16 PAKASITES IN GENERAL 



which has not yet been assimilated and utiHzed, (2) by mechani- 

 cally mjuring its tissues or organs, (3) by the formation of ex- 

 cretions or " toxins," which act as poisons. 



The first method of damage is of least importance, though it 

 is obvious that the amount of food abstracted by some parasites, 

 e.g., large tapeworms which may reach a length of several yards 

 and grow at the rate of several feet a month, must be considerable. 



Much more serious are the various kinds of mechanical injury 

 to tissues or organs. This damage is done by the blood-sucking 

 parasites, such as hookworms, flukes, leeches and blood-sucking 

 arthropods and the tissue-devouring forms, such as dysenteric 

 amebae, malaria parasites, lung flukes, and fly maggots, which 

 may not only devour the cells of the body, but may also cause 

 hemorrhages, give portals of entry for other infections, and per- 

 forate the intestine. Here also belong the obstructing parasites, 

 which by their presence block bloodvessels, as do subtertian 

 malaria parasites or blood flukes; stop up lymph vessels, as do 

 adult filarial worms; or partially or completely close up such 

 ducts as the bile duct and pancreatic duct, as do liver flukes. 

 There are also parasites which damage and inflame the tissues by 

 boring through them, as does Trichinella, the guinea-worm, itch 

 mites and fly maggots. 



The third type of injury, by excretion of toxic substances, is 

 done to some extent by practically all parasites. In external 

 parasites the damage is usually done by an excretion, usually 

 of the salivary glands, which prevents the coagulation of blood, 

 and tends to inflame the tissues with which it comes in contact. 

 In the case of internal parasites the toxic substances are prob- 

 ably in most cases the waste products of the parasites, voided 

 into or absorbed by the blood or neighboring tissues. In many 

 cases these toxins have specific actions on particular tissues or 

 organs, so that parasites in one part of the body may do their 

 chief damage to an entirely different part. Intestinal worms, 

 for instance, may produce considerably greater derangements 

 of the blood or of the nervous system than of the intestine; the 

 trypanosome of Chagas' disease produces, by means of toxins, 

 specific effects on the thyroid gland and gives rise to the symp- 

 toms which result from interference with the gland, even though 

 the parasites may not be located in the gland itself; the bite of 

 certain ticks along the line of the spinal cord or on the middle 



