THE EFFECT OF PARASITES ON THE HOST 33 



circumstance may expose the host to a very high infection or permit 

 the uncontrollable multipHcation of the parasites within the body 

 which generally spells disaster. In order to soften Pharaoh's heart no 

 new parasites were created but the numbers of existing ones were 

 temporarily increased. When the dust turned to lice the importance of 

 this fact was no doubt appreciated by the Egyptians — " Then the 

 magicians said unto Pharaoh, This is the finger of God." It is this 

 problem of numbers upon which the host concentrates most of its 

 efforts and which it strives desperately to solve. Birds have developed 

 fidgeting, preening, dust-bathing, blinking and, in certain species, 

 "anting" (see p. 127) to such a fine art that ecto-parasites are generally 

 kept within reasonable bounds. Temporarily, at the end of the nesting 

 season, or if the host falls sick, their numbers may increase until they 

 assume menacing proportions, but this is unusual. The host has also 

 developed two main types of resistance to internal parasites. In some 

 cases, where small organisms are concerned, it imposes a sort of birth- 

 control on the invader and in other cases a curb on overcrowding, 

 like the housing act, and thus prohibits the development and establish- 

 ment of any further individuals of the same species in the same 

 individual host. This is a particularly effective method where helminths 

 are concerned. In fact, in many cases it is the early worm — and often 

 only the early worm — which gets the bird. 



Both types of resistance can be demonstrated in the case of avian 

 malaria. During the ten days following infection the parasites are 

 found to accumulate very rapidly in the bird's blood. Then the host 

 falls ill and displays symptoms characteristic of the disease. During 

 this crisis the number of parasites in the peripheral blood stream in- 

 creases to a peak, but after about five days the bird recovers and the 

 parasites apparently disappear. However, a few are able to hang on in 

 the bone-marrow and the spleen where they continue to reproduce but 

 only in a very discreet manner. Then suddenly, for some reason which 

 remains obscure, the host's power of imposing birth-control on the 

 parasite seems to fail and there is a sudden increase in its numbers and 

 the sufferer has a so-called relapse. This feature is also characteristic of 

 malaria in man. 



The reason for the sudden initial fall in the number of parasites is 

 principally due to their destruction by certain other blood cells of the 

 bird known as the phagocytes. A curious sort of armament race takes 

 place within the bird's veins and arteries. The phagocytes begin to 



