Lyme borreliosis (Lyme disease) is an infection caused by the spirochete bacteria Borrelia burgdorferi sensu lato, which are transmitted by bites from infected ticks of the Ixodes genus (mainly Ixodes ricinus in Europe). Lyme borreliosis is the most prevalent arthropod-transmitted human infection in northern Europe, North America and temperate Asia. In Europe, ticks are usually found in mixed deciduous woodland or moorland areas where the microclimate supports their life cycles, and there are small mammals and birds serving as hosts. Four genospecies (B. burgdorferi sensu stricto, B. afzelii, B. garinii and possibly B. valaisiana) cause Lyme borreliosis and all have been detected in Europe. There is evidence for varying clinical presentations of Lyme borreliosis caused by these different genospecies [1].

Like syphilis, Lyme borreliosis is a multisystem infection, which has several stages and symptoms that mimic other infections. After an initial infection, which is localised to the site of the tick bite, and commonly characterised by an red rash expanding around the area of the tick bite (erythema migrans), the bacteria may spread to other sites in the body. Some patients also have non-specific influenza-like symptoms, such as tiredness, headaches, and aches and pains in the muscles and joints. Secondary sites of infection may include the nervous system, joints and heart and may lead to severe neurological complications (neuroborreliosis) and chronic arthritis [2]. Early detection of erythema migrans is crucial, as the disease is easily treated with antibiotics at this stage, preventing the development of later, more severe stages of the disease.

Detection and reporting of Lyme borreliosis
Characteristic clinical signs of Lyme borreliosis such as erythema migrans, if present, are a good indicator of the disease. Diagnosis takes into account the risk of tick exposure. Supporting evidence is provided by laboratory investigation, usually antibody tests.

Although this infection is most commonly diagnosed during the summer, coinciding with greatest tick activity, cases are reported throughout the year, probably because unspecific symptoms cause delays in both consultation and diagnoses, and also because of the long incubation period of some clinical manifestations. Different case definitions are in use throughout Europe. Misdiagnosis (mainly overdiagnosis) can occur because clinical presentations are not unique to Lyme borreliosis [2].The European Concerted Action on Lyme Borreliosis network (EUCALB, http://www.oeghmp.at/eucalb/) has published a consensus case definition [3].

Problems with comparing data in Europe
Approaches to collecting data on the incidence of Lyme borreliosis vary considerably across Europe. Very few countries have made Lyme borreliosis a mandatorily notifiable disease, and there are currently no plans to add this to the listed diseases covered by European Community-coordinated disease surveillance [4].

In the majority of countries, data is collected mainly through diagnostic laboratories reporting available details of patients with positive test results. Drawbacks of laboratory data include under-reporting of erythema migrans, varying patterns of test referrals, varying criteria for serological diagnoses and inclusion of seropositivity which may be due to past infection. Other sources of incidence estimates include voluntary reporting, GP/physician surveys, and hospital in- and out-patient diagnoses. Differences in data sources used, and in biases in those sources, make it difficult to draw meaningful comparisons between countries. At present national surveillance systems and routine diagnostic reporting is probably the best way of obtaining epidemiological data throughout Europe; however, this should be complemented by specific human seroprevalence studies.

Based on available data, the highest reported incidence of Lyme borreliosis is found in central Europe, with an estimated incidence of 206 per 100 000 population in Slovenia (based on laboratory reports) and 135 per 100 000 population in Austria (based on physician surveys) (Table). In southern Europe, incidence appears to be much lower, with an incidence of less than 1 per 100,000 (fewer than 30 cases per year) in Portugal and Italy. However, focal areas of higher incidence can occur in countries where the incidence is generally low.

Table. Reported cases or estimated cases and incidence by European country, source Eurosurveillance Editorial Advisors and others.
large variation in methods used to acquire data in different European countries

Note:
Methods used to acquire data vary in different European countries. Incidence is the number of new cases per 100 000 population per year
*estimated number of erythema migrans case-patients [5]
**voluntary reporting
***estimate based on physician survey

Trends in incidence of Lyme borreliosis
The data presented here, and also the results of some other studies, indicate that the incidence of Lyme borreliosis may be increasing in certain European countries, assuming that surveillance systems have been stable [6]. Nine of 16 European countries with time-series data available (Table) show evidence of increasing incidence of Lyme borreliosis, although these data are only for a relatively short time period of five years, and may not reflect the longer term trend. Increases have been seen in Poland, eastern Germany, Slovenia, Bulgaria, Norway, Finland, Belgium, Britain (England & Wales and Scotland) and the Netherlands. The fairly large increase that occurred between 2003 and 2004 was 44% in Belgium, 74% in Norway, 51% in Finland, and 73% in Bulgaria.

Tick numbers and activity levels affect the number of cases of Lyme borreliosis. The prevalence of ticks infected with B. burgdorferi and the incidence of Lyme borreliosis is higher in central and eastern Europe and lower in western Europe. Changes in human behaviour (in some European countries this may include visiting summer houses, hiking, mushroom or berry picking, hunting or fishing) and the ecology of tick hosts (eg, increase in populations of animals such as deer) may affect exposure of humans to ticks. Climate change, resulting in milder winters and thus more ticks, is also hypothesised to contribute to increasing Lyme borreliosis [7]. At the same time, increased awareness of ticks as vectors, and increasing interest in Lyme disease have undoubtedly stimulated protective measures and surveillance [8]. A number of other infections can also be transmitted through the bite of a hard-bodied (ixodid) tick, including ehrlichiosis, babesiosis, bartonellosis, tickborne encephalitis, tularemia and possibly Q fever. Changes in the density and geographic distribution of ticks may eventually be reflected in changes in the incidence of both Lyme borreliosis and other tickborne infections in some European countries.

Tick bite avoidance is currently the most effective prevention
There is currently no vaccine for Lyme borreliosis. Effective prevention relies on increased public education about the disease and avoidance of tick bites (eg, using protective clothing, insect repellents, and early detection and removal of ticks).

Workers in high-risk occupations, such as farming and forestry, and long-term residents of highly-endemic areas are likely to recognise ticks and to have some awareness of the symptoms of Lyme disease. Visitors from cities and non- or low endemic regions (for example, on activity holidays, walking, trekking and mountain biking in these areas) often have little or no knowledge of ticks and the disease. Physicians from these areas may be relatively unfamiliar with, and unsuspecting of, the disease. Such people may therefore be vulnerable to both infection and to under-diagnosis and development of disseminated or chronic infection after returning home.

Footnote
Websites recommended by the European Union Concerted Action on Lyme Borreliosis network (EUCALB) for public and/or professional information are:

• American College of Physicians Online. Initiative on Lyme disease. http://www.acponline.org/lyme/
• United States Centers for Disease Prevention and Control. Learn about Lyme disease. http://www.cdc.gov/ncidod/dvbid/lyme/index.htm
• National Reference Laboratory for Borreliae, Munich University. Lyme borreliosis. http://pollux.mpk.med.uni-muenchen.de/alpha1/nrz-borrelia/miq-lyme/frame-summary.html

Acknowledgements:
With particular thanks to Professor Jeremy Gray at EUCALB, Dr Reinhard Kaiser, Ms Agnetha Hofhuis, Dr Sarah Randolph, Dr Robert Hemmer, Dr Bohumir Kriz, Dr Irina Lucenko, Dr Paula Vasconcelos, Dr Mira Kojouharova, Dr Stefania Salmaso, Dr Rasa Liausediene, Dr Tanya Melillo Fenech, Dr Alenka Kraigher, , Dr Reinhild Strauss, Dr Germaine Hanquet, Dr Koen de Schrijver, Dr Ines Steffens, Dr Malgorzata Sadkowska, Dr Judite Catarino, Mr Norman MacDonald, Dr Lelia Thornton, Dr Chryso Gregoriadou, Dr Olga Kalakouta, Dr Markku Kuusi, Ms Aase Sten, Dr Eva Maderova, Dr Hilde Kløvstad, Dr Maria Grazia Pompa, Dr G Stanek