322 
East Coast Fever 
trace the morphological changes undergone by the parasite having thus 
been negative, we performed some experiments on the effect of various 
conditions on the infectivity of the ticks, in order to understand more 
fully the biology of the parasite in the tick. Our results, although still 
incomplete, are of interest, as they show the nature of some of the 
conditions affecting the infectivity of R. appendiculatus that have fed 
on cattle infected with East Coast Fever. 
Theiler (1904), and Lounsbury (1906), have shown that Theileria 
■parva may be transmitted by Rhipicephalus appendiculatus, simus, 
evertsi, nitens, and capensis, respectively. In every case the infection 
is conveyed by the tick only in the stage following engorgement on an 
infected animal. 
Accordingly, a tick having ingested infected blood as a larva will 
only be infective as a nymph, or, having sucked infected blood as a 
nymph, it will be infective as an adult. A tick which is infective 
as a nymph necessarily will be iion-infective as an adult, the infection 
only persisting from one stage to the next. It has been shown that 
infected nymphs lose their infection and become “clean” by engorging 
themselves, not only on cattle, but also on rabbits and sheep, animals 
which are not susceptible to East Coast Fever (Gonder, 1911). 
I. Experiments on the effect of incomplete feeding on the infectivity 
of R. apjjendiculatus infected with Theileria parva. 
Our attention was first directed to this subject by Colonel H. 
Watkins-Pitchford, F.R.C.V.S., who informed us that, in Natal, he had 
been able to infect cattle with East Coast Fever by the bites of ticks 
that had previously fed for two or three days on a normal animal. His 
experiments, which have never been published, showed that ticks do 
not necessarily become “clean” as a result of biting a vertebrate host, 
and the importance of this discovery is such that we decided to repeat 
and also, if possible, extend Watkins-Pitchford’s observations in Natal. 
Our experiments entirely confirm his discovery and, moreover, 
demonstrate that an infected tick does not become infective until it 
has fed oti a bovine for at least two days. 
The experiments were as follows: 
N umerous larvae of R. appendiculatus were allowed to gorge them¬ 
selves on an infected calf shortly before the animal died of East Coast 
Fever. The gorged larvae dropped off the calf February 6 and 7, and 
were kept at 30° C. until the nymphs started to emerge six or seven 
