This organism is now classified in a separate genus, Francisella and is called Francisella tularensis. Like other members of the other two groups, this is a small Gram-negative, encapsulated, coccobacillus. It is 0.7 pm by 0.2 pm when first isolated but larger, up to 1-5 pm long, when in culture. It has a marked tendency to pleomorphism in artificial culture. Francisella tularensis cannot be cultured on ordinary media but requires the addition of pure egg yolk or pieces of sterile rabbit spleen to fluid media or of cysteine and glucose to human blood agar.
Francisella tularensis produces a disease (tularaemia) in wild rodents with lesions reminiscent of those found in plague-infected animals. The organisms are found in large numbers within the cells of the liver and spleen, suggesting that it may multiply as an intracellular parasite. Infection in man may result in a prolonged febrile illness with glandular lesions and ulcers of the skin, but the clinical manifestations may be mild or influenza.
Human infections are usually diagnosed by inoculation of laboratory for example guinea pigs or mice, with exudates from the glands or ulcers (and isolation of Francisella tularensis from infected tissues, e.g. spleen, on glucose cysteine blood agar). The patient serum should he tested for agglutinating antibodies. It should be noted that the serum of persons infected with BruceIla species may agglutinate Francisella tularensis,
The disease has a worldwide distribution and has been reported from several European countries. The infection, which is a typical zoonosis and which is largely tick-borne among its natural hosts (lagomorphs and rodents) is trans-mitted to man during the handling of infected animals, e.g. rabbits and hares; or laboratory cultures (it is highly infective for laboratory workers). Tularaemia is sometimes water-borne as a result of pollution of water with the excreta of infected rodents such as water rats.