Epidemiology

Lyme disease can occur at any age, but two peak age groups have been identified: 10 to 19 years and 50 to 59 years.[6]​ Men and women are equally affected.[6]​ Onset is usually between May and October, mostly related to increased outdoor activity compared with the rest of the year.[6]​​ Early Lyme disease usually occurs in late spring and early summer, although can sometimes occur in the autumn. Lyme disease occurs in temperate regions in the northern hemisphere including North America, Europe, and Asia. High incidences are reported in endemic areas in the US (Northeast, mid-Atlantic, and upper Midwest regions), Scandinavia, Germany, Austria, Slovenia, Sweden, Russia, China, and Japan.[6]

In Europe, Lyme disease is most common in central European and Scandinavian countries. In 2006, the World Health Organization (WHO) estimated that 85,000 cases of Lyme disease occur each year in the European Union (EU), although it is likely that there is substantial underestimation of the disease burden.[7] The population-weighted incidence rate in Western Europe has been estimated to be approximately 22 cases per 100,000 per year.[8] There is a large level of heterogeneity in the incidence data between European countries, and even between different regions of the same country, however.[9] This is likely due to differences in case definitions and collection methods, as well as the fact that very few countries include Lyme as a compulsorily notifiable disease. Over- and under-diagnosis of Lyme disease is another important factor with a significant impact on the number of reported cases.[8]​​​​

Lyme disease is the most common vector-borne infection and one of the most common notifiable diseases in the US. A total of 275,589 cases of Lyme disease were reported between 2008 and 2015 in the US.[6] Of those patients, 72.2% had erythema migrans, 27.5% had arthritis, 12.5% had neurological manifestations, and 1.5% had carditis.[6]

Not every diagnosis is reported; while approximately 35,000 cases of Lyme disease are reported in the US each year, an estimate based on insurance records suggests that the true number of new cases may actually be around 476,000 annually.[10][11]

Incidence of Lyme disease appears to be increasing over time in both the US and Europe.[1]​[12]​​ Potential reasons for this include improved reporting, increased human development in wooded areas, spread of ticks to new areas, growing deer populations, and climatic changes, which result in expansion of the latitude, altitude and seasonality at which ticks are found.[1][13]​​​​​​

Co-infection with babesiosis or ehrlichiosis (anaplasmosis) may occur. This is because the Ixodes scapularis tick may also transmit Babesia microti and Anaplasma phagocytophila. One study found that in patients with Lyme disease, approximately 2% were infected with B microti and 2% were infected with A phagocytophila.[14]

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