740 TEXTBOOK OF ZOOLOGY 



fore more abundant. Some parasitologists consider pathogenic 

 parasites to be imperfect parasites because they are not quite per- 

 fectly adapted for successful parasitic life, while nonpathogenic 

 species are considered perfect parasites. However, no hard and fast 

 line can be drawn between the two. Many parasites which are so 

 perfectly adapted to their customary host that they produce no ill- 

 effects have been found to be strongly pathogenic to other hosts 

 where the adaptation is less perfect; for example, certain trypano- 

 somes which are harmless to the antelopes of Africa, their natural 

 hosts, produce the highly fatal African sleeping sickness when in- 

 jected into men. 



Means of Infection and Transmission 



Many different means of transfer from host to host have been 

 developed by the various kinds of parasites. These may be classified 

 as below: 



A. Passive transmission. 



1. In food or in water. 



2. By bite of insects. 



3. By sexual intercourse. 



4. By direct contact. 



B. Active invasion under own power. 



By ** passive transmission" is meant the transfer of eggs or larvae 

 from one host to another without any action of their own. For 

 example, the eggs of cestodes and of some nematodes, such as 

 Ascaris, pass out of the host's intestine in the feces; if food of 

 other animals is contaminated by these feces, animals which eat 

 this food will swallow the eggs, which hatch into larval worms 

 within the digestive system of the second host and thus establish 

 a new infection. Sheep may become infected with liver flukes by 

 eating the encysted larvae on grass or swalloAving encysted larvae 

 while drinking water. Trichina larvae encysted in hog meat de- 

 velop into adult worms in the human, intestines if infected pork is 

 eaten raw or improperly cooked. 



The second means of passive transmission is also widely used. 

 Malaria parasites, the trypanosomes which cause African sleeping sick- 

 ness, and many parasites of domestic and wild animals are carried 



