4 ASHBUBN AND CRAIG. 



The disease, since the work of Wilson and Chowning in 1902, has each 

 year been the subject of careful investigation and report, and the pros- 

 pects now are that its etiology will soon be well understood. 



The total number of cases occurring each year is small and this fact, 

 considered in connection with the further ones that the disease is encoun- 

 tered in such a limited and relatively isolated region and has been known 

 for so short a time, makes it remarkable that the present knowledge 

 concerning it should be as great as it is. 



IV. ETIOLOGY. 



Tsubsugamushi disease occurs along certain limited parts of the banks 

 of a few rivers on the west coast of the main island of Nippon, being 

 limited to Echigo and Akita Provinces. The distribution of the in- 

 fected areas is irregular and, up to the present time, unexplainable. 

 They are all subject to submergence by floods which occur in June, 

 but not all flooded districts are infected, nor does the relative location of 

 an infected district, up or down stream, seem to influence a noninfected 

 one. The floods usually occur in June and last but a few days. Im- 

 mediately ' after their subsidence the infective regions are not dangerous, 

 but after a few weeks or a month and synchronously with the appearance 

 of the ahamushi or red mite, they become so, and any person entering 

 them takes a considerable risk of contracting the disease. Consequently, 

 these regions are avoided at this time by all whose poverty does not 

 drive them there to work. As a general rule no right of ownership is 

 exercised over such land and the very poor do, therefore, cultivate hemp 

 on it in some places and in other parts visit it to gather mulberry leaves 

 to feed silkworms. In either case they are apt to be bitten by red 

 mites and it is customary for them to search carefully for their bites 

 after leaving the places where they are encountered. However, the 

 mites are so small as to be very difficult of detection and if the bite does 

 not cause pain, it is frequently overlooked. Not all mites are infective 

 and many bites therefore cause no trouble. However, a certain propor- 

 tion of them do, the point bitten becoming an eschar and later an ulcer. 

 The neighboring lymphatic glands become enlarged and painful and an 

 attack of fever succeeds. 



It is the experience of practically all who have carefully studied the 

 disease, and they are numerous, that every case of it is preceded by the 

 bite of a mite, and in the great majority of instances this is located by an 

 examination of the region drained by the lymphatic glands, which first 

 become enlarged and tender. So far as could be learned in Japan, Balz's 

 contention that such is not the case has not received support, and later 

 investigators agree that the above method is the sole means of infection. 

 The mite in question is the larval form of a Trombidium, species un- 

 known. The larva bears a great resemblance to that of Leptus autum- 

 nalis. It is so small as to be almost invisible to the naked eye, it is 



