group were found. This species was responsible for the high Rickettsia infection rate of 43.8% in D. reticulatus ticks, suggesting high rates of transovarial transmission. Borrelia burgdorferi s.l. was detected in 9.4% of ticks, whereas the other pathogens Anaplasma phagocytophilum, Coxiella burnetii, Francisella tularensis subspecies, species of Babesia and Bartonella were less frequently detected. We identified hotspots with high tick infection rates, which should be included in future surveillance studies, especially when Francisella tularensis and Coxiella burnetii are involved. Our survey revealed a high burden of tick-borne pathogens in questing and feeding I. ricinus and D. reticulatus ticks in different regions in Belarus, indicating a potential risk for humans and animals.
We also reported on the identification of two new Francisella-like endosymbionts (FLE), found in three different tick species from Bulgaria. The FLEs were molecularly characterized and seem to be facultative secondary endosymbionts of ticks. However, further studies are necessary to determine the pathogenic potential of these species.
In order to investigate the role of migratory birds in the spread of SFG Rickettsiae and Babesia species, 236 wild birds comprising of 8 species of Passeriformes were collected at Curonian Spit in Kaliningrad enclave of North-Western Russia. In total, 126 ticks were removed and analysed for the two pathogens. Rickettsia species were detected in 15.1% and Babesia species in 1.6% of ticks. The survey indicates that migratory birds may become a reservoir for Babesia spp. and SFG Rickettsiae and are involved in their geographic dispersal. Future investigations need to characterize the role of birds in the epidemiology of these human pathogens in the region.
Our studies highlight the need for continuous surveys on the prevalence, distribution and spread of tick species and tick-borne pathogens. In the course of climatic change permanent changes in abiotic prerequisites like temperature and relative humidity may enhance survival of non-endemic tick species. Timely detection of invasive tick species and application of countermeasures can possibly prevent the establishment of exotic zoonoses. Tick surveillance is an important measure for the better understanding of the epidemiology of tick-borne pathogens and the population dynamics of the main vector ticks. However, the establishment of guidelines for the design of future studies is warranted.