CHAPTER VIII 

 EPIDEMIOLOGY AND SANITARY MEASURES FOR THE CONTROL OF NEMIC PARASITES OF MAN 



WILLIAM W. CORT, Department of H.lminthology, School 

 of Hygiene and Public Health, the Johns Hopkins Univer- 

 sity, Baltimore, Md. 



ELOISE B. CRAM, Division of Zoology. National Institute of 

 Health, U. S. Public Health Service, Washington, D. C. 



DONALD L. AUGUSTINE, Department of Comparative 

 Pathology and Tropical Medicine, School of Medicine and 

 Public Health, Harvard University, Boston, Mass. 



General Discussion 

 W. W . C. 



.Xt'tor the disfdvi'ry of :i liuiiuiii piujisito the next st("]> in flic 

 si'<|»oiUH' loading to ofFective control must he the detcrniiiuitioii 

 of its life cvclc and method of luinian infection. Such iiifor 

 mation suggests the liroad lines along which control measures 

 can be developed, but needs to be sii|i|ilcmented by eiiidemi 

 olugic studies to gain information on tlic vjirious factors in- 

 volved in dissemination in population groups. These factoi's 

 differ greatly for the different nemic jiarasites of man, which 

 vary in their host relations and life cycles. 



In those species in which eggs or larval stages have a free 

 life, vis., the hookworms, asearis, trichuris, enterobius, etc., 

 knowledge is essential on the effect of general environmental 

 factors, such as temperature, moisture, and physical and chemi 

 cal conditions of the soil; for only when the environment out- 

 side the host is favorable can these free stages persist and in- 

 fect nuiu. The relations of these factors depend on the anu)unt 

 of development <mtside the host necessary before the infective 

 stage is reached and vary greatly between snch species as en- 

 terobius and the hookworms. In those si)ecies with interme- 

 diate hosts the relations outside the definitive host are still 

 more complicated, since they involve all the factors related to 

 the infection of the intermediate host and the transmission by 

 it of the parasite. Thus all the relations of Trichi/irlla spiralis 

 to the rat and pig become of vital significance in its tranmis- 

 sion ; with the guinea worm, Dractinculus medinensis, the Cy- 

 clops is brought into the picture: and in the filariae the rela- 

 tions of mosquitoes and certain other blood sucking flies must 

 be considered. 



Equally significant in epidemiologic studies of nemic para- 

 sites is the consideration of human habits in relation to trans- 

 mission. For those species in which the eggs pass out with the 

 feces, habits of excreta disposal are of great significance. 

 Kqnally important also are all the human habits that make 

 jiossible the entrance of the free stages into the human body. 

 With those nematodes which have intermediate hosts the human 

 habits that are related to spread are entirely different and vary 

 greatly with the species. In the filariae, habits that bring about 

 exposuie to the bites of the insect vector are important both 

 in relation to the infection of the insect and in the transmission 

 of the parasite l)ack to man. In certain cases the human rela 

 tions may be very peculiar. As for example with TricliiiitlUi 

 .spiralis where the methods of feeding pigs and habits in rela- 

 tion to pork eating have to be considered; or with the guinea 

 wonn where transmission depends on the drinking of water 

 containing cyclops, in which infected individuals have waded 

 or washed their feet. 



.\nother phase that cannot be neglected is the host parasite 

 relations. For example, the development of a specific immunity 

 or the presence of an age resistance may be important in de- 

 termining the distribution of the parasite in the population ; 

 or undernourished individuals may be more snsceiitible than 

 are those on a good diet. We know least about these factors 

 and in the present state of our knowledge their relation to 

 epidemiology is difficult to evaluate. 



It is evident that the more extensive is the understanding of 

 the epidemiology of a parasite, the more effective the control 

 program can be made; thus, weak links in the cycle of trans- 

 mission can be more effectively discovered and mistakes avoided. 

 Most effective in control are ,'ittempts to ch.ange human habits 

 that make possible transmission. In fecal borne infections the 

 improvement of sanitation to prevent soil pollution is most 

 important. Where transmission is by insect vectors, control 

 measures are chiefly concerned with the jirotection of tlie peo 

 [lie from their bites and with their eradication from areas near 

 liuman habitations. Where treatment is eflFective and easily ap- 

 plied to large groups, mass treatment may be an efficient con- 

 trol measure in breaking the cycle at the stage passed in the 

 human body. 



In the following discussion only the most important and best 

 kiu)wn of the nemic iiarasites of man will be considered, viz., 

 the hookworms, .liiciirlosloma iliioilinalr and Ncrator amrri- 

 callus: the large round worm, Asearis liimbricoitlis ; the whi|) 

 worm, Trichuris trichiiira: the pin worm, Eiitrrohiiis rcrnii- 

 ciilaris; the flesh worm, 'I'ricliiiii lla siiiralis; the guinea worm, 

 DraciDiciilus iiiiiliiiciisis ; and the most impoitant of the filariae, 

 ll'iiclicrcria baiicrofti. Onchocerca volvulus, and Microfilaria 

 nialaiii. For most other human nematodes there is little in- 

 formation on epidemiology or control methods, and they are 

 for the most part of minor significance as human parasites. 

 .\lso, the knowledge iiresented on the more important forms 

 gives a backgiound for understanding similar relations for the 

 other sjiecies. 



The Hookworms 

 W. W. C. 



In ancient Egyptian, .\rabian, and Greek writings are founl 

 descriptions that may possibly have referred to hookworm dis- 

 ease. .-Vlso, accounts in medical treatises of the 17th and ISth 

 centuries from Brazil, Guadeloupe, and Jamaica almost cer- 

 tainly referred to this disease. Modern knowledge dates from 

 the description of Ancylostoma diiodrnalr by Dubini (1S43). 

 In 1S7S Grassi and the brothers Parona demonstrated that 

 hookworm infection could be diagnosed by fecal examinations. 

 Leichtenstern in 1887 demonstrated experimentally that infec- 

 tion could be brought about by the ingestion of larvae. Looss 

 (1898) first discovered infection by skin penetration with mi- 

 gration through the lungs and over a period of years carried 

 out extensive investigations on hookworm biology which cul- 

 minated in his Iftll monograph. 



By tlic beginning of the lIlUli century the scene had shifted 

 to the Western Hemisphere, where Lntz in Brazil, .\shford in 

 Puerto Rico, and .Stiles in the Ignited States had demonstrated 

 the importance of hookworm disease. Of especial significance 

 was the discovery by Stiles (1902) of the second species of 

 human hookworms, \rcator americaniis. Very important also 

 was the work of the Puerto Rico .\nemia Commission from 

 1904 to 1908 (Ashford and Gutierrez, 1911) which carried out 

 the first extensive investigation and control program in a trojii 

 cal country. Later (1909 to 1914) came the campaign of the 

 Rockefeller Sanitary Commission in the southern United States, 

 which was followed in 1914 by the establishment of the In- 

 ternational Health Board of the Rockefeller Foundation, which 

 in the next few years extended hookworm control campaigns 

 widely into other parts of the world. 



The true human liookworms, Aiici/lostoma ihiodcnalr and Xic 

 ator americaniis, are widely distributed between the 3t5th paral 

 U'l north latitude and the 30th parallel south. Within this belt 

 there are extensive regions where the combination of favorable 

 temperatures and rainfall make possible the development of 

 widespread heavy infections and clinical disease in populations 

 living close to the soil under iiriniitive conditituis of sanitation. 

 Such populations are still found in very limited areas in the 

 sonthein t'nited States and more extensively in the West In 

 dies. Central America, northern South .\merica. Tropical -Af- 

 rica, and in certain parts of southern Asia and the East In- 

 dies.* While A. (liioilenale and N. americaniis are present to- 

 gether in a considerable part of the hookworm belt they show 

 important differences in geographical distribution and appear 

 to have origiimted in different parts of the world (Darling, 

 1920). 



A. diiodcnale and N. americaniis differ greatly in the mor- 

 phology of the adults. The former is larger, appears to be more 

 injurious to its host and is harder to eliminate with anthelmin- 

 tics (Darling, Barber, and Hacker, 1920). The female of A. 

 duodcnalc produces about 22,000 eggs every 24 hours while 



*For a detailed discussion of the geographical distribution of hook- 

 worm disease see Chandler, 1929. pp. 18-54. 



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