260 HOOKWORMS 



from the lungs they pass by way of the trachea to the oesophagus, 

 and thence to the stomach and intestine. Experiments show, 

 however, that the larvae may reach the intestine by other routes, 

 leaving the trachea and oesophagus out of the circle of migration, 

 but in any case they follow a rather roundabout path in the 

 bloodvessels. Probably in cases of infection by food or drink 

 the worms bore through the mucous membranes of the mouth or 

 oesophagus during the swallowing of the food and thus, even 

 when eaten, reach their ultimate destination by an indirect route. 

 The larvae shed their skins twice more after entering the human 

 body, each time attaining more and more of the adult character- 

 istics and growing in size at the expense of the blood and mucous 

 membranes on which they feed. After the last moult the sexes 

 are differentiated but the larvae are still less than a fourth their 

 full size and require five or six weeks from the time of infection 

 to become fully mature. The length of life of individual hook- 

 worms in the intestine is variously estimated in months or years. 

 The readiness with which reinfection usually occurs makes this a 

 difficult point to determine. 



The Disease. — The disease to which hookworms give rise 

 varies to a very great extent in different individuals, and is not 

 always dependent upon the number of worms present. It was 

 formerly supposed that the anemia and loss of vitality produced 

 by hookworms was due solely to the loss of blood devoured by 

 the parasites. In cases of severe infection, where perhaps several 

 thousands of worms may be harbored by a single patient, the 

 amount of blood devoured must be sufficient to account for a 

 considerable degree of anemia. However, in cases of infection 

 with relatively few worms the symptoms are sometimes fully as 

 marked and cannot be explained on this basis. The injuries 

 from hookworm infection result apparently from a number of 

 causes which may be summed up as follows; (1) ulceration or 

 infection of the skin from wounds made by the boring of the 

 parasites, often giving rise to an extensive affection of the feet 

 in the form of pimples or sores called " ground itch," " water 

 sores," etc., caused partly by entrance of bacteria into the wounds, 

 and partly by the irritation produced by the boring of the worms; 

 (2) loss of blood devoured by the parasites; (3) loss of blood from 

 the bleeding of wounds into the intestines, sometimes very con- 

 siderable, due to a secretion from the mouth of the worm which 



