28 AN INTRODUCTION TO THE STUDY OF VIRUSES 
on the diseased or healthy plant, might be expected to increase the 
flow of enzymes and so reduce the insect’s efficiency as a vector. 
Effect of Viruses on their Vectors 
There is very little evidence to suggest that an arthropod vector 
of viruses is in any way affected by the virus it may transmit. Even 
when there is good evidence that the virus multiplies inside the body, 
as is the case with certain mosquito-borne viruses, the insects do not 
seem to be adversely affected. 
The only case known of an arthropod vector being affected by the 
virus it transmits is that of the louse (Pediculus) which is, itself killed 
by the rickettsiae of typhus fever. But the suggestion has been made 
that typhus fever, like other rickettsial diseases, was originally a disease 
of insects which has become adapted to the human host. 
Transmission of Viruses to the Progeny of Infective Vectors 
The inheritance of a virus by the progeny of infective vectors is a 
rare occurrence and there are only two authentic records in the plant 
viruses. In the animal viruses there are one or two instances of infection 
passing through the eggs of certain mites and ticks, and in some cases 
the larvae of a tick can pass on infection to the nymphae, and freshly 
infected nymphae can transmit infection to the adults. 
Fukushi (1939) presents data showing that the virus of the dwarf 
disease of rice is passed from parent to offspring to the third generation 
without recourse to a fresh supply of virus. For the virus to be inherited 
it is necessary for the female parent to be infective; no virus was passed - 
on to the eggs when the male parent only was virus-carrying. 
Recently another case of virus-inheritance has been described: 
Black (1948) gives evidence of the transmission of the clover club-leaf 
virus through the eggs of its insect vector. Twenty-four out of 
twenty-seven insects arising from eggs laid by an infective (i.e. viru- 
liferous) female, transmitted the disease (89 per cent). None of the 
insects transmitted the virus until at least three weeks after hatching. 
The best transmissions occurred during the seventh to eleventh weeks 
inclusive. 
As regards the inheritance of infection by vectors of animal viruses, 
this seems to be confined to rickettsial diseases of which the vectors 
are mostly mites and ticks. 
The life history of the trombiculid mites, as pointed out by Kohls 
(1947), is of special interest in relation to disease transmission since 
