Ecology, Host and Vectors of Francisella Tularensis

Francisella tularensis is a gram-negative facultative intracellular bacterium that can cause a fatal disease, tularaemia, in human and animals. Francisella tularensis survives for months on very low temperatures, in water, mud or dead animals. If stored on –70° C or lyophilized, virulence can be kept for years. It can also survive heating up to 42-65° C after food or water processing. Francisella tularensis is an intercellular parasite able to infect a great number of cell types while the macrophages are considered as the key cells for manifestation of the disease in humans. There are four subtypes of Francisella tularensis: tularensis (type A), holartica (type B), mediastatica and novicida. F. tularensis type A is found only in North America and is highly virulent. F. tularensis type B is less virulent for humans, and exists in North America and Europe. F. tuleransis subsp. holartica is less virulent, especially for humans, because its dissemination is slow and it causes a mild state of illness. F. tuleransis subsp. mediastatica has unique biochemical characteristics and is isolated only in Kazakhstan and Turkmenistan in central Asia, and shows virulence for organism similar to F. tuleransis type B. The primary reservoir of tularaemia are mammals like rabbits and other rodents while the vector of disease are flies and mosquitoes. The bacteria can enter the organism by different routs of infection and the clinical symptoms of the disease depend on these entry.