Eurosurveillance

ECDC

Increase in Incidence of Tick-borne Encephalitis in Lithuania in the 1990s

 Rediger
 1 Published: 07.05.04 Updated: 20.08.2004 10:23:43
A.Juceviciene1, A Laiskonis1, P.Leinikki2 1 Kaunas Medical University of Lithuania, Clinic of Infectious Diseases, Kaunas, Lithuania, 2 Department of Infectious Disease Epidemiology, National Public Health Institute (KTL), Helsinki, Finland
Neurological infections of tick-borne encephalitis virus (TBEV) etiology cause significant morbidity in Lithuania. Several hundred clinical cases verified by laboratory diagnosis are reported annually. While the number of reported cases have increased rapidly during the 1990s, the rise may have been due to improved laboratory methods and increased awareness among both the public and the doctors.

Hospital records from four hospitals were examined for three different categories, bacterial meningoencephalitis or meningitis, viral neuroinfections and tick-borne encephalitis (TBE). The overall numbers of bacterial infections remained rather constant while viral neuroinfections increased significantly. A sharp increase of TBE cases was observed between 1992 and 1993 and the increase continued in the 1990s. Significant differences were observed between the hospitals.

The results of the investigation indicate a significant increase in the occurrence of clinically manifested TBE cases in the 1990s. This increase may be due to change in the risk of infection, climatic changes, and warrants a re-evaluation of the current vaccination policy.

Introduction

Notification data show an increase of TBE cases in several countries in the Baltic Sea region (1). In Lithuania the increase started in the early 1990s and was most significant in the years 1993-1995. At the same time, commercial enzyme immunoassays (EIA) became available for the specific diagnosis of the disease and replaced the other more conventional and cumbersome tests. It is also possible that during the concomitant reform of the health system, changes in clinical practice or case definitions may have influenced the reported incidence of TBE.

On the other hand, several other factors may have increased the risk for TBEV infections. Climatic changes have been documented during the 1990s that may have affected the distribution of ticks (2). A link has been reported between certain climatic features and the increase in TBE cases in Sweden (1). Socioeconomic changes in Lithuania during the transition period have resulted in the dissolution and reorganisation of the agricultural and industrial structures and increased unemployment. This may have influenced disease epidemiology through behavioural changes of people and families.

TBE has remained geographically restricted; certain areas are well-known "hot spots" of infection while new risk areas have been detected only on occasion (3,4). A vaccine is available and has been recommended to people living or visiting areas known to carry high risk for TBE (5,6). If the virus is detected in new areas, the vaccination policy must be adjusted.

We have analyzed data concerning neurological infections and other clinical conditions from clinical records in four hospitals in Lithuania in 1984-1999. Hospital records indicate that real increase in the incidence of TBE has occurred during the 1990s in Lithuania.

Material and methods

Hospital records from Kaunas Clinic of Infectious Diseases, Vilnius Hospital for Infectious Diseases, Panevezys Hospital for Infectious Diseases and Department of Infectious Diseases, Siauliai Hospital were investigated. Numbers of patients with a diagnosis of neurological infection were collected from four hospitals and patient records with a discharge diagnosis of neurological infection were examined. The number of patients having viral meningoencephalitis or meningitis, tick-borne encephalitis, or bacterial meningitis were collected from all hospitals in 1984-2000. The TBE case definition had remained the same throughout the observation period and included typical clinical picture, relevant epidemiological information and laboratory examinations. Since 1994, specific laboratory diagnosis was confirmed in most cases by demonstrating antibodies (a significant increase in antibody titres between acute and convalescent sera, a single high titer or demonstration of antibodies in CSF) by the haemagglutination inhibition (HI) method and a commercially available EIA test. Vilnius was the reference laboratory. In 1996, a diagnostic laboratory was established in Kaunas and tests were thereafter performed in this facility. Various hospitals implemented the tests in slightly different way. In Vilnius HI and EIA (Immunozym, Progen Immuno tests) were run in parallel until 1996 after which only EIA was used. In Kaunas an EIA for total antibodies was used from 1997. IgM antibodies were tested from 1998. Since 2000, all hospital laboratories started to use IgM EIA.

Results

There has been a significant increase in the number of hospitalised patients with neurological infections during the 1990s (Fig 1).

Fig 1. Annual number of reported cases of tick-borne encephalitis (TBE), bacterial meningoencephalitis (BME) and viral meningoencephalitis (VME) in Vilnius, Kaunas, Panevezys and Siauliai districts of Lithuania, 1984-2000

Cases of viral meningoencephalitis increased sharply from 1992 to 1993 with two peaks in 1997 and in 1999. Laboratory data suggest that these peaks were due to outbreaks of enteroviral infections that were observed concomitantly in other European countries (7).

Annual numbers of bacterial meningoencephalitis cases remained constant from year to year in the observation period. TBE "appeared" to the statistics in 1993 and has increased year by year ever since. In Vilnius, which contributes almost half of the annual cases in the country, a very modest increase was observed while in Kaunas, Siauliai and Panevezys a high increase was reported (Table 1).

Table 1. Comparison of the number of annual cases of tick-borne encephalitis, bacterial meningoencephalitis and viral meningoencephalitis in two major hospitals in Lithuania, 1984-2000

Year

Bacterial meningoencephalitis

Viral meningoencephalitis

Tick-borne encephalitis

 

Kaunas

Vilnius

Kaunas

Vilnius

Kaunas

Vilnius

1984

16

32

121

48

17

0

1985

16

27

41

54

2

0

1986

5

23

37

58

7

0

1987

8

13

104

122

7

0

1988

6

10

118

95

0

0

1989

4

7

86

31

4

0

1990

7

17

55

25

3

0

1991

7

32

42

39

5

0

1992

27

19

45

37

2

0

1993

19

23

92

48

50

0

1994

21

29

111

58

34

7

1995

23

17

127

44

110

7

1996

16

26

154

50

97

15

1997

27

21

151

118

148

38

1998

24

21

115

118

146

31

1999

21

11

145

127

90

13

2000

22

21

144

76

175

10

There is no indication that changes occurred due to the availability of specific laboratory tests. In 1993-1995 all samples were sent to the Vilnius laboratory. Since 1997 tests for the Kaunas district were performed in the laboratory at the Kaunas Infectious Disease Hospital using the same commercial test (Table 2).

Table 2. Application of laboratory testing for the diagnosis of tick-borne encephalitis in Kaunas Hospital in 1990-2000

Year

Method used*

No of TBE cases

1990

HI**

3

1991

HI**

5

1992

HI**

2

1993

HI**

50

1994

HI and EIA***

34

1995

HI and EIA***

110

1996

HI and EIA***

97

1997

EIA/total

148

1998

EIA/IgM

146

1999

EIA/IgM

90

2000

EIA/IgM

175

* All TBE cases until 1997 were confirmed in the Vilnius laboratory

** Hemagglutination Inhibiton

*** Hemagglutination Inhibition, EIA on HI-reactive samples

In 1997, 1998 and 2000 the disease was particularly prevalent. Comparison of the patient records and, in particular, the lumbar puncture tests did not indicate any changes in the practice of requesting specific laboratory tests.

Discussion

Our analysis suggests that in the Kaunas region there was a significant increase in the incidence of neurological infections in the 1990s. A similar increase was observed in Panevezys and Siauliai regions. The case definition was based on clinical features and lumbar puncture and did not change during the observation period indicating that the increase of reported cases was real. A recent sero-epidemiological study lends support to this conclusion. The age-specific sero-positivity rate did not increase in a population-based survey that could be explained by an increased exposure of younger people (7). An possible explanation could be climatic changes that influenced the infectivity of ticks (8).

A similar increase could not be observed in the Vilnius region suggesting that the increase was geographically restricted. The disease seems to remain in the same areas over long periods; new foci are occasionally observed (6). This has been well documented for instance in Finland, where Åland has remained a "hot spot" for decades (9).

Several factors may have contributed to this increase. First, a change in the ecological system may have increased the number of infected ticks in certain areas. An analysis of various tick species in Lithuania was recently published (10). Further studies are needed to assess whether changes in tick ecology have taken place in Lithuania during recent years. Clinically manifested cases of borreliosis have also increased in the Kaunas region (personal communication).

TBE is a prevalent disease in Lithuania causing annually several hundred clinical cases as reported to the epidemiological centre. A considerable proportion of patients suffer from short or long term sequelae increasing the burden imposed to the health care system. An effective vaccine is available, but it is expensive and needs frequent boosting thus increasing the total costs and need for an effective infrastructure. An alternative strategy would be to use selected vaccinations based on assumptive risks for infection in different occupations or areas of residence. To assess the feasibility of this approach, a careful mapping of the incidence and prevalence of the disease in Lithuania is needed. Cows and sheep could serve as sentinels to map the locations of greater than average risk areas.

References

  1. Lindgren E and Gustafson R. Tick-borne encephalitis in Sweden and climatic change. Lancet 2001; 358: 16-18.
  2. Randolph SE and Rogers DJ. Fragile transmission cycles of tick-borne encephalitis virus may be disrupted by predicted climate change. Proc R Soc London B 2000; 267: 1741-44.
  3. Kaiser R. The clinical and epidemiological profile of tick-borne encephalitis in southern Germany 1994-98: a prospective study of 656 patients. Brain 1999; 122: 2067-78.
  4. Haglund et al. A 10-year Follow-up study of TBE in the Stockholm Area and a review of the literature: Need for a vaccination strategy. Scand J infect Dis 1996; 28: 217-24.
  5. International scientific working group on TBE. http://www.tbe-info.com
  6. Immuno Ag . Tick-borne encephalitis (TBE) and its immunoprophylaxis. Vienna: Immuno Ag, 1997.
  7. Juceviciene A et al: Seroprevalence of tick-borne encephalitis virus in Lithuania. J Clin Virol. In press.
  8. Randolph SE, Gern L and Nuttall PA. Co-feeding ticks: epidemiological significance for tick-borne pathogen transmission. Parasitology Today 1996; 12: 472-9.
  9. Brummer-Korvenkontio M, Saikku P, Korhonen P. Isolation of tick-borne encephalitis virus from arthropods, vertebrates and patients. Amer J Trop Med Hyg 1973; 22: 382-9.
  10. Zygutiene M. The entomological and acarological situation in Lithuania. EpiNorth 2001; 2: 10-11.

Acknowledgement

AJ was the recipient of a research grant from the Nordic Council of Ministers through the project "Infectious Disease Control in the Barents and Baltic Sea Regions". We thank S. Panavas, E. Preidys, A. Gulbiniene, and I. Ru_kuviene for valuable help in collecting data from the hospitals of infectious diseases in Lithuania and V. Kilciauskiene for consultation regarding laboratory testing.


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