l] INTERMEDIATE HOST 5 



protozoal parasites, the sexual part of their life-cycle takes 

 place in the invertebrate carrier, and therefore the latter is 

 the definitive host. On the other hand, the sexual cycle of 

 Filaria bancrofti takes place in its vertebrate host, man, and in 

 this case man is the definitive host. 



The host in which the parasite merely multiplies asexually 

 is called the intermediate host, and in the case of protozoal 

 infections is the vertebrate, but in Filaria is the invertebrate, 

 host. 



The term " intermediate host " has given rise to much con- 

 fusion, for many writers still persist in considering the expression 

 synonymous with " invertebrate host." As a matter of fact, 

 in the majority of cases the vertebrate is the intermediate 

 host, and it is only obscuring the true relations of parasite 

 and host to persist in the erroneous application of this term. 



In all cases the equilibrium between parasite and host is 

 much better established in the definitive than in the inter- 

 mediate host. As a general rule parasites do not have a very 

 harmful action on their definitive hosts, whether inverte- 

 brate or vertebrate, whilst on the other hand there are many 

 examples of parasites seriously affecting the health of their 

 intermediate hosts. Thus the malarial parasite does not seem 

 to affect injuriously the health of its definitive host, the mos- 

 quito, whilst in the intermediate host, man, it produces malaria, 

 a serious and often fatal disease. Similarly, the presence of 

 Filaria bancrofti in its definitive host, man, does not produce 

 .any obvious ill-effects, whilst a large proportion of the infected 

 mosquitoes succumb to the infection. 



Although so many parasites have an injurious effect upon 

 their hosts, sometimes causing death, it is obviously a short- 

 sighted policy for a parasite to kill its host, as by so doing it 

 also destroys itself. A parasite which invariably killed its 

 host would eventually die out, owing to the extermination 

 of the latter, and it is not inconceivable that, in this way, 

 excessively fatal infections have disappeared from the world, 

 together with the animals they affected. 



At the present time the great majority of blood parasites 

 do not cause the death of their hosts. An equilibrium between 



