The cause is unknown, but has been attributed by some to various bacteria and by others to a protozoon.
Nagayo attaches importance to piroplasm-like forms found in the spleen and lymphatic glands.
Kawamuro and his colleagues could not demonstrate any organism by any method of examination. The virus is in the blood and even as small an amount as 0.001 cc. may infect a monkey although 0.1 cc. is usually required. The virus is present also in the enlarged lymph glands. Heating the blood at 50°C. for 10 minutes destroys the virus. Recently Hayashi has reported as cause minute rod, ring-shaped or spheroid bodies which when stained with Giemsa’s stain are brought out in the lymphocytes and endothelial phagocytes of lymph nodes and spleen. They also occur in the blood plasma and in severe cases in red blood cells. He has transmitted the disease to monkeys, guinea pigs and rabbits. These bodies resemble bacteria, in this respect resembling the organisms of typhus and spotted fever of the Rocky Mountains. Hayashi regards the organism as resembling Theileria parva and Bartonella bacilliformis, and believes he has shown the field mouse to be a reservoir of the virus.
The disease is not communicable from person to person and only follows the bite of a larval mite, Trombidium akamushi (Leptotrombidium akamushi). This is a minute orange-red arachnoid which can scarcely be seen with the naked eye. This mite is only a source of danger in the region of the inundated river banks, its bite not producing the disease elsewhere.
Persons harvesting hemp during August are liable to contract the disease if bitten by the larval mite.
The mite is found in large numbers on the ears of field mice, these hosts, however, not appearing to be suffering from any particular disease.
Pathology
Other than the local ulcer and the swollen regional glands, there is little that is definite. The spleen shows enlargement and there is also swelling of the mesenteric glands. The lower part of the ileum may show injection.
Symptomatology
A Typical Case.—About one week after receiving the bite of the larval mite, which may not have been noticed by the patient, there develop chilliness, giddiness and headache, with a rising temperature. In two or three days from the onset, painful glands are noticed in certain regions as of groin, axilla or neck. From these glands we can often by following inflamed lymphatics find the small necrotic ulcer which is often located in the armpit or in the region of the genitals. There is a dark red areola about the ulcer which is only slightly tender. The glands are not very much enlarged and are not excessively tender. There may be general glandular enlargement following that of the primary swellings. The pulse rate is only from 80-100, notwithstanding the rise of the fever to 104°F. or even 105°F.