Ticks - public health risks in urban green spaces

Abstract

Background: Urban green spaces are important for human health, but they may expose visitors to tick-borne diseases. This not only presents a potential public health challenge but also undermines the expected public health gains from urban green spaces. The aim of this study is to assess the public health risk of tick-borne diseases in an urban green space used for recreation in Stockholm, Sweden.

Methods: We used a mixed method approach identifying both the magnitude of the tick hazard and the extent of the human exposure to tick-borne diseases. At six entry points to an urban green space, we sampled ticks and documented microhabitat conditions from five randomly assigned 2 m × 2 m plots. Surrounding habitat data was analyzed using geographical information system (GIS). Nymphs and adult ticks were tested for Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum using TaqMan qPCR. Positive B. burgdorferi (s.l.) ticks were further analyzed by nested PCR amplification and sequence analysis. Population census data and visitor count data were used to estimate the degree of human exposure to tick-borne diseases. To further understand the degree to which visitors get in contact with infected ticks we also conducted interviews with visitors to green spaces.

Results: High tick densities were commonly found in humid broadleaved forest with low field vegetation. High pathogen prevalence was significantly correlated with increasing proportions of artificial areas. Integrating the tick hazard with human exposure we found that the public health risk of tick-borne diseases was moderate to high at most of the studied entry points. Many of the visitors frequently used urban green spaces. Walking was the most common activity, but visitors also engaged in activities with higher risk for tick encounters. Individual protective measures were connected to specific recreational activities such as picking berries or mushrooms.

Conclusions: The number of visitors can be combined with tick inventory data and molecular analyses of pathogen prevalence to make crude estimations of the public health risk of tick-borne diseases in urban green spaces. The risk of encountering infected ticks is omnipresent during recreational activities in urban green spaces, highlighting the need for public health campaigns to reduce the risk of tick-borne diseases.

Keywords: Ixodes persulcatus; Ixodes ricinus; Exposure; Habitat; Hazard; Human activity; Microhabitat; Recreation; Tick-borne diseases.

Fig. 1

Location of the study area in Sweden (a) and Stockholm (b), and positions of the 6 sampling sites in Nacka nature reserve (c)

Fig. 2

Examples of photo-documented sampling plots. The proportion of different field layer categories was used to determine microhabitat conditions

Fig. 3

Correlations between human population size close to the entry point (9 km²) and very close to the entry point (1 km²), and number of visitors, tick density and pathogen prevalence. The color shows the strength of the correlation with green color indicating a strong positive correlation and pink color, a strong negative correlation. Absence of color indicate a non-significant correlation (p > 0.05). In this correlation matrix Sickla entry point was divided into two independent sites as in the habitat models

Fig. 4

Risk of exposure to tick-borne pathogens at the differ entry points to Nacka nature reserve

Fig. 5

Activities performed by visitors in Nacka nature reserve. Foraging includes picking berries and mushrooms, and family time includes having a picknick or having family leisure time. Error bars indicate 95 percent confidence interval