14th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 2000

Jan. 29 - Feb. 5, 2000
Marilleva, Trento, Italy

RT-75

Bagging as a predictive method for landscape epidemiology of Lyme disease

Cesare Furlanello, Stefano Merler, Annapaola Rizzoli*, Claudio Chemini*, Claudio Genchi**.
ITC-IRST, Trento, *Centro di Ecologia Alpina, Trento, **Institute of General Pathology and Parassitology, Veterinary Medicine, MilanoUniversity, Italy

Introduction

The spirochete Borrelia burgdoferi is the primary agent of Lyme disease. Risk exposure is due to the bite of an infected tick, which may act as a vector also for other important infections such as human babesiosis and granulocytic ehrlichiosis¹, Cardiac complications (Lyme carditis), which may occur in up to 8% of patients^2,3, must be given serious consideration⁴; with more than 16000 cases in 1996 alone, Lyme disease is now the most common vector-borne illness in North America⁵, as well as being very common in Europe, where the tick Ixodes ricinus has been acknowledged as the main vector of the disease together with I. persulcatus. Rodents, birds in frequent contact with the ground, and medium sized mammals act as a reservoir for the spirochete genospecies B. burgdoferi s.s., B. afzelii and B. garinii and as a result infections transmitted by ticks are increasing in most European countries⁶. Known endemic areas for B. burgdoferi are Southern Sweden Slovenia, Lithuania, certain hot spots in Germany, Austria, Northern Italy, and, in general, Central-Eastern Europe⁴. Borreliosis should be suspected in all patients with unexplained cardiac symptoms (and especially in atrioventricular (AV) block) and who may have been exposed to Lyme disease in regions invaded by the epidemic⁷. Idiopathic AV block is a primary manifestation of Lyme carditis^2,8-12, also present in young patients⁹. Myopericarditis, ventricular extrasistoles, and intermittent tachyarrhythmias have been reported as manifestations of Lyme carditis, including a case of fascicular tachycardia associated with an underlying complete heart block¹⁰. The association with borreliosis has been demostrated from the discovery of spirochaetes by subendocardial or endomyocardial biopsy^13,14. Borrelia burgdoferi infection may also play an active role in the development of dilated cardiomiopathy^15,16 and standard antibiotic treatment may reverse dilated cardiomiopathy in patients with symptoms of borreliosis¹⁵. Prompt recognition and treatment of Lyme disease may indeed strongly reduce the risk of cardiac complications: in a population-based retrospective cohort study, people with a history of previously treated Lyme disease did not show a higher prevalence of cardiac abnormalities than people without any apparent history of previous Lyme disease¹⁷. Given these findings, a predictive risk map of exposure to Lyme disease may be used for the correct diagnosis of the disease either in the early phase or in case of possible cardiac complications, where a high amount of suspicion is due in endemic areas.

The risk of transmission of B. burgdoferi s.l. can be assessed through spatial models of risk exposure to tick bites and measures of the prevalence of the infection in ticks¹⁸. One should note that the prevalence of infestation of Borrelia in sampled ticks may be high in endemic areas for Lyme disease, with rates of infection of 25-35% of nymphs and 50-70% of adult ticks¹⁹, with more than 40% of the adult I. ricinus ticks found infected in hot spot regions of Slovenia²⁰. In Trentino, the prevalence ranging from 15 to 40% of B. burgdoferi in I. Ricinus has been reported^21,22.

In this paper, updated results are presented in risk assessment of human exposure to tick bites in Trentino based on a digital map model developed in joint research by ITC-irst and the Centre of Alpine Ecology^4,23,24. The new model is based on the bagging methodology, and it improves over previous published results⁴.