that of N. americanus only about 8,000 to 10,000 (Super, 1927) 

 Also the infective larrae of the first species are slightly larger, 

 of different structure, and more resistant to environmental con- 

 ditions than those of the second (Svensson, 1925). In experi- 

 mentally infected human volunteers, the adults of A. duodenale 

 lived almost 7 years and those of X. americanus over 5 years 

 (Kendrick, 1934). In these infections, however, the egg counts 

 in the individuals infected with the first species fell to a very 

 low level in less than 2 years, and in those harboring the sec- 

 ond species they were greatly reduced in about a year. In spite 

 of all these differences, both species are very similar in their 

 host relations and life cycles; and the symptomatology, epi- 

 demiology, treatment, and control of the diseases they produce 

 are alike in all essential particulars. 



Although in a few cases the human hookworms have been re- 

 ported incidentally in other hosts, and A. americanus appears 

 to be a normal parasite of anthropoid apes, there is at present 

 no good evidence that such animals serve as true reservoir hosts. 



Besides the true human hookworms there are several others 

 that have some relation to man. The dog and cat hookworms, 

 .4. caninum, A. braziliense, and Uucinaria sienocephaia have 

 been extensively used in studying host-parasite relation prob- 

 lems. The larvae of the last two have been shown to produce 

 linear skin lesions in man, and A. braziliense is the causative 

 agent of creeping eruption which is especially prevalent in 

 certain parts of the southern United States (Fiilleborn, 1928; 

 Kirliy Smith, Dove, and White, 1929). N. suiUus, described 

 by Ackert and Payne (1923) and by some workers considered 

 as a synonym of A. americanus, has received consideration in 

 relation to'the possibility that its host, the domestic pig, may 

 serve as a reservoir host for humau hookworm disease. 



FACTORS AFFECTING THE FREE STAGES OF THE 

 HOOKWORM LIFE CYCLE 



Development of hookworm larvae can be completed at tem- 

 peratures ranging from about 12° to 37° C, with the optimum 

 from about 2.5° to 30° C. (Stiles, 1921; McCoy, 1930). Below 

 22° C. the development is greatly slowed up; and at tempera- 

 tures approaching 37° C, although development is very rapid, a 

 considerable proportion of the larvae either fail to develop or 

 soon die. The eggs and larvae are quickly killed by tempera- 

 tures above 40° C. and have little resistance to temperatures 

 close to freezing (Looss, 1911; Svensson, 192.")). The injurious 

 effect of low temperatures on hookworm eggs and larvae is the 

 determining factor in limiting the distribution of hookworm 

 disease almost entirely to tropical and subtropical regions. 



All the free stages of the hookworm life cycle are quickly 

 killed by desiccation. Therefore, in regions of low rainfall in- 

 fection is absent or kept at a low level (Chandler, 1926-1928; 

 Sawyer, 1923; Docherty, 1926). On the other hand, while the 

 eggs and infective larvae will live for a considerable period 

 under water, they will not develop eitlier under water or in cul- 

 tures that are saturated with moisture. Therefore, in areas 

 where the soil is flooded for a part of the year hookworm in- 

 fection may be kept at a low level (Chandler, 1926; Barnes and 

 O'Brien, 1924). Hookworm larvae require the presence of 

 oxygen for development (McCoy, 1930) and it is probably its 

 absence that prevents their development in a saturated medium. 

 Also, they require a loose porous culture medium and do not 

 develop well in clay soils (Stoll, 1923b). Soil relations are 

 very important in the southern United States where infection 

 is almost entirely absent in areas with clay soil and is par- 

 ticularly intense in those with a loose sandy soil (Augustine 

 and Smillie, 1926; Rickard and Kerr, 1926). 



The developing larvae can apparently feed normally only on 

 living bacteria, which must be present in considerable numbers 

 for development (McCoy, 1929). It seems probable that the 

 growth of enough bacteria for the needs of the larvae depends 

 chiefly on the mixture of feces with the soil and if the eggs 

 become separated from the fecal material in which they are 

 passed development will be checked. 



Epidemiologic studies of recent years have given illustrations 

 of the types of field conditions that are suitable or unsuitable 

 for the development of the hookworm larvae in the soil. Loose 

 porous humus, sandy, or loam soils that are well shaded give 

 the best development. Places of intense soil infestations under 

 such conditions have been reported in fields of sugar cane in 

 Trinidad (Cort and Payne, 1922). in coffee groves in the hills 

 of Puerto Rico (Cort, Riley, and Payne, 1923), and in fields 

 of cultivated mulberry trees in the Yangtse Delta region of 

 China (Cort, Grant, and Stoll, 1P26). In clay soil not covered 

 b.v a layer of humus or a growth of grass almost no larvae 

 will develop even where the rainfall is considerable (Cort and 

 Payne, 1922). Even on soils of loose texture in regions of 

 abundant rainfall, development of soil infestation will be great- 

 ly inhibited if there is no shade, since exposure of the soil sur- 

 face to the sun's rays produces alternate periods of wetting and 



drying which quickly destroy a large proportion of the larvae 

 (Augustine, 1923c). Unshaded areas covered with a thick 

 growth of grass have in some cases been reported as very favor 

 able for development (Korke, 1925). In hookworm infected 

 population groups, therefore, significant sources of infection 

 may be limited, even where there is extensive soil pollution, to 

 the Comparatively few places where the eggs are deposited on 

 a loose soil that is well shaded. 



When the larvae develop in the soil they migrate toward the 

 surface and are found frequently singly or in clumps extending 

 from the particles (Augustine, 1922b; 1923b). In only a few 

 cases have they ever been reported at depths below the super- 

 ficial surface layers (Baermann, 1917b). When covered with a 

 loose soil they can migrate vertically from considerable depths 

 (12 to 36 inches), while in a water soaked or stiff clay soil 

 almost no upward migration occurs (Payne, 1922 and 1923). 

 Lateral migration is very restricted and they will not spread out 

 from the place of development unless carried by water or ani- 

 mals (Augustine, 1922a; Chandler, 1925). After the second 

 molt they no longer feed and will continue to live only as long 

 as their reserve of food material lasts. Therefore, the more 

 active they are the shorter will be their life. Under artificial 

 conditions in water, however, infective hookworm larvae have 

 been kept alive for as long as 18 months (Ackert, 1924). In 

 the soil in the tropics their life may be limited to only 6 to 9 

 weeks, with the great majority dying in 3 or 4 weeks (Augus- 

 tine, 1922c and lS23c). Under conditions less favorable for 

 activity they may persist in the soil for periods up to 4 to 6 

 months (Hirst, 1924; Baermann, 1917b). There is also evi- 

 dence that the larvae of A. duodenale live somewhat longer 

 than those of X. americanus (Svensson, 1925). 



A consideration of the activities of the infective hookworm 

 larvae in the soil lead to certain practical considerations iii 

 relation to the epidemiology and control of hookworm disease. 

 The larvae tend to remain in "nests'' where the stools are 

 deposited ; so only limited places are sources of infection. 

 Further, the burying of feces except under a very stiff clay 

 soil is dangerous because the larvae will soon reach the surface. 

 There is no evidence, however, that they will migrate out of 

 latrines (Payne, 1922). Finally, where soil pollution is 

 stopped, sources of infection will be naturally sterilized in a 

 comparatively short time b.v the death of the larvae. 



HOST RELATIONS TO HOOKWORil INFECTION 



The penetration of the infective hookworm larvae through 

 the skin produces lesions which are commonly known as ground 

 itch. Secondary bacterial infection frequently increases the 

 severity of this condition. Also, the type of reaction suggests 

 in many cases an allergic condition associated with the pres- 

 ence of immunity. Thus Sarles (1929) noted a much more 

 severe skin reaction to hookworm larvae in old resistant dogs 

 than in susceptible puppies. 



Lung symptoms produced by the migrations of the larvae 

 have frequently been noted. They are only occasionally at all 

 severe except in extremely heavy infections, suggesting that the 

 larvae usually enter a few at a time. 



In the intestine, the hookworms bite deeply into the mucosa 

 and appear to suck blood constantly throughout their adult 

 life (Wells, 1931; Nishi, 1933). It seems evident that they 

 feed chiefly on elements derived from the blood (Hsii, 1938). 

 They move" from place to place and, therefore, when numerous 

 injure the intestinal wall over considerable areas. Blood con- 

 tinues to flow from the lesions even after they have moved 

 away. Disturbances of the digestive system which are com- 

 monly present in moderate as well as heavy infections have 

 been "explained chiefly in relation to the injury of the intes- 

 tinal mucosa produced by the worms. 



Anemia is the most prominent symi)tom of hookworm disease. 

 Indeed, most of the long train of symptoms found in chronic 

 hookworm patients can be related to the presence of long 

 standing anemia. The etiology of hookworm anemia has been 

 the subject of considerable controversy. A review of the liter- 

 ature indicates that there is no convincing evidence that it is 

 caused by toxic products of the worms. Recent investigations 

 have emphasized the importance of blood loss in the production 

 of the anemia. In experimentally infected dogs the blood pic- 

 ture follows exactly that produced by artificially induced hem- 

 orrhage (Foster and Landsberg, 1934; Landsberg and Cross, 

 1935; Landsberg, 1937). Apparently, blood loss produced by 

 the worms is only one factor in the production of the anemia 

 in hookworm infected populations. Disturbances produced by 

 dietary deficiencies, particularly lack of iron, have been em- 

 phasized as important additional factors (Rhoads, Castle, Payne, 

 and Lawson, 1934 a & b; Cruz, 1934). More recent work, 

 however, stresses general dietary deficiency rather than lack of 

 iron alone (Otto and Landsberg, 1940; Payne and Payne, 

 1940). Anemia produced by other diseases especially malaria 



310 



