THE EFFECT OF PARASITES ON THE HOST 35 



birds, one attack confers a lifelong resistance or immunity against that 

 particular organism. Generally, in the case of the larger parasites 

 immunity is such that it prevents reinfection of the same species but 

 only while the infective organisms are still in the body of the bird. Thus 

 complete recovery — that is to say the death of the parasites — exposes it 

 to a fresh invasion. This state of affairs is so advantageous for the 

 worm or protozoan concerned that it might well be considered an 

 adaptation of the parasite rather than the host. In other words, the 

 host protects the parasite for life from intra-specific competition and 

 offers itself once again for spoliation by some other member of the race 

 if death removes the intruder. Simultaneously, of course, it also protects 

 itself from the crushing, possibly fatal, burden of over-population. 



In the case of many endo-parasites of insects, superparasitism 

 frequently results in a battle between the larvae, only one surviving per 

 host. Thus intra-specific competition limits the number of parasites 

 and in this way achieves much the same results as the temporary im- 

 munity developed by the host itself. In the case of birds, if a heavy 

 initial infection occurs, and many larval worms are ingested simulta- 

 neously, intra-specific competition may also occur between the 

 developing adult worms. 



This balance of power or mutual adjustment of parasite and host is 

 regarded as the hallmark of successful parasitism. Caullery considers 

 that in the case of such an infection the parasite and host together form 

 a functional balanced system which is placed in opposition to the ex- 

 ternal environment. In other words both parties make the best of a bad 

 job. The host's reactions — the result of selection — tend to reduce the 

 inconvenience to a minimum and the parasite has to live as unobtrusive- 

 ly as possible in a hostile environment. Together they must face the 

 dangers and hazards of the outside world. 



In the case of larval flukes it is worth remembering that there can 

 be no gradual adaptation between host and parasite. Selection is 

 entirely one-sided. The parasite castrates the host, or in the case of 

 young snail hosts inhibits the growth of the gonads, and therefore the 

 more susceptible snails, and even those which survive infection the most 

 successfully, do not reproduce themselves and are eliminated from the 

 population. Consequently adaptation can only be on the side of the 

 parasite. In the case of larval flukes this situation is very obvious, but 

 it probably exists in many other cases of parasitism, when it is wrongly 

 assumed that adaptation is mutual. 



FFC— D 



