Risk for human tick-borne encephalitis, borrelioses, and double infection in the pre-Ural region of Russia.

We assessed the risk for human tick-borne encephalitis (TBE), ixodid tick-borne borrelioses, and double infection from 1994 to 1998 in Perm, which has among the highest rates of reported cases in Russia. We studied 3,473 unfed adult Ixodes persulcatus ticks collected from vegetation in natural foci and 62,816 ticks removed from humans. TBE virus and Borrelia may coexist in ticks.

The prevalence of Borrelia burgdorferi sensu lato in Ixodes persulcatus and I. ricinus ticks in the zone of their sympatry.

A total of 7210 unfed adult Ixodes persulcatus Schulze, 1930 and I. ricinus (L., 1758) ticks were collected from the vegetation by flagging in 35 study sites located in the zone of their sympatry (mainly in Leningrad region, Russia). Borrelia infection in ticks was estimated by the dark-field microscopic analysis of gut contents in standard vital preparations at a magnification of x600. No correlation was revealed between the series of parameters characterising the abundance of each tick species (tau = -0.13) and between the series of these parameters and the prevalence of Borrelia in each vector. It is concluded that in the broad zone of I. persulcatus and I. ricinus sympatry, the presence and proportion of one vector in the ecosystem does not have any significant effect on the extensity of infection and on the epizootic and epidemic significance of the other vector. Each tick species has its independent (of the other species) and relatively original functional role in the focal ecosystem.

Mixed infection by tick-borne encephalitis virus and Borrelia in ticks.

To investigate the relationships between tick-borne encephalitis (TBE) virus and the bacterial spirochaete Borrelia burgdorferi sensu lato in vectors with mixed infections, unfed adult Ixodes persulcatus ticks were collected by flagging from vegetation in southern-taiga forests of the Pre-Urals region of Russia where both infections circulate sympatrically. Prevalences of TBE and Borrelia infections in a total of 4234 ticks were compared over 5 years. No significant differences were revealed between the prevalence of Borrelia infection in ticks with and without TBE virus (29.4+/-7.8% vs 23+/-3.6%), or between the prevalence of TBE virus infection in ticks with and without Borrelia (24.0+/-6.6% vs 18.4+/-3.4%). In ticks with mixed infection (40/689 = 5.8%), concentrations of TBE virus and Borrelia were not significantly correlated with one another. Field observations showed parallel trends in the prevalence of these pathogens in tick populations from year to year (1993-1997) indicating that, in I. persulcatus with mixed infection, Borrelia and TBE virus do not seem to interfere with each other and are apparently not involved in any antagonistic relationships.

Main features of tick-borne encephalitis eco-epidemiology in Russia.

Tick-borne encephalitis (TBE) is a classic obligatorily transmissible viral infection with natural focality, which is widespread mainly in Eurasian forests of the temperate zone. Virus was discovered in the Russian Far East in 1937. Its main long-term reservoirs and vectors in natural foci are Ixodes persulcatus and I. ricinus ticks. Boundaries of the virus range and the location of natural foci within it are closely associated with the distribution pattern of these ticks. The largest categories of TBE range zoning are described. Each natural focus functions as a relatively autonomous parasitic system. The principal scheme of TBE virus circulation related to the development of three successive generations of the main vector is presented. The main parameter of epizootic activity and epidemic manifestation of natural foci and their long-term fluctuations in different regions are discussed. TBE morbidity in Russian and its dynamics during approximately 45 years are analysed.

Small mammals as reservoir hosts for Borrelia in Russia.

In 1992-1993, a total of 29 isolates of Borrelia were obtained from small mammals captured in taiga forests of the Perm' region: 26 isolates were obtained from Clethrionomus glareolus and one each from C. rufocanus, Microtus oeconomus, and Apodemus sylvaticus. Isolation of Borrelia was performed by plating the animal material on BSK-II medium. Most isolates (86.2%) were obtained from the urinary bladder, the rest from the heart and spleen. According to the results of IFA with a set of monoclonal antibodies, isolates did not contain Borrelia burgdorferi sensu stricto. Preliminary data suggest that they can belong to B. garinii and B. afzelii. On the basis of the abundance of animals, it is suggested that C. glareolus, a background species among small mammals inhabiting the study region, is the main reservoir host for Borrelia.

Differences in Borrelia infections in adult Ixodes persulcatus and Ixodes ricinus ticks (Acari: Ixodidae) in populations of north-western Russia.

Dark-field microscopy was used to determine the number of Borrelia spirochetes in 630 standard preparations obtained from adult ixodid ticks (344 Ixodes persulcatus and 286 I. ricinus) collected in 1989-92 in the Leningrad region of Russia. The average numbers of Borrelia in I. persulcatus and I. ricinus preparations were 34.7 and 23.3 per 100 microscopic fields, respectively. The maximal individual values registered each year for ticks of both species were several hundred times greater than the minimal values. Ticks carrying relatively small numbers of Borrelia generally predominated. Proportions of more heavily infected ticks varied considerably from year to year. These parameters were significantly higher in foci with predominance of I. persulcatus ticks. As a consequence, risk to acquire Lyme borreliosis in such foci is considered greater than in foci where I. ricinus predominates.