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.
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.
The louse infesting jack rabbits Haemodipsus ventricosus, can transmit the infection from rabbit to rabbit so that the infection in the jack rabbits is probably kept up by this agency. Recent experiments have shown that the infection in white mice can be transferred by the bites of the mouse louse and also by bedbugs. Again if the mice are fed on infected bedbugs transmission is found to take place by such feeding experiments. The faeces of such bedbugs prove infectious. Guinea pigs injected subcutaneously with the urine of white mice suffering from the disease die acutely with typical lesions of tularaemia.
Mice fed on the liver of a rabbit dying of the disease succumbed to the infection within five days.
As noted under etiology the most important method of spread of the disease to man is by the bite of an infected horsefly (Chrysops discalis). From experimental studies it would seem that any biting arthropod might prove a transmitting agent. In view of the fact that the infected deer fly tends to lose its infectivity after about five days Francis is of the opinion that the fly is only a mechanical transmitter of the organism.