S. Hadwen and Gr. H. F. Nuttai.l 
301 
pathogenic effects follow. Undei- natural conditions, freshly attached 
ticks are frequently removed from their seats of attachment to man and 
animals, consequently many subjects that have been bitten do not suffer 
from serious after-effects. 
On the hypothesis that the disease is infective, the negative results 
which follow the premature removal of the ticks may be explained on 
the assumption that the hypothetical parasite undergoes a development 
in the tick whilst the latter is attached to the host, and that the parasite 
only enters the vertebrate host after the tick has remained attached to 
this host for some days. This supposition is strengthened by the 
observations of Nuttall and Hindle upon East Coast Fever, which are 
published in this number of Parasitology, wherein it is demonstrated 
that the tick RMpicephalus appendiculatus is non-infective during the 
first two or three days of its attachment to susceptible cattle. 
We hope, in the course of further investigations, to throw more light 
upon this interesting affection, and to determine if it is due to an 
infective process or not. 
We can offer no explanation of the deaths observed in guinea-pigs 
following the application of P. venusUis larvae and nymphs (Washington 
strain); they were certainly attributable to the ticks although but few of 
these were placed upon the animals. The examination of the guinea- 
pigs at autopsy proved negative. One of us (S. H.) has not observed 
any ill effects in rabbits upon which D. venustus larvae and nymphs were 
raised in Canada. As far as our records go it is only the adult ticks 
which attack man, or larger animals, in Canada.* 
