Analysis of the long-term dynamics of tick-borne encephalitis (TBE) and ixodid tick-borne borrelioses (ITBB) morbidity in Russia

As early as in the 1960s, Professor M.P. Chumakov suggested that primary tick-borne encephalitis (TBE) diagnoses made by practicing physicians should be verified by serological or virological methods. Verification showed that a large proportion of TBE cases (included in the official statistics of TBE morbidity) was accounted for by the so-called seronegative forms of the disease. Ixodid tick-borne borrelioses (ITBBs), or infections of the Lyme borreliosis group, which proved to be sympatric to TBE in Russia accounted for an even higher morbidity in past years. Some other tick-borne diseases were still unknown at that time. We analyzed 1062 case histories of patients with febrile diseases acquired through tick bites in the Udmurt Republic, Russia (one of the most hazardous regions with respect to TBE morbidity), recorded from 1965 to 1968, and 1509 similar cases recorded from 1983 to 1988. In the first period, 97 out of 684 cases (14.2%) included in the official statistics of TBE morbidity could be classified as ITBB according to modern case criteria. In the second period, 464 out of 1509 cases recorded as TBE (30.7%) were retrospectively diagnosed as ITBB. No specific diagnosis could be made in 7–17% of the total number of cases interpreted as ‘tick-borne’ diseases. A similar situation existed in other regions of Russia before 1996 and, as a consequence, the official parameters of annual TBE morbidity in different periods were overestimated by a factor of 1.2–1.6. Here, the long-term dynamics of TBE morbidity in Russia are presented as they appear according to official data and after correction with regard to estimations.