34 FLEAS, FLUKES AND CUCKOOS 



increase in numbers soon after the malaria parasite is inoculated into 

 the bird. So do the parasites. Both increase more and more rapidly. 

 Sometimes the parasites occur in vast numbers — one infected corpuscle 

 to every ten healthy red blood cells is not uncommon. A desperate 

 running fight ensues — the phagocytes killing and ingesting the parasites 

 as quickly as they appear in the blood. Sometimes the parasites win and 

 their uncontrolled reproduction kills the host, but generally, in the 

 case of avian malaria, the phagocytes are victorious. The parasites are 

 destroyed — except, as we have pointed out, for a few isolated pockets of 

 resistance in the bone-marrow and the spleen. When the emergency is 

 over and the parasites have vanished from the outer peripheral stream 

 the number of phagocytes returns to normal. The standing army which 

 remains appears sufficient to cope with the ordinary situation. But 

 if the resistance of the host is lowered and the parasites again tempor- 

 arily get the upper hand, wholesale mobilisation of the phagocytes 

 occurs all over again. For the continuation of their life-cycle it is of 

 vital importance that Plasmodium should appear periodically in 

 large numbers near the surface of the host's body. Without these 

 occasional outbursts of reproductive activity they would never find 

 their way into the proboscis of the insect carrier. On the other hand it 

 is not in their interest to kill the host outright. 



A great number of unknown factors may be involved in the so- 

 called lowered resistance of birds. The weather, particularly humidity 

 and low barometric pressure, the phases of the moon, the amount of 

 sugar present in the blood, exposure to ultra-violet rays, have all been 

 implicated and may be the direct or indirect cause of a relapse. In the 

 case of many parasites a bird's resistance varies with age. Tuberculosis 

 is pathological chiefly in old birds, whereas only young birds fall 

 victims to the attacks of certain worms. 



In addition to phagocytosis the birds react to the presence of Plas- 

 modium by developing certain specific substances known as antibodies 

 in the blood serum and other body fluids. Their presence renders the 

 environment difficult or unsuitable for the parasite. This keeps the 

 numbers down after the initial reduction by the phagocytes — at any 

 rate any new infection by the same species is destroyed or unable to 

 develop. This phenomenon is known as partial immunity. Immunity 

 is the most widely studied of all the effects which parasites produce on 

 their host, for it has a wide practical application in medicine. In the 

 case of certain virus diseases such as small-pox in man and fowl-pox in 



