1. Experiments conducted to demonstrate the susceptibility of horses and cattle are described.
2. The appearance of virucidal antibodies in the serum of these animals after infection is demonstrated by a technique of in vitro neutralization of virus.
3. The technique of tick feeding investigations is briefly described.
4. It is shown that-
(a) larvae of R. appendiculatus will pick up infection for transmission as nymphae;
(b) nymphae will pick up infection for transmission as adults;
(c) the virus does not pass through the egg to the next generation;
(d) infective nymphae tend to lose their infection after feeding
on immune animals though some of the resulting adults may still be infective ;
(e) the same occurrence was noted after feeding infective nymphae on susceptible animals but, particularly if sheep are the hosts, there is a danger of the nymphae reacquiring infection before detachment;
(f) clean ticks do not acquire infection when feeding on an immune animal simultaneously with infective ticks.
5. The significance of the work is discussed.
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