Lyme borreliosis is an endemic zoonotic disease in Estonia. Infections are typically observed between April and October depending on the seasonal activity of ticks. The distribution of the majority of cases coincides with the distribution of Ixodes ticks. Endemic foci exist in Western, Eastern and Southern Estonia.
Lyme disease has been a notifiable disease since 1992 in Estonia. Research concerning the endemic foci of the disease started in 1988. The following gives an overview of the epidemiology of Lyme borreliosis in Estonia.
Material and methods
Ticks were collected by the flag-method. Smears of tick intestines were investigated for Borrelia burgdorferi sensu lato using dark-field microscopy. In 1988 – 2000 a total of 12,535 Ixodes ticks collected all over the country were investigated: 7,967 were Ixodes ricinus (5,105 larval and 2,862 nymphal) and 4,568 Ixodes persulcatus (4,213 larval and 355 nymphal) ticks. All ticks collected were registered using the Universal Transverse Mercator (UTM) grid system.
Lyme disease diagnosis was currently based on the typical clinical syndrome Erythema chronicum migrans and confirmed by serological analysis (ELISA).
In 1992 – 2000 a total of 2,595 Lyme disease cases were reported. 1,499 persons were questioned to identify risk areas and regions.
Results and discussion
Ixodes ricinus biotopes can be found all over the country. Ixodes persulcatus is distributed only in Eastern and Southern Estonia while there are also several areas of mixed distribution.
Three percent of the Ixodes ricinus adult ticks and 5% of the Ixodes persulcatus adult ticks investigated were infected with B. burgdorferi. In some geographical areas the contamination of ticks was higher: 31% in Narva-Jöesuu (NE58), 12% in Western Virumaa (MF47) and 14% in Tartumaa (ME58). The contamination of nymphal ticks was less extensive: 0.4% Ixodes ricinus and 1.4% Ixodes persulcatus.. These results indicate the possibility of transovarial transmission of borrelias.
The incidence rate of Lyme disease was annually on average 22 per 100,000 population. The illness started typically in April and continued until October depending on the seasonal bioactivity and abundance of the tick population in different areas. Most affected persons were women (67%) and aged over fifty (47%) The areas of highest risk are currently concentrated along the Finnish Gulf coast in Harjumaa and Western Virumaa, in Pärnumaa and Eastern Virumaa counties and on the island of Saaremaa. These areas are also risk areas for tick-borne encephalitis. In 1998 – 2000 37 cases of Lyme borreliosis and tick-borne encephalitis virus mixed infections have been reported. These cases have been caused by bites from doubly-infected ticks, predominantly in endemic foci along the Finnish Gulf coast.
Fig 1. Lyme disease endemic foci, 1993-2000
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