36 FLEAS, FLUKES AND CUCKOOS 



It is of course a well known fact that micro-parasites such as 

 bacteria and the viruses can, under certain circumstances, show an 

 increase or decrease in the pathological or poisonous effect they exert 

 on their host. The same phenomenon can be observed in various 

 parasitic Protozoa. For example, if the spirochaete which causes 

 relapsing fever in man is inoculated into mice, and passed rapidly 

 through a series of these animals, the organism loses its power to infect 

 man at the end of a few years. It is also noticeable how in many 

 epidemics, such as the influenza epidemic of 1948-49, the virus increases 

 in virulence as it passes from host to host. By the spring the disease was 

 far more serious than at the beginning of the winter. Certain try- 

 panosomes, such as the species which attack the big game of Africa, 

 exert no ill effects on those animals which are considered to be their 

 normal hosts, but prove virulent and fatal if they are passed to domestic 

 cattle. In other cases, such as pigeon-pox, the effect on unusual hosts 

 like the chicken is negligible in comparison with the effect on the normal 

 host. Again the introduction of other parasites may lower or raise the 

 virulence of an infection. Thus mild and chronic avian malaria in 

 canaries can be stimulated by the presence of spirochaetes, and 

 develop virulence and toxigenicity which soon kills the host. These 

 variations are often considered to be the result of selection acting upon 

 certain types already present in the infection or developed by muta- 

 tions. In bacteria they are frequently associated with morphological 

 changes. Such evolutionary trends can be observed in the case of 

 unicellular organisms owing to the numerous generations which follow 

 one another in rapid succession. The phenomenon is one of great 

 interest and practical importance but despite an immense amount of 

 research it is still not properly understood. 



Turning now to brood parasites, such as the European cuckoo, the 

 attack on the host is relatively easy to observe. In the first place the 

 female destroys at least one egg of the host which she replaces by her 

 own. Subsequently, if the egg is accepted and incubated, the young 

 cuckoo, on hatching, destroys all the other eggs or nestlings which may 

 be present. Henceforth the entire efforts of the foster parents are 

 directed towards feeding and rearing the intruder chick. Frequently 

 birds desert their nests after a visit from the cuckoo, but even such cases 

 entail a considerable loss of time and effort on the part of the victim. 

 It must be remembered that a female cuckoo can, under favourable 

 conditions, lay over twenty eggs in different nests during a single 



