In recent publications, Francis, Lake and Mayne, and Wayson have recorded the transmission of the disease experimentally by the house fly (_Musca domestica_), the horsefly (_Chrysops discalis_), the stable fly (_Stomoxys calcitrans_), the rabbit louse, the mouse louse and the bedbug, and have shown that the freshly recovered organism can be grown on other than the special media previously recommended. =Geographical Distribution.=—The first case of infection occurring in man was reported from Ohio by Wherry and Lamb in 1914. Since then a number of cases have been reported by Francis among rural residents of Utah and by Francis and Lake among laboratory workers handling animals infected with _B. tularense_. ETIOLOGY AND EPIDEMIOLOGY =Etiology.=—_Bacterium tularense_ is a small nonmotile Gram-negative cocco-bacillus, from 0.3 to 0.7µ long and gives the appearance in stained preparations of being surrounded by capsular material. It is very difficult to cultivate and until recently it has been grown only on coagulated egg yolk as used by McCoy and Chapin. [Illustration: FIG. 70.—_Chrysops discalis._ The transmitting agent of tularaemia.] Recently Francis has shown that the organism will grow scantily on serum or blood agar. However, by adding a piece of fresh sterile spleen to such media he has obtained a more satisfactory culture medium, although for routine work the egg yolk medium is preferred. The organism refuses to grow on ordinary media such as nutrient broth or agar. Material from a culture, or pus from bubo, or an emulsion of the spleen of an infected guinea pig, when rubbed into the abraded skin of an experimental animal brings about infection, in this respect showing a similarity to plague infection. Upon autopsy of such an experimentally infected guinea pig we find haemorrhagic oedema at the site of inoculation with caseation of lymph glands and small necrotic foci in spleen and liver. Smears from the spleen show the organism in varying numbers. In infected rodents the organism is often found abundantly in the blood and in man a bacteriaemia may occur. =Epidemiology.=—The infection of ground squirrels with this organism was noted by McCoy in his plague work in California. Francis in studying the disease in man in Utah found infection of the jack rabbits and ground squirrels of the region and showed that the transmission of the disease could be carried out by a biting fly (_Chrysops discalis_) and reported this fly as the common transmitting agent of the human infection. He regarded the sick and dying jack rabbits as the reservoir of virus. Tularaemia is chiefly a disease of the rural population attacking those at work in the fields. As a result of accidental laboratory infections it would seem that almost 100% of those carrying on extensive animal experimentation with this organism become infected.