MODE OF INFECTION 259 



(Fig. 103D). The larva has by this time grown to several times 

 its original size, being over 5 mm. (^V of an inch) in length, and is 

 now in the infective stage and ready to begin its parasitic life. 

 No further food is taken but the parasite begins an active migra- 

 tion in the neighboring soil or water. If even a trace of moisture 

 is present in the soil the larvae are capable of traversing consider- 

 able distances and may thus give rise to infection far from the 

 place where the fgeces were originally deposited. They are said 

 to be able to travel through moist soil at a rate of probably not 

 less than five feet per hour, which, if kept up constantly in a 

 straight line would mean a wandering of forty yards in twenty- 

 four hours. While such continued travel in a straight line prob- 

 ably would never occur, it is evident that a single infective stool 

 would easily be able to infect the ground for several square yards. 

 Complete drying up is fatal to both eggs and larvae in all stages. 



The larvae may remain just under the surface of moist soil or 

 mud or in water for a long time, awaiting an opportunity to enter 

 a human host. They have been kept alive in the laboratory 

 in plain water at a temperature of about 60° F. for 18 months 

 and unless attacked by predaceous insects or other animals 

 would undoubtedly live fully as long under outdoor conditions. 

 They are much more resistant to unfavorable conditions than 

 are the eggs or newly hatched larvae. They can exist under de- 

 privation of air for a long time and may survive burial in snow 

 for at least six days. 



It was formerly thought that infection occurred by way of the 

 mouth only, the larvae entermg with impure food or water. It 

 is now believed, however, that this means is not only not the 

 usual one, but that direct infection by swallowing may never 

 occur, since there is evidence to show that the parasites are un- 

 able to resist the acid juices of the stomach before they have 

 first passed through the blood and tissues of the body. It was 

 discovered purely by accident that the hookworm larvae can 

 readily penetrate the skin and bore through the tissues until 

 they reach a vein. The feet of plantation laborers are often in 

 a bad state of soreness and ulceration due to the boring of the 

 larvae and to subsequent infection by bacteria. Walking on in- 

 fected ground with bare feet is undoubtedly the mode of infection 

 in the majority of cases. By the blood or lymph vessels the 

 worms are carried eventually to the heart and thence to the lungs; 



