27 
AN HEREDITARY RICKETTSIA- LIKE PARASITE OF 
THE BED BUG (CIMEX LECTULARIUS). 
By J. A. ARKWRIGHT, E. E. ATKIN and A. BACOT. 
(From the Lister Institute of Preventive Medecine.) 
K 
(With Plate II and 1 Text-figure.) 
The Known Species of Rickettsia. 
The forms under consideration resemble Rickettsia prowazeki —the supposed 
cause of typhus fever—which occurs in lice that have fed on typhus fever 
patients. 
It may therefore not be amiss to describe briefly the organisms which have 
been grouped by various workers in the same category as Rickettsia. 
The general characters of “Rickettsia'’ may be summarised as follows: 
(a) Bodies of minute size, usually 0-5/x in diameter or less, of round or 
diplococcal shape, though very minute bacillary and even thread-like forms 
occur. 
(b) Though resembling very small bacteria in general appearance, they 
stain much less readily than ordinary bacteria but can be coloured with 
Giemsa’s stain; they are readily decolourised by Gram’s method. 
(c) Absence of motility. 
( d ) Resistance so far, with one exception, to attempts made to cultivate 
them on artificial media in vitro. 
(e) Their occurrence in very large numbers in the gut and in some cases 
in other organs of blood-sucking insects. 
The known organisms apparently belonging to this group are: 
(1) The organism found in the tick— Dermacentor venustus —the inverte¬ 
brate host of the parasite causing Rocky Mountain spotted fever. 
This was first described by Ricketts and has since been very thoroughly 
studied by Wolbach (1919) and called by him Dermacentroxenus rickettsi. 
It is found in very large numbers in the alimentary canal, salivary glands, 
muscles and other organs of the tick and is passed on to the next generation 
of ticks. Similar forms have been described in the blood of men and other 
animals infected with this disease; Wolbach records its regular occurrence in 
the tissues of the mammalian host. This organism varies considerably and 
according to Wolbach, has three distinct morphological forms. 
(2) R.prowazeki described in detail by da Rocha-Lima (1916) and previously 
by Ricketts (1909) and Sergent, Foley and Vialatte (1914). This is found in 
