Risk for Human Tick-Borne Encephalitis, Borrelioses, and Double Infection in the Pre-Ural Region of Russia

Edward I. Korenberg, Gamaleya Research Institute for Epidemiology and Microbiology, Russian Academy of Medical Sciences, Moscow, Russia; Lidiya Ya. Gorban', Perm Center of State Sanitary-Epidemiologic Inspection, Perm, Russia; Yurii V. Kovalevskii, Gamaleya Research Institute for Epidemiology and Microbiology, Russian Academy of Medical Sciences, Moscow, Russia; Vladimir I. Frizen, Perm Center of State Sanitary-Epidemiologic Inspection, Perm, Russia; Andrei S. Karavanov, Chumakov Institute of Poliomyelitis and Viral Encephalites, Russian Academy of Medical Sciences, Moscow, Russia.

Disclosures

Emerging Infectious Diseases. 2001;7(3) 

In This Article

Abstract and Introduction

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.

Tick-borne encephalitis (TBE) and infections of the Lyme borreliosis group, or ixodid tick-borne borrelioses (ITBB)[1], are widespread in Russia. In 1996, 1997, and 1998, cases of these diseases totaled approximately 16,650, 11,350, and 14,700, respectively. Most cases were acquired from the bite of an adult Ixodes persulcatus tick. In some regions west of the Volga River, the I. ricinus tick may also transmit these infections. The geographic distribution and epidemiology of TBE and ITBB in Eurasia are almost identical[2].

These data suggest that double infection by TBE virus and Borrelia may result from transmission of both pathogens from double-infected ticks to humans[2]. Such ticks are present in natural foci, and the occurrence of TBE virus and Borrelia is independent in ticks rather than mutually exclusive. The prevalence of Borrelia in unfed I. persulcatus ticks with and without TBE virus is virtually identical, and the same is true for TBE virus prevalence in ticks with and without Borrelia infection. Concentrations of virus and Borrelia in double-infected ticks are not correlated. In the natural mixed foci of TBE and ITBB, interannual changes in the prevalence of virus and spirochetes in ticks are virtually parallel. The coexistence of these microorganisms in their principal vectors, which promotes simultaneous infection in humans bitten by ticks, is apparently an important precondition for the relative autonomy of conjugate parasitic systems formed by TBE virus and Borrelia[3]. However, prevalence of both pathogens in ticks collected from vegetation has not previously been compared with their prevalence in ticks removed from humans.

Cases of double infection were identified for the first time during 1986 to 1988 in Austria and northwestern European Russia[4,5]. They have been described in the species range of the I. ricinus tick[6] but occur more frequently in areas where I. persulcatus ticks are abundant[7,8]. We describe long-term studies assessing the risk for human double infection compared with that for separate TBE and ITBB infection in a region with a consistently high level of both diseases[1].

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