ENVIRONMENTAL DISEASE 



tecting livestock, does not hold much 

 promise for human use. More effec- 

 tive repellents are being sought, but 

 prospects for compounds appreciably 

 more effective than those now used 

 are dim. 



Vector-control programs through 

 the application of residual insecticides 

 have had distinct success; there have 

 also been failures. In part, the failures 

 have resulted from development of 

 resistance to insecticides; but in 

 greater part failures have been due to 

 biological behavior patterns of the 

 anopheline species in question, pre- 

 cluding effective exposure to residual 

 insecticides. Larviciding techniques, 

 particularly including low-volume 

 aerial application of insecticides, are 

 in a phase of reassessment. 



New techniques of vector control, 

 using genetic manipulation, insect 

 pathogens, antimetabolites, and insect 

 hormones are currently attracting 

 much attention. Genetic manipulation 

 includes male sterility induced by ir- 

 radiation of chemosterilants, cytoplas- 

 mic incompatibility, and translocation 

 semisterility. Successful application of 



such techniques will require much 

 more comprehensive knowledge of 

 the biology of each anopheline species 

 under consideration than now exists. 

 In this connection, the biology of the 

 nonbiting males of the many species 

 has received little attention in the 

 past but may well be critical in at- 

 tempts at genetic manipulation of 

 populations. 



A related approach involves at- 

 tempts to replace a vector population 

 of one species by a nonvector popu- 

 lation of a different species through 

 competitive displacement. Such an ex- 

 periment is now under way on a Pa- 

 cific atoll, attempting to displace the 

 filaria vector species. The concept 

 could also be applicable to displacing 

 a parasite-receptive clone of a vector 

 species by a parasite-resistant clone of 

 the same species. 



Epidemiology — Studies relating to 

 the central problem — the under- 

 standing of the epidemiology of ma- 

 laria in human populations — are 

 indicated at various points in the pre- 

 ceding discussion. It must be further 

 pointed out that epidemiological 



studies today are greatly embarrassed 

 by the various types of partial control 

 which may be operating in a field lo- 

 cality, including partially effective 

 drug therapy with many drugs, chang- 

 ing agricultural and living habits of 

 populations, and partially effective 

 vector-control programs. In the proc- 

 ess of measuring, variables change 

 and the picture changes. This situa- 

 tion cannot be controlled and will not 

 change. 



Need for Trained Manpower 



Especially important is continuing 

 training of field epidemiologists, with 

 enough background to permit them to 

 work effectively on actual field prob- 

 lems of malaria in overseas locations. 

 This should include medical person- 

 nel, entomologists, and control ex- 

 perts. Most of the medical schools in 

 the United States and in the world do 

 not meet this problem adequately, and 

 attention should be given to the 

 strengthening of several centers that 

 can be recognized as training centers 

 for tropical diseases in general and 

 malaria in particular. 



Other Parasitic Diseases 



Many parasitic infections are, in 

 fact, zoonoses with significant inter- 

 relation between man and domestic or 

 wild animals — e.g., hydatid disease, 

 American trypanosomiasis, leishmani- 

 asis, and fascioliasis. Study and con- 

 trol of such parasitic diseases are 

 seriously neglected though they cause 

 immense losses — both social and 

 economic. 



These are diseases of the poor and 

 ignorant, which can, in part, explain 

 the neglect since those people have 

 little political leverage. Nevertheless, 

 the fund of information on the dis- 

 eases and their control has run ahead 

 of the development of sound and use- 

 ful control programs. Most existing 



control programs are weak and inade- 

 quate despite the gravity of the prob- 

 lems. Somehow this pattern of neglect 

 has to be broken. 



Schistosomiasis 



Schistosomiasis is a worldwide 

 scourge in regions containing about 

 592 million people. (See Figure X-20) 

 About 125 million people are infected. 

 About 2.6 million are totally disabled 

 by it and 24.8 million are partially dis- 

 abled. In Brazil alone, approximately 

 15.5 million people live in affected re- 

 gions and 5.8 million are infected; 

 116,600 are totally disabled and 1.4 

 million are partially disabled. The 



estimated economic loss to Brazil due 

 to the "loss of resources" (i.e., loss 

 due to reduced productivity of goods 

 and services alone) is estimated to be 

 about $106 million per annum. 



The disease is out of control in al- 

 most all endemic areas and has spread 

 or increased in prevalence in Africa, 

 the Philippines, and Brazil in recent 

 years. In these areas, the increase has 

 been due to migration of infected peo- 

 ple, opening up of new areas for 

 settlement, or water resources devel- 

 opment schemes. 



Schistosomiasis demonstrates par- 

 ticularly well the complex feedback 

 among human health, agriculture, in- 



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