Lyme Disease in Humans
Justin D. Radolf, Juan C. Salazar and Raymond J. Dattwyler
from: Borrelia: Molecular Biology, Host Interaction and Pathogenesis (Edited by: D. Scott Samuels and Justin D. Radolf). Caister Academic Press, U.K. (2010)
Lyme disease is a rapidly emerging tick-borne, complex, multi-system infectious disorder caused by the spirochetal bacterium Borrelia burgdorferi. The ailment, which affects adults and children alike, is widespread in the Northern Hemisphere and it continues to expand as humans encroach on the sylvatic habitat of the spirochete's mammalian reservoirs. Since first identified in the 1970s the incidence of Lyme disease has increased more than 30-fold and it is now considered the most prevalent arthropod-transmitted infection in both the United States and Europe. B. burgdorferi is transmitted by ticks of the Ixodes ricinus complex, including I. scapularis, I. ricinus and I. persulcatus. In North America, B. burgdorferi sensu stricto is the only species proven to be pathogenic for humans. In Europe, both B. afzelii and B. garinii are most commonly associated with human disease. The spirochete's genomic features, as well as its unique molecular architecture, are considered to have a seminal role not only in how it is transmitted from ticks to humans, but also how it triggers immune responses in afflicted individuals. Inflammatory manifestations associated with the disease result from the host's innate and adaptive immune responses to the bacterium, rather than from toxigenic molecules, which borrelia cannot produce. Indeed, the deposition of spirochetes into human dermal tissue generates a local inflammatory response that becomes manifest as erythema migrans (EM), the hallmark skin lesion of Lyme disease in North America. In Europe, two additional dermatologic disorders, borrelial lymphocytoma and acrodermatitis chronicum atrophicans (ACA) are frequently associated with infection. EM is frequently accompanied by 'flu-like' symptoms, including myalgias, arthralgias and fever, which are generally believed to be cytokine-mediated in response to hematogenous spread of the bacterium. If treated appropriately, the prognosis is excellent; however, if untreated, patients may develop a wide range of inflammatory clinical manifestations, most commonly involving the central nervous system, joints and heart. Within days of treatment, the signs and symptoms associated with the disease typically begin to subside, although in some individuals a complete recovery can take several weeks or even months. A minority of treated patients may go on to develop a poorly defined fibromyalgia-like illness, which is not responsive to prolonged antimicrobial therapy. Below we integrate current knowledge regarding the ecological, epidemiological, microbiological and immunological facets of Lyme disease into a conceptual framework that sheds light on the disorder that healthcare providers encounter read more ...