1917 ENTOMOLOGICAL SOCIETY. 59 



Insects as Diiject IxocuL.VToits of Disease. 



This is another simple relation, and the insects which carry disease in this 

 way are piercing species, taking up germs and inserting their contaminated month 

 parts into their' healthy victim. This transfer is precisely analogous to blood- 

 poisoning from the prick of a contaminated needle or pin. A little earlier this 

 method of carriage of disease was considered to be more easily possible than it is at 

 present. A study of the hal)its of many of these blood-sucking insects indicates 

 tliat. while they take a very full meal, they frec^uently wait for many hours before 

 attempting another bite, and in the meantime ingested bacteria may be digested 

 or excreted and the beak become cleansed or the micro-organisms dried up. 



Xevcrthdess this method still holds, and it is in this way that certain biting 

 flies carry the disease known as anthrax or malignant pustule, and m the same 

 way the very destructive disease of domestic animals in oriental regions, known as 

 surra, is carried by gadflies. In this same way also the disease of cattle in Africa 

 long known as fly sickness or nagana is carried by one or more of the tsetse flies 

 of the genus Glossina. Although, while it is possible for this disease to be almost 

 immediately inoculated after the first bite of a diseased cow, by simple transfer, 

 the fact that after a term of days has elapsed inoculation again becomes possible 

 indicates that the parasitic organism may undergo a sexual development in the 

 body of the fly. This will be brought out later in speaking of sleeping sickness. 



In the case of the rat fleas and bubonic plague, about which so much has been 

 written of late years, there occurs also something more than a passive carriage, 

 although the causative organism of bul)onic plague is one of the bacteria, and is 

 known as BnciUus pestis. The story of the discovery of the carriage of this dread 

 disease by fleas is a most interesting one, but cannot be told "at length. Any flea 

 which attacks both rodents and man may be an agent in the transmission of the 

 disease, and several species are thus implicated. Inasmuch a^. the causative organism 

 of the disease is a Bacillus, and is not dependent upon any insect for the completion 

 of its development, theoretically any blood-sucking insect which feeds upon a 

 plague-infected man or animal, and then passes to a healthy individual may carry 

 tlie disease. Thus bacilli have been found in a head-louse taken from an infected 

 man, and in a louse taken from an infected squirrel. 



^loreover, it has been found that in bubonic plague the disease may be 

 sjn-ead from man to man without any intermediary whatever. Conclusive evidence 

 to this eiTect was fouiul Ijy Dr. Strong and Dr. Teague during the Man- 

 churian epidemic of 1010-11. This type of the disease, however, forms a very 

 small percentage of the human cases, and in the great majority of cases of a 

 plague epidemic fleas are the responsible carriers, and as a rule rats or other 

 rodents, such as the ground-sqiiirrels of California, form the other end of the 

 chain. So practically all the measures in the modern cities are leased upon the 

 destruction of rats, and we in the United States recall with pride the campaign 

 against the rats and the plague carried on so successfully only a few years ago in 

 San Francisco, under th;.^ direction of the ]>resent Surgeon-General of our-Public 

 Health Service, Dr. Rupert Blue. 



While feeding, fleas are in the habit of squirting blood from the anus, and 

 where they have been feeding upon mice ^nd rats dying of plague, this excreted 

 blood is found to be full of the plague bacilli. Thus, not only may the disease be 

 caused by the bite, but by sul)sequent scratching, ^[oreover, Bacot and ^Martin have 

 shown very recently that plague-infected fleas regurgitate blood through the 

 mouth, and that the disease mav l)e thus transmitted. 



