CHAPTER VIII 

 VIRUSES AND INSECTS 



1. The Multiplication of Viruses in Insects 



Many virus diseases are transmitted by insect vectors, animal 

 viruses usually by blood-sucking insects such as mosquitoes, lice, 

 and sometimes by arthropods such as ticks, while plant viruses by 

 aphides and leafhoppers. It is worthy of note that an insect which 

 has been contaminated with a virus, as a rule, is endowed with no 

 infectivity immediately after the contamination, but that only after a 

 certain period of time becomes capable of transmitting the virus. In 

 the case of yellow fever a kind of mosquitoes, for example, attains 

 the infectivity 12 days after blood-sucking, and in dengue fever there 

 is an incubation period of 7 to 10 or more days before the mosquitoes 

 are infective to man. Also with a variety of plant viruses and their 

 vectors the existence of similar incubation periods have been well 

 established. 



The presence of these incubation periods can be readily elucidated 

 by the assumption that viruses multiply in the insect bodies thereby 

 the properties of the virus are changed and activated. In corroborat- 

 ing this assumption, it has been confirmed by a number of workers 

 that some viruses actually multiply in the insect vectors. For instance, 

 Trager (89) has shown by cultivating the virus of equine encephalo- 

 myelitis in sterile mosquito tissue that the virus can under certain 

 circumstances undoubtedly multiply in the mosquito. Merrill and 

 Ten-Broeck (90), working with the same virus, macerated the bodies 

 of infected mosquitoes, and fed the juices to non-infective mosquitoes, 

 and found that after ten transfers the mosquitoes still become infec- 

 tive and since the dilution at each transfer was at least 1 : 100 it 

 seems conclusive that multiplication must have taken place. Whitman 

 (91) has presented evidences that a kind of mosquito is capable of 

 multiplying yellow fever virus in its body. Following the injection 

 of human blood, the content of virus falls for several days, reaching 

 a minimum during the first week. It then increases rapidly until 

 quantities of virus greater than those previously encountered can be 

 demonstrated. 



