88 AN INTRODUCTION TO THE STUDY OF VIRUSES 
The method of setting a thief to catch a thief or, to give it its proper 
name, biological control, may have some slight bearing on the control 
of virus vectors. The tick, Dermacentor venustus, transmits the rickettsial 
disease, Rocky Mountain spotted fever, and attempts have been made 
to reduce its numbers by breeding and liberating vast numbers of a 
Chalcid wasp which parasitizes it. 
3. [Elimination of Virus Reservoirs — 
The question of virus reservoirs is an important one and although 
little is known about them, there is good evidence that they exist 
for at least three virus diseases of ee 
These reservoirs of infection are usually other species hick either 
react very slightly to the virus or do not react at all but still retain the 
virus in the blood. Such animals are known as carriers. 
In the case of African horse sickness it is thought that some species 
of wild game may serve as a carrier. As regards jungle yellow fever, 
a careful survey was made of a large number of miscellaneous wild 
animals of the jungle to find out if any were susceptible to infection. 
More than two thousand wild animals were captured and inoculated 
and it was found that none of the animals were killed by the virus 
and few showed any sign of illness. Nevertheless many species, after 
inoculation, had virus circulating in the blood stream whilst the animals 
were running about. This is important because it would allow time 
for the mosquito to pick up the virus even if it circulated in the blood 
for only a few days. Once the mosquito itself was infected, however, 
it would retain the virus for the rest of its life which may be several » 
months under favourable conditions. It is theoretically possible, 
therefore, for many types of animal to act as carriers of the virus of 
jungle yellow fever, but whether, during the above survey, any of the 
animals tested were naturally infected with the virus is not known. 
Many attempts have been made to find a natural reservoir of the 
mosquito-borne virus of equine encephalomyelitis in the U.S.A., 
and there is some evidence that birds may be symptomless carriers. 
At the moment, the so-called “prairie chicken” seems to be implicated. 
That a subject may be his own carrier of a virus is shown by the 
existence of the so-called latent viruses which rest quiescent in the 
body and are stimulated into action under circumstances which 
presumably lower the resistance in some way. Herpes simplex is a 
good example of this and presumably, also, the virus of the common 
cold. 
