RICKETTSIA BODIES 039 
belief that the Rickettsia are living entities, and this viewpoint is 
adopted here. 
Some of the Rickettsia bodies appear to be harmless parasites living 
in the mid-gut of insects; others, notably Rickettsia prowazeki, appear 
to invade the tissues of their insect host, and even lead to its death. 
The occurrence of these parasites within the lumen of the gut of the 
insect would seem to afford ready access to other hosts, because the 
Rickettsia are doubtless voided with the feces. Those Rickettsia 
that are parasitized upon a blood-sucking insect, as a louse, also 
liave ready access to the mammalian carriers of the insect, and may 
readily pass into the blood stream through the abrasion made by the 
bite of the louse, either directly by implantation, or, somewhat less 
readily, through the act of scratching by the animal or the man. If 
the particular Rickettsia thus introduced into the tissues of an animal, 
or man, can grow in its tissues, infection may occur. At least three 
Rickettsia, those of typhus, of Rocky Mountain spotted fever and of 
trench fever, appear to be transmitted thus to man through infected 
lice. 
The study of the Rickettsia is fraught with many difficulties. These 
organisms, with the possible exceptions of Rickettsia melophagi,^ 
R. prowazeki' and R. pediculi^ have not thus far been cultivated in 
artificial media. Furthermore, they stain with difficulty, and are 
Aery small. Several observers'* have reported the presence of the 
Rickettsia in the alimentary canals of lice gathered from normal per- 
sons. These observations are few however, and the associations were 
such that human infection of the lice was not at all impossible. On 
the other hand, Arkwright, Bacot and Duncan,* Hindle,'' and Wolbach, 
Todd and Palfrey,^ found very few naturally infected lice among their 
stocks that had been kept under observation for several months, and 
away from infected humans. It should be stated, however, that 
ArkAvright, iVtkin and Bacot^ found Rickettsia-like bodies in bed-bugs 
(Cimex lectularius) obtained from sources free from suspicion of in- 
fected human cases. It is by no means improbable, therefore, that 
certain blood-sucking insects do have Rickettsia in their alimentary 
tracts quite apart from human derivation. It is quite obvious that 
much careful experimentation must be performed before this question 
can be answered definitely. 
General Characteristics of the Rickettsia. —The Rickettsia thus far 
studied show the following rather uniform characteristics. 
il/orp/^o/of/?/.— Minute, round or ovoid bodies, 0.5 micron or less in 
diameter: bacillary and filamentous forms occur, but less commonly. 
1 Noller: Arch. f. Schiffs. ii. Tropn.-Hvg., 1917, 21, 53. 
= Kuczynski: Med. Klin., 1920, 16, 706, 73.3, 759. 
» Sikora: Arch. f. Schiffs. u. Tropen.-Hyg., 1921, 25, 123. 
■< Brumpt: Bull. Soc. Path. Exotique, 1918, 9, 249. Munk and da Rocha-Lima: 
Miinchen. med. Wchnschr., 1917, 64, 1422. 
s Jour. Hyg., 1919, 18, 76. e Parasitology, 1921, 13, 152. 
7 Etiology and Pathology of Typhus, 1922. « Parasitology, 1921, 13, 27. 
