Tick-borne Encephalitis and Lyme Disease Epidemiology in Saint-Petersburg
Published: 17.11.04 Updated: 17.11.2004 11:26:45
L.P. Antykova, V.I Kurchanov
State Sanitary-Epidemiological Surveillance Centre, Saint-Petersburg, Russia
The tick-borne encephalitis (TBE) and Lyme disease (LD) morbidity of residents of Saint-Petersburg is connected to the presence of natural foci in the Leningrad region: 80,3% of the TBE cases and 74% of the LD cases have been infected in the region. Ticks carrying TBE virus and Borrelias range from 5 to 27% and from 20 to 46%, respectively. Mixed TBE and LD infections were notified in 15% of the TBE cases
The formation or activisation of anthropurgic natural-focus diseases is a peculiarity of the urbanisation process. Of such diseases, Tick-borne encephalitis (TBE) and Lyme disease (LD) are found in Saint-Petersburg and its suburbs. This is connected to the intensive recreational-domestic activity of the population.
The first confirmed case of TBE in Saint-Petersburg was registered in 1940. During the post-war period between 5 and 15 cases were registered in the city annually. From 1963 to 1972, 152 cases of TBE were reported, and 30-65 cases a year were registered in the following years. Laboratory tests for LD have been performed at medical institutions in Saint-Petersburg since 1988.
Materials and methods
The incidence of TBE and LD were analyzed for the years 1985 to 2001. Ticks were tested for antibodies to TBE virus and the presence of Borrelia with an ELISA test. Ticks were examined for the presence of Borrelia by dark field microscopy of native specimens.
Patients’ serum samples were analyzed with an ELISA test and by hemagglutination inhibition (HIA). LD was diagnosed on the basis of clinical symptoms and the results of blood serum analysis in an indirect immunofluorescence test with corpuscular Borrelia antigen.
Results and discussion
Non synchronous peaks every 3 to 4 years characterize the incidence of TBE in Saint-Petersburg (fig. 1).
Fig 1. Number of cases of Tick-borne encephalitis in Saint Petersburg 1985-2001
916 cases of TBE were registered in the city from 1985 to 2001, including 216 cases among children (23%). 80.3% of the patients had been infected in the Leningrad region, while 16.6% of the patients were infected in other regions of Russia.
4.572 cases of LD were registered in Saint-Petersburg from 1985 to 2001, of which 448 were among children (10.6 %). The number of cases differs between the years from 80 to 700, but there was no tendency in peak incidence levels (fig. 2).
Contact with ticks was reported in 74% of the cases in Leningrad region, in 9 % in the suburbs of Saint-Petersburg and in 17% in other Russian regions. TBE and LD infection in Leningrad region occurred in six districts and was associated with the peculiarities of recreational-domestic activity.
The territories where TBE and LD infections occurred almost fully overlap. The main carriers of the infection in suburb foci are Ixodes persulcatus and Ixodes ricinus.
The presence of humoral antibodies to TBE virus among the local population differs from 9.9% to 30.6%. Ticks bearing TBE virus and Borrelias range from 5 to 27% and from 20 to 46%, respectively. Consequently, the infection among inhabitants of Saint-Petersburg by TBE and LD is connected to the presence of natural foci in the Leningrad region.
Registration of the TBE and LD incidence among the population of Saint-Petersburg suburbs began in 1971. Twenty eight patients with TBE and 415 cases of LD were registered from 1985 to 2001. Ixodes ticks are common in forest and forest-bush landscapes of suburbs, the main carrier among them being Ixodes persulcatus. In the districts along the coast of the Finnish Gulf and to the south and lower streams of the Neva-river, there have been a low number of Ixodes ticks for many years. Over the last three years, the number of ticks has increased from 0.5 to 12 ticks per 1 flag-hour in the period of their maximum activity.
In the districts along the northern coast of the Finnish Gulf where the main recreational zones of the city are situated, average figures of ticks differ from 7 to 25 ticks per 1 flag-hour during the year. In the suburbs of Saint-Petersburg the share of ticks with TBE viral antigen varied from 1 to 3%. The percentage of ticks infected by Borrelias ranged from 4 to 37%. Humoral immunity to TBE virus among the local population of Saint-Petersburg suburbs was found in 4.3 to 17.5% of those examined.
Cases of TBE and LD are registered in the city from April to October with the maximum level in May-June, which correlates with the peak of Ixodes persulcatus activity. Eighty two percent of the TBE and 94% of the LD patients mentioned being bitten by ticks. In 12% of the patients there was a nutritional route of contamination after drinking fresh goat milk.
Among patients with TBE, persons aged 20-39 (29.6%) and 40-59 (28.3%) predominated, whereas the majority of patients with LD were bewteen the ages of 40-59 (42.7%) or 60 years and older (27.9%).
Fig 2. Number of ixodus tick borreliosis 1985-2001
The clinical picture of TBE was dominated by meningitis (57.2%), meningoenchephalitis (9.3%), encephalitis (1.6%), poliomyelitiformis (1.9%) and fever-formis (29.6%). No patients were vaccinated against TBE. Deaths due to TBE were registered in 12 cases during this period: 10 cases of meningoenchephalitis and 2 cases of poliomyelitiformis forms. The TBE diagnosis was confirmed by serology in 75% of the cases. LD diagnosis was confirmed in 79 to 94% of the cases. Mixed infections of TBE and LD were registered in 138 patients, accounting for 15% of the TBE cases.
91.240 patients asked for medical attention for tick bites during the period from 1985 to 2001. The correlation between those who suffered from tick bites to TBE and LD patients was 110:1:6.
According to the reported data it appears necessary to investigate the epidemiology of TBE and LD in the conditions of a big city with it’s zone of recreational-domestic influence further in order to find the best possible means of prevention.