H. L. Duke 
425 
X. THE PART PLAYED BY EACH OF THE ABOVE TWO METHODS OF 
TRANSMISSION IN THE PERPETUATION OF TRYPANOSOME STRAINS. 
The trypanosomes on entering the gut of a fly suited to their cyclical 
development multiply enormously by division. This process is continued for 
at least 17 days and finally the forms infective to a new host are found in the 
salivary glands of the fly. The gulf separating these salivary gland forms from 
the original infecting trypanosomes is, biologically speaking, enormous. It is 
reasonable to suppose that none but specific and deep-rooted characters could 
survive such a drastic ordeal. The question of the transmission of acquired 
characters through the insect intermediary has not yet been properly worked 
out for the haematozoa. Gonder’s experiments with T. lewisi are, however, 
most instructive in this connection (li). He found that a strain of lewisi whose 
resistance to arsenic had survived 20 passages, lost this property when trans¬ 
mitted cyclically by lice. The same strain when transmitted by the lice from 
rat to rat by the direct method retained its arsenic resistance. 
Laboratory strains of trypanosomes are almost invariably kept up by 
various methods of subinoculation, which may be said to correspond nearly 
to direct transmission by the insect intermediary. Many of these laboratory 
strains acquire in the course of time atypical characters. In the case of Yorke 
and Blacklock’s T. vivax strain, the organism eventually acquired the power 
of survival in rabbits, which animals were wholly resistant to the earlier 
passages (12). 
Instances of increase in virulence of laboratory trypanosomes are not un¬ 
common; this is notably the case with certain gambiense and brucei strains. 
We know, however, of no evidence to show that these characters, acquired in 
the course of prolonged direct transmission, can persist when exposed to the 
searching processes involved in cyclical development in the insect host. 
There is no apparent reason why such specially virulent strains should not 
be developed in nature whenever mechanical transmission is sufficiently free. 
It may be that an example of this is afforded by the direct transmission of 
cattle disease, such as that caused by T. pecorum , by insects incapable of 
cyclical transmission of the trypanosome but which, none the less, cause 
outbreaks away from tsetse fly. 
The conditions necessary for direct transmission are primarily the presence 
of large biting flies (small biting flies such as mosquitoes, Simulium , etc., are 
not suspected to transmit trypanosomes in this manner), and, incidentally, in 
view of the difficulty of transmission, the presence of many large biting flies 
and of many potential hosts of the parasite in close juxtaposition. 
These conditions are met with when an infected ox is admitted to a herd 
grazing in a densely fly infested pasture, and it is only under such conditions, 
so far as we are aware, that epidemics of fly sickness occur. They would also 
be met in the case of game animals, such as buffalo, which run in herds, and 
which are subject to attack by large biting flies of “common'' species. 
