In this retrospective study, Google AdWords Keyword Planner was used to measure the search volume of terms related to tick bites and borreliosis across Germany from January 2015 to December 2018. The Keyword Planner is often used by advertisers to improve Google marketing campaigns and provides monthly search volumes estimated by Google. The term search volume applies to the number of searches for a topic or search term. To assess search volume within a specific field, words are initially entered into the Keyword Planner; thereupon, the program provides keywords that are most relevant to the topic. This process may be used both to answer scientific questions and for medical research [10,11].

In addition, search terms related to tick bites were identified using a keyword cluster for the German words for “tick bite” (“Zeckenbiss”) and “borreliosis” (“Borreliose”). Based on this cluster, Google AdWords Keyword Planner determined search terms to be analyzed. This data included only Google users with a German internet protocol address who used the German language. Furthermore, the German Climate Data Centre [21] was used to relate Google search volume to weather data by analyzing mean monthly temperature in degrees Celsius. Due to seasonal differences in tick activity as well as tick bite incidence, we defined summer months as April to September and winter months as October to March.

In 9 of 16 federal states in Germany (Bavaria, Berlin, Brandenburg, Mecklenburg-West Pomerania, Rhineland-Palatinate, Saarland, Saxony, Saxony-Anhalt, and Thuringia) covering 42% of the total German population, mandatory notification for the three most common Lyme borreliosis manifestations (erythema migrans, acute neuroborreliosis, and Lyme arthritis) has been achieved since 2013.

To assess whether the Google search volume correlates with registered cases of borreliosis, all registered cases from the federal states of Brandenburg, Mecklenburg-West Pomerania, Saxony, and Saxony-Anhalt were considered in the analysis, as complete statistics of registered data were only available for these on the website of the German federal government agency and research institute for disease control and prevention (Robert-Koch Institut).

The search volume data of the identified search terms was assessed using Excel version 15.23 (Microsoft Corporation). To describe the relationship between the investigated variables, we used SPSS version 24.0 (IBM Corp) to calculate the Pearson correlation coefficient (r) [22].

The analysis of Google search volume related to tick bite and borreliosis identified an annual pattern that people tended to search more frequently during summer months. Therefore, a high correlation with average temperature was observed. Furthermore, a high correlation between registered cases of borreliosis in four German federal states was revealed.

One of the top five key terms of tick bite was borreliosis and vice versa. Therefore, we compared the two most common lay terms, tick bite and borreliosis. Interestingly, the search volume for the latter was higher. Especially in 2017 and 2018, a greater divergence between the keywords was observed, which can be explained by a greater awareness of associated diseases. This might be because of celebrities diagnosed with Lyme disease, such as Bastian Schweinsteiger and Justin Bieber, or because health education programs have taught people to make more accurate searches. Furthermore, media like smartphone apps and video games significantly improve knowledge of the disease and preventive measures [23,24]. However, a Finnish survey showed that, regarding knowledge, attitudes, and practice toward ticks and tick-borne disease, 65% of participants relied on newspapers and magazines as the main source of information [25]. Pharmacy health magazines, radio, or TV shows start media coverage of ticks and tick-borne diseases in late spring when average temperatures rise and ticks begin to appear. However, we did not find data to support this well-known approach.

Correlating weather data with the Google search volume showed seasonal trends, which were described in previous works concerning pruritus and identified inhabitants’ needs [12]. Especially during the German winter months October 2016 to March 2017 and October 2017 to March 2018, the Google search volume showed a distinctive increase for tick bite and borreliosis compared to data from October 2015 to March 2016. The first hypothesis was that winters get milder in Germany due to climate change so that ticks have a longer active period. Ticks are active when average temperatures are between 4-10 °C (median 7 °C) [19]. However, as during each winter, there were a comparable number of months below this temperature. This does not explain the recognizable increase in search volume that we can see. Potentially, the increase was due to the awareness of tick bites and associated diseases, as well as media campaigns starting earlier in those years.

In some German federal states, it is mandatory to report borreliosis cases. Comparing numbers from Google searches and registered borreliosis cases shows a discrepancy. For example, in Brandenburg, the highest number of registered borreliosis cases in the reviewed years was 1743 in 2017. In comparison, the 2017 Google search volume for “borreliosis” in Brandenburg was 38,200, which is 22 times higher. This might be because the number of tick bites are much greater than the development of borreliosis symptoms. Additionally, not only affected people but also their relatives might search for information online, which explains a considerably higher number of search queries.

Walker [20] posed the question of whether Google trends can be used to study parasitic (ie, tick-borne) diseases [20]. Their results showed seasonal patterns in search volume but no significant correlation between mean monthly temperature and internet searches for “tick.” Additionally, they tried to use the internet search volume to estimate parasitic occurrence. However, there was no apparent relationship between the annual number of tick-borne encephalitis cases and mean annual internet searches for either tick or tick-borne encephalitis [20].

We found statistically high correlations between registered borreliosis cases and Google search volume in four federal states. Previous studies identified Google data as a predictor of infectious disease outbreaks [26,27]. Nevertheless, to the best of our knowledge, this is the first work that shows a high correlation between incidence of an infectious disease and Google search volume of the implied disease. These results could help estimate incidences of borreliosis in German federal states where registration is not mandatory. Furthermore, Google search volume could be used to estimate incidences of diseases that are not required to be reported.

This study has some limitations. In Germany, Google accounts for 95% of search engine use, so Google data can depict the interests of the population as a whole. To transfer our findings to other countries, different market shares of Google over alternative search engines need to be taken into account. Although it is common among the whole population to make health-related searches, younger people tend to use the internet more often [28]. Furthermore, the automatic completion of search terms by Google may influence people’s search behavior. It may promote an understanding of the health problem and the need to seek necessary medical help; however, priming by autocomplete has the potential to make incorrect associations [29]. Another limitation to our study is that we solely used German key terms.

Our study provides insight into terms and fields of interest associated with tick bites and borreliosis, relevant to the German population. We found statistically high correlations between Google searches for borreliosis and registered cases of borreliosis across four German federal states. Accordingly, these results could help to estimate the incidence of borreliosis in the remaining 12 German federal states where it is not mandatory to report borreliosis. Furthermore, this approach could aid in the development and implementation of effective and sustainable awareness campaigns.