incidence of the pig and human asearis are very great under 

 conditions that would seem to favor cross infection (Payne, 

 Aekert, and Hartman, 1925; Caldwell and Caldwell, 192(3; Mar- 

 tin, 1926; Roberts, 1934). Therefore, until some evidence can 

 be 'presented of human infection with asearis from the pig, it 

 hardly seems reasonable to consider the domestic pig as a 

 reservoir host of any significance in the dissemination of asearis 

 in human populations. 



There is some evidence that a specific immunity is acquired 

 to infection with A. himbricoiiles. Some of the studies showing 

 this have been made on abnormal hosts and, therefore, involve 

 only the stages of the cycle through the lung migration (Kerr, 

 1938). Other workers have reported experiments that sug- 

 gested the development of immunity in pigs (Morgan, 1931; 

 de Boer, 193(ib; Roberts, 1934). In pigs abo infection is very 

 much greater in young than in old animals (Ransom and Fos- 

 ter, 1920; Roberts, 1934). Such differences might be explained 

 as the result of an immunity produced by repeated infection. 

 There is some suggestion also that in older animals poor nutri- 

 tion may increase susceptibility (Morgan, 1931; Hiraishi, 

 1928). Possibly in man a part at least of the reduction of in- 

 fection in adults as compared with children may be due to the 

 development of an acquired immunity, although difference in 

 habits cannot be excluded. Also, it seems not unlikely that 

 undernourishment or other debilitating factors may influence 

 susceptibilitv to this parasite. 



There is some suggestion that A. Iiimbricoidcs is not well 

 adapted to its host. This has been suggested by several work- 

 ers because of the difficulty of producing experimental infec- 

 tions in pigs (Ransom and Foster, 1920; Martin, 1926; Hirai- . 

 shi, 192S; Roberts, 1934). A similar relationship in man may 

 explain the rarity of heavy infections. Another significant 

 host relation is the rapid turnover of the infection and the con- 

 stant loss of worms in infected populations (Otto, 1930). In- 

 dividuals frequently pass worms; heavy worm burdens are only 

 kept up by constant reinfection; and groups removed from ex- 

 posure to "reinfection soon lose their worms (Keller, 1931). It is 

 not clear whether this instability of infection is due to lack 

 of attachment of the worms or to immunity reactions of the 

 host. Finally, it seems probable that host reactions have a 

 part in keeping asearis infections in human populations at a 

 low level except under extreme conditions of exposure to infec- 

 tion, and in establishing the peculiar age distribution of this 

 parasite. 



GENERAL ENVIRONMENTAL F.\CTORS 



The eggs of A. lumbricoidrs live for long periods of time 

 and are remarkably resistant to most external conditions. They 

 have been kept alive for 4 to 't years (Davaine, 18.")8 and 1863; 

 Martin, 1920) and under natural conditions will live for 1 to 

 2 years and survive the winter (Brown. 1928; Roberts, 1934). 

 Under field conditions, where they would be exposed to a larger 

 variety of factors, it seems probable that they remain viable 

 for somewhat shorter periods, although it is evident that in- 

 fested soil renuiins dangerous for a very much longer time 

 than is the case with hookworm. 



Asearis eggs have been shown to have a remarkable resistance 

 to a wide variety of chemical agents (Yoshida, 1920; Ransom 

 and Foster, 1920). It seems evident, therefore, that in nature 

 they would rarely if ever meet chemical conditions in the soil 

 that would be unfavoralile. The eggs require a constant supply 

 of oxygen for their development, they can, however, live for 

 several weeks under anaerobic conditions and can develop in 

 cultures where oxygen tension in the surrounding water is only 

 a fraction of saturation (Brown, 1928a). It seems evident, 

 therefore, that under natural conditions they can readily find 

 oxygen enough for development except in polluted water or 

 saturated media where bacterial growth would use up the 



supply. 



Asearis eggs in all stages of development can withstand freez- 

 ing temperatures for surprisingly long periods of time (Cram, 

 1924; Nolf, 1932) and will develop slowly at temperatures as 

 low a's 12° C. The optimum temperature for development seems 

 to be about 30° to 33° C. and development is almost completely 

 inhibited at temperatures of about 37° C. Higher temperatures 

 are very iniurious to the eggs and at temperatures above W° C. 

 they are killed in a short time (Ogata, 192."i; Nolf, 1932). 

 Desiccation is also an important factor in killing asearis eggs, 

 although they will remain viable for several days when dried on 

 glass slides and kept at a relative humidity of about ."JO percent 

 (Otto, 1929; Roberts, 1934). On the dry surface of soil they 

 survive much longer (Caldwell and Caldwell, 1928; Brown. 

 1928b). Their resistance to desiccation is also greatly increased 

 by low temperatures (Martin, 1920). They will develop nor- 

 mally when air dried on glass slides and kept in an incubator 

 with" a relative humidity above 80 percent (Otto, 1929). A 

 number of authors have reported that direct sunlight is lethal 

 to asearis eggs, although in many of the experiments the effect 



of high temperature vias not excluded. There is, however, defi- 

 nite evidence that sunlight per sc docs in.iure the eggs since 

 Nolf (1932) demonstrated that they were quickly killed by ultra 

 violet light. Under conditions in the field, a combination of 

 high temperatures with desiccation is probably most important 

 in killing the eggs as is shown by the rapidity with which they 

 die when exposed to direct sunlight on certain types of soils 

 (Brown, 1927b; Otto, 1929). 



The resistance of the eggs of the human asearis to external 

 environmental conditions accounts for its wide geographical dis- 

 tribution. Studies in the United States (Otto, Cort, and Keller, 

 1931) have shown that they can develop and persist on the hard- 

 packed clay soil of unshaded dooryards where the eggs of tri- 

 churis and hookworm are soon killed. Certainly, A. lumbri- 

 coides is less restricted in its spread by clinuitic and soil con- 

 ditions than any other human parasite with free stages. Of 

 course, tropical and semitropical countries with a high rainfall 

 oft'er the most favorable conditions for its spread; but where 

 human habits are particularly favorable a high incidence with 

 heavy infections may occur in regions, such as in certain places 

 in North China, where there is low lainfall and a long cold 

 winter (Cort and Stoll, 1931; Winfield, 1937a). 



HUMAN HABITS AND SOURCES OF INFECTION 

 Studies of the last few years in such widely separated regions 

 as tropical America, the southern United States, North China, 

 and the Philippine Islands, have shown that the chief sources 

 of asearis infection are from eggs deposited Ijy young children 

 in the yards, under the houses, and even within the houses them- 

 selves (Brown, 1927a; Cort, Stoll, Sweet, Riley, and Schapiro, 

 1929; Cort and Stoll, 1931; Cort, Otto, and Spindler, 1930; 

 Nair, 1935; Otto and Cort, 1934a; Tubangui, Basaca, and Pas- 

 co, 1934; Winfield, 1937 a & b). This household pollution by 

 young children results in the accumulation of large numbers of 

 viable eggs in the dooryards which are frequently carried into 

 the houses. Under these conditions eggs can easily contaminate 

 food and water and also infect directly by hand-to mouth trans- 

 fer the youngest children who play in the dirt and are most 

 careless in their habits. More general areas of concentrated 

 soil infestation are frequently found such as those near unsani- 

 tated schools or in the yards of institutions (Caldwell, Cald- 

 well, and Davis, 1930). In Egypt the sources of infection ap- 

 pear to be chiefl.v from eggs on the floors of the houses (Scott, 

 1939). The point has been repeatedly stressed that heavy in- 

 fection of a family can only be brought about by the grossest 

 type of soil pollution close to the house combined with very 

 careless habits, especially in the children. Families without 

 infection have frequently been found living next door to heav- 

 ily infected "asearis families." Also, in certain regions, as 

 for example western Tennessee, where there is little or no 

 sanitation in some of the rural areas, asearis infection may be 

 at a low level or absent where the stools are deposited at some 

 distance from the dooryards (Otto, Cort, and Keller, 1931). 

 Sucli relations, and the rarity of heavy infections, can only be 

 explained by postulating that in man, as has been shown in 

 the pig, infection is difficult. The ingestion of large numbers 

 of eggs is evidently necessary to produce even moderate infec- 

 tions of adult worms. When the constant loss of worms is also 

 considered, it is easy to understand why constant exposure to 

 intense infection is necessary to produce heavy infections. 



The contamination of drinking water has been frequently 

 suggested as a method of infection with asearis. In most of 

 the epidemiological studies that have been made in the United 

 States, Tropical America, and the Orient the possibility of in- 

 fection from this source has been practically ruled out. In cer- 

 tain parts of India, however, evidence was found that the 

 contamination of shallow pools of water was a factor in infec- 

 tion (Chandler, 1928). Recently the suggestion has been made 

 (Lane, 1934) that the breathing in of dust containing viable 

 asearis eggs might be a source of infection of considerable sig- 

 nificance. While infection in this way seems possible, it could 

 hardly be a method of major importance except under very 

 unusual circumstances. 



It has been commonly considered that vegetables fertilized 

 with human feces are an important source of asearis infection 

 (Mills, 1927; Yoshida, 1925; Walker, 1927; Khalil, 1931; Rob- 

 ertson, 1936). This would explain, as suggested b.v Mills 

 (1927), the distribution of asearis among all ages and classes 

 of the population in Korea. Several workers in the Orient have 

 found viable eggs of asearis clinging to vegetables that are 

 used for food uncooked (see summary by Winfield and Yao, 

 1937). Also, where human excrement is used as fertilizer, the 

 storage, transportation, and distribution of night soil on the 

 fields would serve to scatter the eggs of asearis widely in the 

 general environment of the villages. However, definite evidence 

 has been presented in studies in China that pollution by chil- 

 dren in the yards and streets of the villages is a very common 

 and perhaps the most important method of asearis dissemina- 



314 



