324 
East Coast Fever 
ticks) were placed on the animal and allowed to gorge themselves. 
Thirteen days later the calf showed a rise in temperature, parasites 
appeared in its blood after four days, and on June 11 it died of East 
Coast Fever. 
From the above described experiment it seems that R. appendiculatus. 
infected with Theileria parva, do not become infective until they have 
fed on a bovine for more than two days. 
The result of Exp. 3 suggests at least two possibilities with regard 
to the conditions necessary before an infected tick becomes infective. 
It might be supposed that the parasites within the infected ticks require 
a certain degree of warmth in order to complete their development and 
be capable of infecting cattle. The body-warmth of the host might 
afford the necessary degree of heat, and therefore the following experi¬ 
ment was performed in order to test this hypothesis:— 
Exp. 4. Demonstrating that heating unfed infected ticks to 37° G. for 
three days does not render them infective during the first tiuo 
days after they become attached to the host. 
Nineteen infected nymphs were placed for three days in an incubator 
at 37° C., prior to being placed on a calf. After' having fed on the calf 
for two days, the partially engorged ticks were I'emoved (see below, 
Exp. 5) and the aninral kept under observation for two months, during 
which period it never showed any symptoms of being affected. 
This experiment shows the incorrectness of the above-mentioned 
hypothesis. If warmth was the only necessary factor, the ticks, having 
been heated to body temperature for three days and then placed on the 
calf for anotlier two days, should have been infective, as, ordinarily, 
a nymph only requires five days for its complete engorgement. The 
only alternative hypothesis is that the parasites contained within the 
unfed and infected tick are unable to complete their development 
within the invertebrate host until the latter has begun to ingest blood. 
After the tick has commenced feeding, the influx of blood starts some 
development resulting in the infected tick becoming infective only after 
an ipcubation period of at least two days. In this connection, the result 
of the following experiment is of some interest:— 
