158 
On Rickettsia 
R. trichodectae n. sp. Occurs in the alimentary canal of Trichodectes pilosus. 
R. linognathi n. sp. Occurs in the alimentary canal of Linognathus stenopsis. 
Rickettsia sp. Sikora 1918. Occurring in the alimentary canal of Psokus and Ctenopsylla 
musculi, respectively.' 
All the Rickettsia mentioned in the above list inhabit the alimentary canal 
of Arthropods, and in addition two of them also occur in the blood of man 
and are associated with human diseases. 
The examination of their hosts shows that these organisms have been 
found not only in blood-sucking insects, but also in at least two species that 
do not feed on blood, viz. Trichodectes and Psokus. 
In view of these facts, therefore, it seems probable that the Rickettsia 
represent a group of micro-organisms which primarily inhabit the alimentary 
canal of insects and other Arthropods. Some of them live saprophytically in 
the mid-gut, whilst others, such as R. prowazeki , have become true parasites 
and invade the tissues of their host. 
As they multiply in the intestine, numbers of the Rickettsia would be con¬ 
tinually passing to the exterior in the faeces and doubtless this is the way in 
which the infection is usually transmitted from one host to another in the 
absence of any intermediate vertebrate host. In the case of blood-sucking 
insects such as lice, any organisms that are voided are continually being intro¬ 
duced into the blood of the host either through the open wound caused by the 
bite of the insect itself, or through excoriations of the surface of the body 
from independent causes. Under these conditions therefore any such organ¬ 
isms would have the chance of becoming adapted to a parasitic mode of life 
in the vertebrate host. 
With regard to the difficult question of the relation between R. quintana 
and R. pediculi it seems not unreasonable to assume that here we have an 
example of a Rickettsia normally inhabiting the intestine of the louse which is 
becoming adapted to a parasitic mode of existence in the blood of man. 
The presence of Rickettsia in apparently normal lice has been recorded by 
many competent observers and their results are not satisfactorily explained 
by the assumption that in all these cases the lice have come either directly or 
indirectly from patients suffering from typhus or trench fever. It is more in 
unison with the results to accept da Rocha Lima and Topfer’s hypothesis 
that there is a species of Rickettsia, R. pediculi, which lives in the alimentary 
canal of the human louse and is non-pathogenic to man. On the other hand, 
the results of Arkwright, Bacot and Duncan (1918) as well as my own, have 
conclusively proved that some strains of lice are practically free from Rickettsia, 
but when fed on trench fever patients become infected with R. quintana, an 
organism that seems to be indistinguishable from R. pediculi except with 
regard to its physiological character of living in the blood of man. 
To explain these results it is suggested that the human louse is liable to be 
infected with a non-pathogenic variety of Rickettsia, R. pediculi, which, under 
certain conditions, has acquired the capacity of living in the blood of man and 
