Tropical typhus bears a close clinical resemblance to mild typhus fever, but it does not spread from man to man, nor does it give rise to epidemics. Lice are not the vectors of the virus. There are two kinds of tropical typhus, the W. form and the K. form. In the W. form, the serum agglutinates the ordinary strains of B. proteus X. 19, and Wilson's B. agglutinabilis, but it does not agglutinate the non-indologenic strain, Kingsbury. In the K. form the serum agglutinates the non-indologenic strain, Kingsbury, but it does not agglutinate the ordinary strains of B. proteus X. 19, or B. agglutinabilis.
The W. form of tropical typhus is a disease of the house and the town; the majority of those affected are indoor workers, such as clerks and shopkeepers, particularly those who deal with foodstuffs. MAXCY has found that the " endemic typhus " or Brill's disease of the United States, which is clinically identical with tropical typhus, has a similar distribution. The like is true of the typhus-like fever described by HONE in Australia which is probably the same disease as the W. form of tropical typhus.
The distribution of the K. form is very different, it is essentially a disease of the open country and affects outdoor workers. It has a patchy distribution and outbreaks occur particularly in areas which, after being cleared of jungle, are allowed to grow up in weeds and scrub. For this reason, we propose the name scrub-typhus for the K. form of tropical typhus. Some of the cases of typhus-like diseases described in India are probably the same as scrub-typhus.
An account is given of an outbreak of sixty-one cases of scrub-typhus on an oil-palm estate where cases of the tsutsugamushi disease had occurred among the European staff. This outbreak illustrates the limited distribution of the disease. Five adjacent estates, served by the same hospital, were unaffected, and the outbreak was confined almost entirely to one of the three Divisions of the oil-palm estate. This Division differed from the other two in being overrun with weeds and undergrowth, with which scrub-typhus is always associated. An attack appears to confer immunity. The cases occurred among the newly recruited labourers, those who had been on the estate for a long time escaped infection. Coolies recruited in the Federated Malay States were as susceptible as those from India, from which it appears that the Indian population of Malaya has not been generally exposed to infection. The outbreak consisted entirely of the K. form of tropical typhus (scrub-typhus); there were no cases of the W. form (? endemic typhus).
Attention is drawn to the relationship of scrub-typhus and the tsutsugamushi disease by the occurrence of the latter on the same estate. The tsutsugamushi disease of the East Indies and scrub-typhus both occur in circumscribed areas which are covered with undergrowth, and their symptoms are much alike There are, however, the following points of distinction : (a) The fever of tsutsugamushi does not end abruptly at the end of the second week as it does in scrub-typhus, nor is convalescence so rapid. (b) A primary sore and bubo are present in tsutsugamushi, but absent in scrub-typhus. (c) The titre of agglutination with B. proteus, Kingsbury, is low in the tsutsugamushi disease but very high in scrub-typhus. In some cases of the tsutsugamushi disease, the serum agglutinates B. proteus, Kingsbury, in higher dilutions than that of normal persons, but not to titres nearly so high as in scrub-typhus.
It is suggested that the two diseases have a similar ~etiology and that scrubtyphus, like the tsutsugamushi disease, is carried by trombiculae.