47 



mosquitoes cannot be very greatly reduced in number, if 

 only enough money is spent on the job. 



22. Other Methods of Preventing Mosquito-Borne 

 Diseases. Three diseases are known to be carried by 

 mosquitoes, namely, elephantiasis, malarial fever, and yellow 

 fever. In all, the manner in which the mosquito acts is the 

 same. The diseases are really due to small parasitic organ- 

 isms which live in the blood,* and which, when the blood is 

 sucked from a patient by a mosquito, exist in the insect for 

 a time, and then, when the insect bites a healthy person 

 some weeks later, enter his body and set up the infection in 

 him. The mosquito thus acts as a carrier or go-between ; 

 and merely conveys the poison from the sick to the healthy, 

 just as the lancet of a vaccinator conveys the vaccine from 

 the arm of one child to that of another. This seems strange 

 to the uninitiated ; but it is a commonplace with pathologists. 

 Several diseases of animals are carried by insects in a 

 similar manner ; for instance, the famous African disease of 

 cattle which has long been known to be communicated by 

 the tsetse-fly. These facts enable us to prevent mosquito- 

 borne diseases by more than one method, as follows : 



(i). By getting rid of mosquitoes. 



(2). By preventing mosquitoes from biting patients. 



(3). By preventing mosquitoes from biting the healthy. 



(4). By killing the parasitic organisms in patients. 



(5). By living at a distance from people who are likely 

 to have the disease. 



All these methods are more or less effective ; but some 

 are much more practicable under certain conditions than 

 others are. Let us now study the question. 



* We know this by inference only in the ease of yellow fever. 



