A STING from a Tick : Epidemiology, Ecology and Clinical Aspects of Lyme Borreliosis

Lyme borreliosis (LB) is the most common tick-borne disease in the Northern Hemisphere and the number of LB cases is increasing. The infection is caused by spirochetes belonging to the Borrelia burgdorferi sensu lato complex, and is, in Europe, transmitted to humans by Ixodes ricinus ticks. To gain a deeper knowledge of the interactions between ticks, humans and Borrelia bacteria, we investigated temporal differences in exposure to tick bites in different parts of Sweden and the Åland Islands, Finland during the years 2008 and 2009. We also investigated the site of tick attachment on the human body and the time it takes for a person to detected and remove such ticks. Furthermore, the distribution of Borrelia species and the number of Borrelia cells in the ticks were investigated. Sera taken from the tick-bitten persons at study inclusion were analyzed for the presence of Borrelia antibodies. Three months later, the clinical outcome and the serological response of the tick-bitten persons were investigated. A total of 2154 I. ricinus ticks and 1546 participants were included in the studies. Participants were exposed to tick bites between April and November, but temporal and spatial differences in exposure to ticks was found. The majority of the tick bites were caused by nymphs (70%) and most tick bites took place on the legs (50%). The site of tick attachment on the body as well as the age and gender of the participant influenced how soon a tick was detected. The majority of participants removed “their” ticks later than 24 hours of attachment. Of all ticks, 26% was Borrelia-infected, but the prevalence varied between the life stages of the tick and between the studied areas. Six species of the B. burgdorferi sensu lato complex and one Borrelia species that may cause tick-borne relapsing fever were detected. Adult ticks that had fed more than 36 hours contained a lower number of Borrelia cells than adult ticks that had fed less than 36 hours. The seroprevalence among the participants varied between genders as well as between the studied areas. Of all participants, 2% was diagnosed with LB and 2.5% seroconverted without an LB diagnose. A correlation between seroconversion and duration time of tick attachment was found, but the number of Borrelia cells in the tick, did not explain the risk of infection for the bitten person. A deeper knowledge and a better understanding of the interactions between ticks, humans and Borrelia bacteria may contribute reducing the risk for tick bites and the risk of developing LB after a tick bite