INSECTS AS AGENTS IN SPREAD OF DISEASE 537 



INSECTS AS AGENTS IN THE SPKEAD OF DISEASE 



Bx CHARLES T. BRUES 



BUSSEY INSTITUTION, HARVARD UNIVERSITY 



LESS than fifteen years have elapsed since the scientific world enter- 

 tained its first definite suspicion that certain human diseases 

 might be spread through the agency of insects. Twelve have gone by 

 since that suspicion became an established fact, and in this short space 

 of time so much has been learned concerning the pernicious activities of 

 these small animals in disseminating disease-causing organisms among 

 man and the higher animals, that the science of preventive medicine 

 can now be applied to many important diseases which were before 

 utterly beyond its reach. Every year brings forth fresh evidence that 

 insects are important factors in relation to public health, and adds to 

 the list of diseases that are partially or entirely dependent upon certain 

 insects for their spread. 



A brief statement of the nature of communicable diseases and of 

 the general habits of the kinds of insects that are implicated in carrying 

 disease will serve to define roughly the field of medical investigation 

 which is open to the entomologist. Communicable diseases are invari- 

 ably due to parasitic organisms in the body which are capable of induc- 

 ing similar s}Tnptoms in other persons or animals if transferred to 

 healthy individuals from diseased ones. Many conditions modify the 

 transfer of communicable diseases; some individuals are more easily 

 infected than others; some may be immune as the result of a previous 

 attack; and, on the other hand, the virulence of pathogenic organisms 

 often varies greatly in accordance with conditions to which they have 

 been subjected previously. A simple method of spread occurs with 

 many diseases, for example typhoid fever and pulmonary tuberculosis 

 With the former, the Bacillus typhosus which is the disease-producing 

 organism, is present in the dejecta of an infected person and may find 

 its way from these to food, carried by flies or otherwise ; ingested by a 

 healthy person, it may quite likely multiply and induce a second case 

 of typhoid. With tuberculosis, the tubercle bacillus from desiccated 

 sputum may enter the lungs of a healthy person with dust and there 

 reproduce the disease. As we shall see later, certain insects are com- 

 monly associated with the spread of diseases of this type, although from 

 the very nature of such diseases, insects are not exclusive factors, and 

 may be referred to as contaminative carriers. 



A second type of communicable diseases differs from the one just 



VOL. LXXXI. — 37. 



