may hIso W a oonipliiatinK faotnr. It .sivms rvidriil alsi> that 

 the clironic lilood loss imnjiu'cd liy the wmins aiiKlit in ccitaiii 

 t'asos be ouo of tin- t'ai'tors that would (iiially lead to the dovol 

 opiuont of oiif of the • ' idiopalhio " aiiomiaH. In addition, it 

 si'onis oi'itaiii that aiicmlas of a variety of ctioloKii'S are fre 

 (luently referred to hool<\vorm infeetion ia eases where the few 

 worms present have litth> if any part in the prodnetion of the 

 anemie eondition lAndrews, liMlU. 



Reeently it lias lieen shown that a speeifio imninnity is ae 

 ((Uired hy dogs in response to repeated infections with .1. <-<nii 

 iiiim in wliieh antil>odies are formeil ehielly in response to [\[<- 

 secretions and excretions of the worms (Kerr, ISlHti; Otto and 

 Kerr, ISlH't; Otto. 1!I40). It seems practically certain that a 

 similar immunity develo|>s in man in response to hookworm 

 infection. In fact, several workers have recently expressed the 

 view that host immunity must play an important role in the 

 regulation of human hookworm infection ( KiiUeborn, Dios, and 

 Zuccarini, 1!I2S: FiiUcliorn. \'.^-2'.^; fort, 1!132; Pessoa and Pas 

 eale, 1937 a & h; Cort and Otto, \'.)iO) . Such a postulation 

 makes it easy to explain the relatively moderate infections and 

 slight evidence of hookworm disease found in many individuals 

 and groups of jieople who appear to live under conditions giving 

 constant opportunity for the invasion of the larvae. Severe 

 cases, especially in children, might perhaps be explained in 

 part by exposure to infections so extrenu' that the develoimu'iit 

 of the immunity is prevented. Also, it seems probable that 

 uadernourishment or other debilitating factors prevent the de 

 velopment of the immune reactions. In experimental infections 

 in young dogs either undernourishment or too rapid infection 

 which weakens the host from extreme blood loss will prevent 

 the immune response (Otto and Kerr, 1!)39); and the immu 

 nity already developed in highly resistant older animals is 

 easily broken down by placing them on a deficient diet { Foster 

 and Cort. I!i3l2; l!l3.'i). If the same relations hold in human 

 infection, individuals or groups that are badly debilitated liy 

 undernourishment or other factors may be expected to acipiire 

 heavy norm burdens arui will also be less able to compensate 

 by the regeneration of new blood for the losses caused by the 

 norms. It seems probable also that malaria and other diseases 

 are more important than is at present realized in weakening 

 the defense mechanism against hookworm infection. In fact, 

 the hypothesis has reeently been suggested that widespread 

 chronic hookworm disease of the type found specially in tropical 

 countries seldom results from uncomplicated hookworm infec- 

 tion, but is produced by hookworm infection plus undernourish- 

 nu'nt or other debilitating factors that weaken the host defense 

 (Cort and Otto, l<t4ii: Cort, 1940). 



HUMAN HABITS I.\ RELATION TO HOOKWORM 

 DISSEMINATION 



Insanitary methods of excreta disposal and activities bring 

 ing about contact with infested soil are the most important 

 human habits in hookworm dissemination. Careless depositing 

 of stools on the ground (soil pollution) is a widespread habit 

 among most of the i)opuiation of the world especially in tropical 

 and subtropical regions. Recently, epidemiologic evidence has 

 emphasized soil pollution in the general vicinit.v of dwellings 

 as important in hookworm infection. Adults and older chil 

 dren arc apt to go for defecation to protected places not far 

 from their houses and often the most important contact with 

 infested soil appears to come about during the act of defeca- 

 tion (Cort, 192.5; Cort, Stoll, Sweet, Riley, and Schapiro, 1929; 

 Chandler, 1928). Young children usually defecate in the door- 

 yards close to the houses or even under or in the houses them- 

 selves, where the soil conditions are usually not suitable for the 

 development of hookworm larvae. 



It is usuall.v difficult to determine the extent to which field 

 work brings the laborer into contact with sources of infection. 

 People living near cultivated areas such as vegetable gardens, 

 banana groves, sugar cane fields, or coffee groves, may by their 

 defecation habits produce concentrated places of soil infestation 

 that will infect field laV)orers. Usually, however, stools passed 

 by laborers at work are widely scattered and would be only 

 occasionally sources of infeetion, as compared with the con- 

 stant exposure in the defecation areas near the houses. 



Some occupational relations especiall.v important in hook- 

 worm dissemination have been noted. Coffee picking in the 

 hills of Puerto Rico has been shown to be responsible for ex- 

 tremely heavy infection (Ashford and Gutierrez, 1911; Cort, 

 Riley, and Payne, 1923). Here groups of people work in the 

 groves for long hours and spread their stools widely when they 

 pick the coffee at weekly intervals for (! or 7 weeks. Toward the 

 end of the picking season the soil of these groves becomes so 

 impregnated with infective larvae that extensive infection of 

 the workers occurs. In places in the Orient where human excre 

 ment is used as fertilizer, the practices in connection with the 

 cultivation of particular crops determine the extent of hook- 



\Miini dissemination. In regions in China where sericulture is 

 important hookworm infection m:iy be widespread because the 

 mi'thods of f<'rtili/,ing the mulberry trees nnike po.ssible the 

 developnu'nt of intense soil infestation ((^ort, (irant, and Stoll, 

 I!i2(>): fr<nn such places the iieople who pick the mulberry 

 le.ives to feed the silkworms ln'come intensely infected. Other 

 "i-cnpational relationships that |iroduce sources of infection 

 might be cited, but as we consider the cviileni'c it beconu's nu)re 

 .Hid niori' evident th;it, except when' lunnan excrenu'nt is used as 

 feililizcr, soil iiollntion in tin' vicinity of the <lwellings is by 

 I'ai the must irnporlant f;ictcir in hookworm di.sseminatiori. 



iiisrijii-.rTioN or hookwor.m infection within 



I'OITI.ATIONS 



The use of the Stoll dilution egg counting method in the 

 extensive epidemiologic stinlies of hookworm disease of the 

 last two decades has given a large anjount of information from 

 different parts of the world on the distribution of hookworm 

 infection in population groups. Estimates of worm burdens by 

 this niethod have made it possible to compare (|uantitatively 

 the infection according to age, sex, occupation, race, and other 

 categories, as well ;is to comp.-ne the ilistribution in populations 

 living under ilifti'rent conditions. Thus d;it;i can be obtained 

 for a .scientific planning of control progr.-inis ami the results 

 of the campaigns in reducing the intensity of infection can be 

 measured. Attention has, therefore, been turned from the per 

 centages of positive cases ;ind has been focused on the number 

 of worms harbored (worm burden). 



There has been an increasing emiihasis on the importance of 

 a proper evaluation of the lightly infected cases, especially 

 those that might be considered as carriers or subclinical, as 

 compared with the heavier cases. A high incidence of hook 

 woiin infection nuiy occur in groups where the number of 

 worms present is so small that they have little if any injurious 

 c'ffect. Such situations may be found, as in certain parts of 

 North China (Cort, (irant, and Stoll, 192li) and Egypt (Scott, 

 1937) where human habits are favorable for hookworm disseni 

 ination but climatic conditions are unfavorable. Similar wide 

 spread, practically sub-clinical infections are also jiresent 

 where sanitation and treatment have reduced the intensity of 

 infection to a low level, but where widely scattered light 

 sources of infection still exist. It is not possible to indicate 

 definitely the actual number of worms necessary to produce 

 clinical symptoms since this would vary in relation to a variety 

 of factors; also it is not easy to accurately evaluate the injury 

 to a population produced by widespread light infections. It caii 

 be .said, however, that light infections are of but little conse- 

 quence as compared with heavy; and that hookworm disease 

 becomes a real i)ublic health problem only in groups with 

 fairly heavy worm burdens. 



The individual family except when isolated is not nearly so 

 much the unit of hookworm infection as is the case with 

 ascaris. This appears to be due to the fact that sources of 

 infection are fairly widesjiread and because defecation places 

 near dwellings are commonly shared by more than one family. 

 Frequently almost all the individuals in even large populations 

 are infected. 



The relative intensity of infection in the sexes and in differ- 

 ent age groups varies greatly in different iiopulations. Usually, 

 however, infection is almost completely absent in children under 

 3 years of age, gradually increases up to 10 years and reaches 

 the adult level somewhere in the early teens or even later 

 (Smillie, 1922; Payne, Cort, and Rile.v", 1923). It may vary 

 considerably in the different age groups of middle life and 

 most often has a tendency to decline in older people. Females 

 usually have a distinctly lower level of infection intensity than 

 males (Carr, 1926; Hill, 1927a; Cort, Stoll, Sweet, Riley, and 

 Schapiro, 192S). It has been suggested that this type of age 

 and sex distribution is most t.vpical of situations where in- 

 fection comes from soil infestation in the general vicinity of 

 the houses. It can be suggested that in most situations the 

 children only have considerable exposure to infection when they 

 begin to visit adult defecation places. Greater activity of boys 

 than of girls brings greater contact with infection; and adult 

 males usually have more contact with sources of infection 

 away from home than do females. Unusually heavy infection 

 in very young children has been noted in certain groups in 

 Panama (Cort, Stoll, Sweet, Riley, and Schapiro, 1929), in the 

 Argentine (Fiilleborn, Dios, and Zuccarini, 192S), in Puerto 

 Rico (Hill, 1927a), and in southeastern (ieorgia (Andrews, 

 1940). This seems to occur only where soil conditions in the 

 dooryards are favorable for the development of hookworm 

 larvae. Heavier infections in women than in men have been 

 found in a few places like the areas of coffee cultivation in 

 Puerto Rico (Cort, Riley, and Payne, 1923) and in certain 

 groups engaged in sericulture in China (Cort, Grant, and Stoll, 

 192()) where the women are eng;iged to a greater extent than 



311 



