PARASITISM 



inevitably means a smaller chance of infection, which makes the 

 host-parasite ratio to some extent self-regulatmg. It is this 

 which may account for the violent iiuctuations m numbers of 

 many animals, especially rodents. The numbers of voles, for 

 instance, increase steadily until a certain degree of overcrowding 

 is reached, and then an epidemic caused by a parasite spreads 

 rapidly and reduces the numbers. 



Apart from death, the effects on the hosts vary with the 

 parasite and the place where it lives. Ectoparasites seldom cause 

 more than irritation, although secondary infection with another 

 parasite, such as the plasmodium carried by the gnat, may lead 

 to serious results. Endoparasites which Hve in the gut (which, 

 strictly, is outside the body) may do nothing more than deprive 

 the host of some food ; such is the case with a small number of 

 Ascaris or tapeworms ; larger numbers may take so much food 

 that the host cannot supply enough, with general effects on health, 

 and symptoms such as diarrhoea and vomiting are common. 

 Hookworms may cause enough loss of blood to produce anaemia. 

 Parasites hving in the fluid tissues (the blood and lymph) may 

 merely steal some food, as does Filaria perstans, which produces 

 no known pathological effect ; they may mechanically block the 

 vessels, like Filaria bancrofti ; or, like Trypanosoma, and 

 Plasmodium, they may manufacture toxins which cause a 

 violent reaction in the host. Those which live in the soHd tissues, 

 such as the Guinea worm and Trichinella, always cause irritation, 

 and may have more serious effects. A special case is made by 

 those parasites which preferentially attack the gonads, causing 

 parasitic castration. Examples are known in the Protozoa, 

 trematodes and insects, but the most familiar is the cirripede 

 crustacean Sacctdina, parasitic on crabs, in which the castration 

 is accompanied by changes in the secondary sexual characters. 

 Parasites which attack the brain, such as Trypanosoma in its 

 later stages, and the cysticercus of Tcenia cosnurus in sheep, 

 have profound behavioural effects. According to the situation 

 and degree of toxicity of the parasite, various local reactions, 

 from inflammation to tumours, may occur in the host's tissues. 



IMMUNITY 



A matter which is of great importance both to the parasite 

 its efforts to establish itself in a new host and to man in his 



