ANIMAL PARASITISM 433 



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 de- 

 veloped 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 ex- 

 ample, 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 diges- 

 tive 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 swallowing encysted larvae while drinking water. 

 Trichina larvae encysted in hog nieat develop 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 

 sickness, and many parasites of domestic and wild animals are car- 

 ried from infected individuals to new hosts by biting insects which 

 suck up the parasites with the blood and inject them into the new host 

 w4th their salivary secretions. 



