24 
THIRTEENTH REPORT. 
of a disease by a biting insect implies the injection of the virus into the 
wound. While this is certainly the case in malaria, it is not so in plague. 
There the infection is contaminative ; in other words the excreta and not 
the saliva of the flea contains the plague bacillus. Leishman has brought 
forward evidence which indicates that an exactly similar condition exists 
in the case of the tick which transmits the African relapsing fever. And, 
it may be added further, that a like view for the transmission of the 
Trypanosoma leicisi by the rat flea has been suggested by Mincliin and 
Thomson. Future studies will be needed to decide just how frequent is 
this contaminative infection. 
In each of these three examples just referred to, the infective organisms 
develop particularly in the alimentary canal. In other words a veritable 
tube culture may result irrespective as whether the organism belongs 
to the bacteria or protozoa. A certain interval is needed to secure such 
multiplication before infection can be possible. As in the case of man 
and higher animals, the organisms which are introduced into the in- 
testine may in time leave the alimentary canal and pass into the circula- 
tion, eventually reaching distant organs, such as the salivary glands. 
The typical active carrier is where the parasite passes through a 
complex cycle of development in the insect, eventually appearing in the 
salivary gland in a new and minute form. This sporozoite stage is then 
injected into the wound and thus gains direct entrance into the blood. 
The classical example of this cyclical infection is afforded by malaria. 
SPIROCHETES. 
The studies of Sehaudinn (1904) on the blood parasites of the little 
owl (Athene noctua ) may be considered as epoch-making even though 
the conclusions which he reached have not been substantiated by other 
workers. These studies concerned two very common intra-cellular para- 
sites of hawks and owls. One of these, the halteridium, if found within 
•the red blood cells, and Sehaudinn believed that this parasite, after fer- 
tilization, developed in the gut of the common mosquito ( Culex pipiens ) 
into innumerable minute trypanosomes, which, by the bite of the insect, 
were subsequently introduced into the circulation of clean owls and 
became transformed into the original intracellular parasite. 
It is beyond the scope of this address to go into a detailed considera- 
tion of the many researches made since that time to confirm or disprove 
Schaudinn’s view that the intracellular halteridium is but a resting 
stage of a flagellate or trypanosome. In collaboration with Dr. MacNeal 
we showed that the morphologic methods employed were insufficient and 
uncertain and hence quite naturally led to wholly erroneous conclu- 
sions. The method of cultivation of trypanosomes as devised by us 
offered the only means of a crucial experimental test of his conclusions. 
If the halteridium developed into flagellates, these should be cultivable 
in vitro and with such pure cultures the questions raised should be 
readily answered. By this means it was possible to demonstrate that 
cultures could be obtained from the blood of birds even though no intra- 
cellular parasites were present. Moreover, the blood of birds having 
halteridia, frequently failed to develop such cultural flagellates. In 
other words, it was shown that there was no relation between the intra- 
cellular parasite and the flagellate or trypanosome and that the latter, 
