DNA Fragment Analysis and Real - time PCR SNP

1995 through 2005 in 2 locations in Sweden affected 441 persons. We performed an epidemiologic investigation of these outbreaks using a novel strategy, involving high-resolution genotyping of Francisella tularensis isolates obtained from 136 patients (using 18 genetic markers developed from 6 F. tularensis genome sequences) and interviews with the patients. Strong spatial associations were found between F. tularensis subpopulations and the places of disease transmission ; infection by some subpopulations occurred within areas as small as 2 km 2 , indicating unidentifi ed environmental point sources of tularemia. In both locations, disease clusters were associated with recreational areas beside water, and genetic subpopulations were present throughout the tularemia season and persisted over years. High-resolution genotyping in combination with patients' statements about geographic places of disease transmission provided valuable indications of likely sources of infection and the causal genotypes during these tularemia outbreaks. T raditional objectives of investigations of infectious disease outbreaks are to identify ways to control ongoing outbreaks and to prevent future outbreaks. However, a paucity of epidemiologic and ecologic knowledge hampers the investigation of tularemia outbreaks caused by the intracellular bacterium Francisella tularensis, although it is one of the most virulent pathogens known. The Centers for Disease Control and Prevention lists this pathogen as one of the most potentially dangerous bioterrorism bacteria (1). Little is known about natural reservoirs of tularemia, F. tularensis transmission mechanisms to humans, and factors infl uencing the often irregular pattern of outbreaks. Because tularemia is a zoonosis and little ecologic information exists about the causal organism, its prevention and control may require the development of novel outbreak investigation strategies. In nature, F. tularensis is associated with an extremely wide range of hosts and arthropod vectors; a recent review listed 304 susceptible species (2). Transmission to humans may occur through a number of routes; skin inoculation by blood-feeding arthropod vectors is one of the most common routes (3). The infectious dose can be as low as 10 bacterial cells (4). Human tularemia naturally occurs only in biotopes in the Northern Hemisphere. We describe an investigation of a large number of F. tularensis isolates from humans. Patients were infected mainly from mosquito bites and had an infl uenza-like illness, a primary skin ulcer, and enlargement of lymph nodes, the ulceroglandular form of tularemia (5,6). Tularemia is endemic in Sweden, with seasonal outbreaks and a patchy geographic distribution. The number of infected humans ranged from 27 …