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Selected economic issues in helminth control
.%c. .Su Mzd. Vol. 19. No. IO. pp. 1057 1060. 1984 Prmted in Great Brnarn. All nghts reserved
SELECTED
ECONOMIC
Copyright
ISSUES IN HELMINTH
(
0377.Y5WX4 $3 00 + O.(X) 19X4 Pcrpamon Press Ltd
CONTROL
N. PRESCOTT and M. F. JANCLOES Population,
Health
and
Nutrition
Department,
The DC 20433.
World U.S.A.
Bank.
1818 H Street
N.W..
Washington.
Abstract-This
paper examines four main economic issues in the formulation of helminth control policies: whether, what, how and with whose resources to control helminthiasis. The paper argues that (i) although helminth control would have a negligible impact on mortality, its nutrition-mediated effects on improved labor productivity and intellectual performance may be significant; (ii) that reduction of helminth disease rather than infection should be the target of control policy. although the preferred intervention may still be eradication rather than continuous control; and (iii) that although the case for public subsidy of helminth control interventions is strong, the existence of serious liscal constraints and some evidence of private willingness-to-pay for anthelminthic chemotherapy_ indicates a potential for parttal cost recovery which
should
be explored. INTRODUCTION
This paper examines selected economic issues in helminth control with particular reference to roundworm and hookworm which are the two major soiltransmitted helminths. A recent estimate (1978) by Peters [l ] indicates that the global prevalence rates of these infections are about 32 and 247; respectively, amounting to some 1270 and 930 million infections. These figures suggest that the prevalence of the major helminths has hardly changed since Stoll [2] published his global prevalence estimates over 30 years ago (1947). The potential scope for new policy interventions is therefore great. Our purpose here is to draw attention to some important issues which arise in the formulation of helminth control policies. In so doing we aim to stimulate research which can help to improve the design of operational interventions. We examine these policy issues in the context of four types of policy decision : whether, what, how and with whose resources to control helminthiasis. For convenience we treat these as separable problems although they are clearly interrelated. WHETHER
TO CONTROL
The decision whether to control helminthiasis necessarily depends fundamentally on a judgment whether the net social benefit of control, that is the surplus of its benefits over its costs, is greater than the net social benefit of competing health interventions. A crucial issue, therefore, is the evaluation of the benefits of control.
A simple way of assessing the potential benefits of helminthiasis control is in terms of the days of healthy life lost which could be averted,..This measure has been proposed by the Ghana Health Assessment Project Team (1981) as a method of assessing the relative importance of different disease problems in a population [3]. In an extensive application of the method to 55 disease problems in Ghana, hookworm anaemia ranked only 36th in order of importance, contributing 1482 days of health life lost per 1000 population. This was equivalent to only 0.5 ?< of the ssn 19:10-D
total, 5 “6 of the contribution of malaria and roughly comparable to influenza and trachoma. Estimates for roundworm were not included in this assessment. This epidemiological assessment in terms of health impact suggests that hookworm. at least. is a relatively unimportant disease problem. This results principally from the fact that the measure of healthy days of life lost is dominated by mortality effects which are relatively low for hookworm and roundworm compared with their morbidity impact. Yet this conflicts with a widespread notion that helminthiasis is important, primarily because of its nutritional effects which impair physical and mental performance and consequently labor productivity. These effects on nonhealth variables are difficult to integrate adequately into the Ghana-type measure of health impact. A secondary reason is that helminth control may indirectly have beneficial effects on other types of health improvement. Eflects on physical pe$ormance
Considerable evidence exists that hookworm infection can significantly impair physical performance and labor productivity. The basis of this effect is the iron-deficiency anemia which results from blood loss due to the blood sucking activity of hookworms attached to the intestinal mucosa. The severity of the anemia depends on the balance between dietary intake of iron, physiological requirements and the amount of blood loss. The amount ofblood loss is proportional to the intensity of infection and is greater in infections with A, duodenale than N. americanus. The daily blood loss has been estimated at about 4ml per 1000 A. duodenale eggs per g of feces and about 2 ml per 1000 N. americanus eggs per g of feces [4]. A greater differential has been estimated by Ho and Brumpt [5] who report losses of 0.2 ml per A. duodenale worm and 0.03 ml per N. americanus worm. The resulting anemia reduces physical performance capacity by diminishing the maximal capacity of blood to transport oxygen to the tissues, which in turn impairs working efficiency at tasks requiring physical effort. Layrisse and Roche [6] have shown in Venezuela that N. americanus infections are associated with significant reductions in hemoglobin levels above thresholds of 2000 eggs/g of feces in women and
1057
1058
N.
PRESCOTTand M.
children and 5000 eggs/g in men. The reduction in hemoglobin levels is approximately proportional to the degree of infection intensity. Similar evidence does not exist for A. duodenale infections but the greater blood loss associated with this species suggests that the magnitude of the effect would be at least twice as great. Significantly adverse effects of iron-deficiency anemia on physical performance capacity have been demonstrated in several studies. For example, Karyadi and Basta [7] found that anemic (Hb < 11 g/dl) construction workers in Indonesia had significantly lower Harvard Step Test (HST) scores than nonanemic workers; the deficit varied between 22 and 50 On at three different sites. Similarly, Viteri and Torun [S] have shown a significant positive correlation (r = 0.72) between the log of the HST score and absolute hemoglobin levels in male agricultural laborers in Guatemala. Using a different measure of physical performance capacity, maximum aerobic power output, Davies et al. [9] measured performance reductions of 24% in African industrial workers with moderate anemia (mean Hb = 9.2 g/dl) and 34 % in those with severe anemia (mean Hb = 6.7gjdl) compared with normal controls. It is clear that reductions in physical performance capacity can significantly reduce physical labor productivity. Significant positive relationships have been estimated between maximum aerobic power output and the productivity of sugar cane cutters in Tanzania [lo] and Colombia [ll]. Direct estimates obtained in studies supported by the World Bank of the relationship between anemia and labor productivity support these indirect results. Basta et ul. [12] have found significant reductions of about 20 0I, in the mean productivity of anemic (Hb < 13gjdl) rubber tappers and weeders compared to normal controls in Indonesia. Recent results [13] in Kenya are similar, indicating that one standard deviation change in hemoglobin levels (1.30 g/dl) is associated with a change in productivity of approx. 6 ” “. .Efl&s on intellectuul perfbrmancr The possible effects of helminthiasis on adult intellectual performance as well as physical performance arise from evidence of its effects on nutritional balance in children, and of the effects of child nutrition status on adult intellectual performance and productivity. For example. Selowsky and Taylor [14] have estimated for Chile that a 10 point increase in preschool 1Q would yield a 6 point increase in adult IQ and that a 10 point increase in adult IQ is associated with a labor productivity increase of about 6”:,. If these estimates are correct the question is whether helminthiasis significantly impairs the nutritional status and intellectual performance of children. In general the evidence is substantially less clear cut than for effects on adult physical performance, but it is suggestive. For hookworm the evidence is both indirect and direct. A review by Read [ 151 concludes that sizeable adverse effects of anemia do not occur on intelligence as measured by standard IQ tests but that they do on selected behavioral attributes such as attentiveness, irritability, persistence, apathy and fatigue which do affect long run intellectual development. These effects
F. JANCLOES
increase with the severity of anemia, probably not becoming significant until hemoglobin concentration falls below 10 gjdl. The direct evidence is more strongly suggestive but not at all recent. An analysis by Smillie and Spencer [16] of N. umericunus infections in Alabama school children of similar social and economic status found that IQ scores decreased as infection intensity increased, the largest reduction being 15 “;o in the most heavily infected group (501-2000 worms). A similar relationship was observed in Queensland [17]. For roundworms the evidence is principally indirect. Infection with A. lumbricoides has been shown to impair nutrient absorption in children [l&19]. The implication that control would improve nutritional status is to some extent supported in studies of the effect of treatment. Stephenson et al. [20] and Gateff et al. [21] have reported significant increases in weight gain following treatment of infected children. However, it was not supported in a similar study by Greenberg et al. [22] in Bangladesh. Thus the evidence is inconclusive but deserves further study in view of the substantial evidence that early nutrition deprivation adversely affects intellectual performance and subsequent adult productivity. Indirect effects So far we have examined benefits which could be attributed to helminth control per se. However, it is also possible that helminth control would lead indirectly to other health benefits. This possibility stems from the enthusiastic appreciation by the community of the visible expulsion of roundworms following treatment. The integration of helminth control into a more general program of health intervention can help to increase community awareness of other health problems and therefore the demand for other beneficial health services. An example from lower Zaire is suggestive of the magnitude of these effects [23]. Community participation in a program combining helminth control with sanitation yielded an enthusiastic response. Compared to other villages, those with the community organized program experienced an increased rate of utilization of health services: the number of consultations per patient rose from 1.6to 3.3 in two years, the number of consultations for malaria attacks increased by 81 Y;, compliance with immunization programs increased from 69 to 83 7, and the acceptability of screening and treatment for tuberculosis also increased. In addition there was a very significant reduction in the incidence of dermatosis, indicating an increased utilization of water supply. While all of these indirect benefits are not necessarily attributable to helminth control, and also entail additional costs. they do suggest the possible value of helminth control as an instrument for promoting overall health improvement. WHAT TO CONTROL
The planning of helminthiasis control necessarily requires a clear specification of its objective. At the broadest level there is a choice between eradication and some degree of control. Two fundamental considerations bear on this choice. First is the characteristically highly aggregated distribution of
Selected economic
issues in helminth
helminth infection intensity. The majority ofhelminths typically infect only a minority of the population. Second is the characteristic relation between disease severity and infection intensity, with a threshold before infection intensity exerts. significant adverse effects. The nutritional status and resulting labor productivity effects of helminth infections discussed previously all share this attribute. Infections with intensity levels greater than this threshold can be defined as helminth disease. Since only a proportion of total infections cause disease it is obvious that the distribution of helminth disease is even more highly clumped than the helminth distribution. If the ultimate objective is to maximize the benefits of helminth control policy, the straightforward implication of these considerations is that intervention should aim to minimize the prevalence of helminth disease rather than helminth infection. The greater the degree of aggregation of the disease distribution the less relevant is the prevalence of infection as an object of control policy. It is tempting to interpret this observation as suggesting that policy should aim simply at control rather than eradication. In the following section we show that this inference is not necessarily correct. HOW TO CONTROL
The two major techniques of helminth control are mass chemotherapy and improvements in water supply and sanitation. Of course there are many possible variations of these basic techniques. For example, the nature of mass chemotherapy can vary enormously with respect to the frequency, population coverage and selectivity of the intervention. These alternatives are, each associated with different costs, epidemiological effectiveness and resulting benefits. In a cost-benefit analysis the optimal choice of intervention would be that which maximizes the net present value of the benefits from control, that is, the excess of discounted benefits over discounted costs. In a cost-effectiveness analysis the optimal intervention could be defined as the one which minimizes the discounted cost per case of helminth disease averted. In either case the measurement of costs and the role of discounting are both important and we consider these aspects of the analysis here. First we examine the choice between techniques which aim at substantial control of disease and those which aim at eradication. At the outset we reject infrequent mass chemotherapy as a completely inefficient alternative. Empirical results [24] and theoretical transmission models [25,26] have established conclusively that this type of intervention is basically worthless because the rate of recovery to the pre-control equilibrium is so rapid under typical transmission conditions. Only programs of sustained mass chemotherapy will achieve significant benefits. We can easily imagine two types of sustained mass chemotherapy. Alternative A represents a combination of frequency and random coverage which does not result in a dramatic reduction in the prevalence rate but does yield a much greater proportionate reduction in the mean work burden per host to a level which approximates the elimination of helminth disease. However it does not reduce the
control
1059
effective reproductive rate below unity, the breakpoint required to eradicate the infection, and therefore requires infinite repetition of the intervention to maintain the controlled equilibrium. As a result the stream of control costs continues indefinitely. This corresponds to an Ascaris example modelled by Anderson and May [25] where chemotherapy is repeated each month with a random coverage of 20 ‘x of the population. Alternative B represents a more intensive combination of frequency and coverage but does achieve eradication after a set number of years. It is more costly per year than Alternative A but the stream of costs is finite. A simple comparison of the costs and benefits of the two alternatives obviously shows that eradication is preferable. The benefits of control and eradication are an infinite series as are the costs of control, but the costs oferadication are limited in time. Thus the excess of total benefit over cost is higher for the eradication alternative. However, the conventional introduction of discount rates into the economic analysis tends to switch the comparison in favor of control. The two alternatives can be considered to have the same discounted benefit because their infinite benefit series are the same. The solution depends entirely on how the discounted costs compare. With discounting, the costs of the control alternative converge to a finite number because the discount rate assigns a progressively lower value to costs the later they occur. The question is whether the discounted costs of eradication are lower or higher than for control. The lower the cost premium of the eradication policy, and the more quickly its costs are terminated by achieving eradication the more likely it is to be preferred, and conversely. We do not attempt an empirical solution here, but note that joint simulations of the costs and epidemiological outcomes of alternative interventions would be a valuable exercise. A second issue concerns the relative benefits of selective vs random coverage with mass chemotherapy, Anderson and May [25] have demonstrated clearly that increasing the selectivity of coverage has a significant, but diminishing, marginal impact on prevalence and intensity of infection at any given coverage level. However, a complete economic analysis would need to take into account the additional screening costs to assess whether greater selectivity would in fact be more cost-effective. Again, we do not attempt this here but note its potential importance. WITH
WHOSE
RESOURCES
The question of who should finance the provision of health services is a central health policy issue, especially in the low-income developing countries where the prevalence of helminthiasis is most widespread. Extremely scarce budgetary resources for health are being increasingly squeezed between declining rates of growth in overall government revenues and increasingly ambitious national objectives for health improvement. In these circumstances the effective implementation of new health interventions is increasingly dependent on minimizing the fiscal burden which they impose. Here we examine briefly the case for and against public subsidy of helminth control.
N. PRESCOTTand M. F. JANCLOES
1060
Two considerations argue in favor of a degree of public subsidy. One is that, like all communicable diseases, the benefits to society ofindividual treatment, especially on a reasonably large scale, are greater than are received by the treated individual. This is because individual treatment has at least a marginal externality effect on transmission of the parasite to others. The second is that infected individuals, who are the potential consumers of chemotherapy, do not demand treatment because they are ignorant of its potential benefits to themselves. These considerations suggest that the helminth problem really results from a failure of private demand to reach socially optimal levels, the appropriate policy response being to subsidize the price of anthelminthics in order to increase the demand for it. However, informal observation suggests that a complete government subsidy may not be required to stimulate demand for anthelminthics in many communities. A fairly strong demand already seems to exist, the major problem on the demand side being an inadequate appreciation of the need for continuous treatment to suppress reinfection. Difficulties also arise on the drug supply side both in the public and private sectors, a problem which reflects general inadequacies in the pharmaceuticals distribution system in many developing countries. Thus, even where a demand exists for anthelminthics it may remain unsatisfied simply because the drugs are not available or are priced excessively by private traders. The appropriate nature of government intervention in helminth control may therefore lie in a combination of partial subsidy of anthelminthic prices, improved health education and efforts to improve the functioning of the drug distribution system. REFERENCES
9
10
11
12.
13.
14.
15. 16.
17.
18.
19.
20.
21.
1. Peters W. Medical aspects-comment and discussion II. In The Reievance of Parasitology to Human Welfare Todav, Symposia of the British Society of Parasitology (Edited
by Taylor A. E. R. and Muher R.). pp. 2540. Blackwell Scientific, Oxford, 1978. 2. Stall N. R. This wormy world. J. Parasit. 33, 1-18, 1947. 3. Ghana Health Assessment Project Team. A quantitative method of assessing the health impact of different diseases in less developed countries. Inc. J. Epid. 10, 1981. M. and Layrisse N. The nature and hookworm anaemia. Am. J. rrop. Med. Hyg. 1966. 5. Ho T. S. and Brumpt L. C. L’ankylostomiase. Pratn 20, 61-72, 1970. 6. Layrisse M. and Roche M. The relationship anaemia and hookworm infection. Am. J. 279-301, 1964. 7. Karyadi D. and Basta S. S. Nutrition and
8
22.
23.
73-80,
4. Roche
causes of 15. 1032.
24.
Revue
25.
between Hyg. 79,
26.
health
of
Indonesian construction workers: endurance and anaemia. Staff Working Paper No. 152, World Bank. Washington, DC, 1973. Viteri F. E. and Torun B. Anaemia and physical work capacity. Clin. Haemat. 3, 609, 1974. Davies C. T. M. et al. Iron deficiency anaemia: its effect on maximum aerobic power and responses to exercise in African males aged 17-40 years. Clin. Sci. 44, 555, 1973. Davies C. T. M. Relationship of maximum aerobic power output to productivity and absenteeism of East African sugar cane workers. Br. J. ind. Med. 30, 146-154, 1973. Spurr G. B. et al. Productivity and maximal oxygen consumption in sugar cane cutter’s, Am. J. clin. Nutr. 30. 316-321, 1977. Basta S. et al. Iron deficiency anaemia and the productivity of adult males in Indonesia. Am. J. clin. Nutr. 32, 916-925, 1979. Wolgemuth J. C. et al. Worker productivity and the’ nutritional status of Kenyan road construction laborers. Am. J. clin. Nurr. 36, 68-78, 1982. Selowsky M. and Taylor L. The economics of malnourished children: an example of investment in human capital. Econ. Dec. Cult. Change 22, 17-30, 1973. Read M. S. Anemia and behavior. Mod. Prob. Paediat. 14, 189-202, 1975. Smillie W. G. and Spencer C. R. Mental retardation in schoolchildren infected with hookworms. J. educ. Psychol. 17, 314-321, 1926. Waite J. H. and Neilson I. L. Effects of hookworm disease on development of North Greenland children. J. Am. med. Ass. 73, 1877-1879, 1919. Blumenthal D. S. and Schultz M. G. Effects of ascaris infection on nutritlonal status in children. Am. J. trap. Med. Hyg. 25, 682-690. 1976. Brown K. et al. Absorption of a rice vegetable diet before and after treatment of ascariasis in children: relation to intestinal worm burden. Am. J. clin. Nutr. 33, 1975-1982, 1980. Stephenson L. S. et al. Relationship between ascaris infection and growth of malnourished pre-school children in Kenya. Am. J. clin. Nutr. 33, 1165-l 172.1980. GateIT C. et al. Chimiotherapie antihelminthique systematique au thiabendazole en milieu scolaire Africam. Annls Sot. helpe .%f>tl trap. 52, 103-l 12, 1973. Greenberg B. L. et al. Single dose piperazine therapy for ascaris lumbricoides: an unsuccessful method of promoting growth. Am. J. c/in. Nutr. 34, 2508-2516, 1981. Jancloes M. F. Rational organization of primary health services in health policies m developing countries. In Nrrrlth Policies in Dewloping Countries. Royal Society of Medicme International Congress and Symposium Series No. 24. Royal Society of Medlcme. London. 1980. Arfaa F. and Ghadirlan E. Epidemiology and mass treatment ofascariasis in six rural communities in central Iran. Am. J. trap. Med. Hyg. 26, 886-871, 1977. Anderson R. A. and May R. M. Population dynamics of human helminth infections: control by chemotherapy. Nature 297, 557-563. 1982. Croll N. A. et (II. The population biology and control of ascaris lumbricoides in a rural community in Iran. Trans. R. Sot. trap. Med. Hyg. 76, 187-197, 1982.
SELECTED
ECONOMIC
Copyright
ISSUES IN HELMINTH
(
0377.Y5WX4 $3 00 + O.(X) 19X4 Pcrpamon Press Ltd
CONTROL
N. PRESCOTT and M. F. JANCLOES Population,
Health
and
Nutrition
Department,
The DC 20433.
World U.S.A.
Bank.
1818 H Street
N.W..
Washington.
Abstract-This
paper examines four main economic issues in the formulation of helminth control policies: whether, what, how and with whose resources to control helminthiasis. The paper argues that (i) although helminth control would have a negligible impact on mortality, its nutrition-mediated effects on improved labor productivity and intellectual performance may be significant; (ii) that reduction of helminth disease rather than infection should be the target of control policy. although the preferred intervention may still be eradication rather than continuous control; and (iii) that although the case for public subsidy of helminth control interventions is strong, the existence of serious liscal constraints and some evidence of private willingness-to-pay for anthelminthic chemotherapy_ indicates a potential for parttal cost recovery which
should
be explored. INTRODUCTION
This paper examines selected economic issues in helminth control with particular reference to roundworm and hookworm which are the two major soiltransmitted helminths. A recent estimate (1978) by Peters [l ] indicates that the global prevalence rates of these infections are about 32 and 247; respectively, amounting to some 1270 and 930 million infections. These figures suggest that the prevalence of the major helminths has hardly changed since Stoll [2] published his global prevalence estimates over 30 years ago (1947). The potential scope for new policy interventions is therefore great. Our purpose here is to draw attention to some important issues which arise in the formulation of helminth control policies. In so doing we aim to stimulate research which can help to improve the design of operational interventions. We examine these policy issues in the context of four types of policy decision : whether, what, how and with whose resources to control helminthiasis. For convenience we treat these as separable problems although they are clearly interrelated. WHETHER
TO CONTROL
The decision whether to control helminthiasis necessarily depends fundamentally on a judgment whether the net social benefit of control, that is the surplus of its benefits over its costs, is greater than the net social benefit of competing health interventions. A crucial issue, therefore, is the evaluation of the benefits of control.
A simple way of assessing the potential benefits of helminthiasis control is in terms of the days of healthy life lost which could be averted,..This measure has been proposed by the Ghana Health Assessment Project Team (1981) as a method of assessing the relative importance of different disease problems in a population [3]. In an extensive application of the method to 55 disease problems in Ghana, hookworm anaemia ranked only 36th in order of importance, contributing 1482 days of health life lost per 1000 population. This was equivalent to only 0.5 ?< of the ssn 19:10-D
total, 5 “6 of the contribution of malaria and roughly comparable to influenza and trachoma. Estimates for roundworm were not included in this assessment. This epidemiological assessment in terms of health impact suggests that hookworm. at least. is a relatively unimportant disease problem. This results principally from the fact that the measure of healthy days of life lost is dominated by mortality effects which are relatively low for hookworm and roundworm compared with their morbidity impact. Yet this conflicts with a widespread notion that helminthiasis is important, primarily because of its nutritional effects which impair physical and mental performance and consequently labor productivity. These effects on nonhealth variables are difficult to integrate adequately into the Ghana-type measure of health impact. A secondary reason is that helminth control may indirectly have beneficial effects on other types of health improvement. Eflects on physical pe$ormance
Considerable evidence exists that hookworm infection can significantly impair physical performance and labor productivity. The basis of this effect is the iron-deficiency anemia which results from blood loss due to the blood sucking activity of hookworms attached to the intestinal mucosa. The severity of the anemia depends on the balance between dietary intake of iron, physiological requirements and the amount of blood loss. The amount ofblood loss is proportional to the intensity of infection and is greater in infections with A, duodenale than N. americanus. The daily blood loss has been estimated at about 4ml per 1000 A. duodenale eggs per g of feces and about 2 ml per 1000 N. americanus eggs per g of feces [4]. A greater differential has been estimated by Ho and Brumpt [5] who report losses of 0.2 ml per A. duodenale worm and 0.03 ml per N. americanus worm. The resulting anemia reduces physical performance capacity by diminishing the maximal capacity of blood to transport oxygen to the tissues, which in turn impairs working efficiency at tasks requiring physical effort. Layrisse and Roche [6] have shown in Venezuela that N. americanus infections are associated with significant reductions in hemoglobin levels above thresholds of 2000 eggs/g of feces in women and
1057
1058
N.
PRESCOTTand M.
children and 5000 eggs/g in men. The reduction in hemoglobin levels is approximately proportional to the degree of infection intensity. Similar evidence does not exist for A. duodenale infections but the greater blood loss associated with this species suggests that the magnitude of the effect would be at least twice as great. Significantly adverse effects of iron-deficiency anemia on physical performance capacity have been demonstrated in several studies. For example, Karyadi and Basta [7] found that anemic (Hb < 11 g/dl) construction workers in Indonesia had significantly lower Harvard Step Test (HST) scores than nonanemic workers; the deficit varied between 22 and 50 On at three different sites. Similarly, Viteri and Torun [S] have shown a significant positive correlation (r = 0.72) between the log of the HST score and absolute hemoglobin levels in male agricultural laborers in Guatemala. Using a different measure of physical performance capacity, maximum aerobic power output, Davies et al. [9] measured performance reductions of 24% in African industrial workers with moderate anemia (mean Hb = 9.2 g/dl) and 34 % in those with severe anemia (mean Hb = 6.7gjdl) compared with normal controls. It is clear that reductions in physical performance capacity can significantly reduce physical labor productivity. Significant positive relationships have been estimated between maximum aerobic power output and the productivity of sugar cane cutters in Tanzania [lo] and Colombia [ll]. Direct estimates obtained in studies supported by the World Bank of the relationship between anemia and labor productivity support these indirect results. Basta et ul. [12] have found significant reductions of about 20 0I, in the mean productivity of anemic (Hb < 13gjdl) rubber tappers and weeders compared to normal controls in Indonesia. Recent results [13] in Kenya are similar, indicating that one standard deviation change in hemoglobin levels (1.30 g/dl) is associated with a change in productivity of approx. 6 ” “. .Efl&s on intellectuul perfbrmancr The possible effects of helminthiasis on adult intellectual performance as well as physical performance arise from evidence of its effects on nutritional balance in children, and of the effects of child nutrition status on adult intellectual performance and productivity. For example. Selowsky and Taylor [14] have estimated for Chile that a 10 point increase in preschool 1Q would yield a 6 point increase in adult IQ and that a 10 point increase in adult IQ is associated with a labor productivity increase of about 6”:,. If these estimates are correct the question is whether helminthiasis significantly impairs the nutritional status and intellectual performance of children. In general the evidence is substantially less clear cut than for effects on adult physical performance, but it is suggestive. For hookworm the evidence is both indirect and direct. A review by Read [ 151 concludes that sizeable adverse effects of anemia do not occur on intelligence as measured by standard IQ tests but that they do on selected behavioral attributes such as attentiveness, irritability, persistence, apathy and fatigue which do affect long run intellectual development. These effects
F. JANCLOES
increase with the severity of anemia, probably not becoming significant until hemoglobin concentration falls below 10 gjdl. The direct evidence is more strongly suggestive but not at all recent. An analysis by Smillie and Spencer [16] of N. umericunus infections in Alabama school children of similar social and economic status found that IQ scores decreased as infection intensity increased, the largest reduction being 15 “;o in the most heavily infected group (501-2000 worms). A similar relationship was observed in Queensland [17]. For roundworms the evidence is principally indirect. Infection with A. lumbricoides has been shown to impair nutrient absorption in children [l&19]. The implication that control would improve nutritional status is to some extent supported in studies of the effect of treatment. Stephenson et al. [20] and Gateff et al. [21] have reported significant increases in weight gain following treatment of infected children. However, it was not supported in a similar study by Greenberg et al. [22] in Bangladesh. Thus the evidence is inconclusive but deserves further study in view of the substantial evidence that early nutrition deprivation adversely affects intellectual performance and subsequent adult productivity. Indirect effects So far we have examined benefits which could be attributed to helminth control per se. However, it is also possible that helminth control would lead indirectly to other health benefits. This possibility stems from the enthusiastic appreciation by the community of the visible expulsion of roundworms following treatment. The integration of helminth control into a more general program of health intervention can help to increase community awareness of other health problems and therefore the demand for other beneficial health services. An example from lower Zaire is suggestive of the magnitude of these effects [23]. Community participation in a program combining helminth control with sanitation yielded an enthusiastic response. Compared to other villages, those with the community organized program experienced an increased rate of utilization of health services: the number of consultations per patient rose from 1.6to 3.3 in two years, the number of consultations for malaria attacks increased by 81 Y;, compliance with immunization programs increased from 69 to 83 7, and the acceptability of screening and treatment for tuberculosis also increased. In addition there was a very significant reduction in the incidence of dermatosis, indicating an increased utilization of water supply. While all of these indirect benefits are not necessarily attributable to helminth control, and also entail additional costs. they do suggest the possible value of helminth control as an instrument for promoting overall health improvement. WHAT TO CONTROL
The planning of helminthiasis control necessarily requires a clear specification of its objective. At the broadest level there is a choice between eradication and some degree of control. Two fundamental considerations bear on this choice. First is the characteristically highly aggregated distribution of
Selected economic
issues in helminth
helminth infection intensity. The majority ofhelminths typically infect only a minority of the population. Second is the characteristic relation between disease severity and infection intensity, with a threshold before infection intensity exerts. significant adverse effects. The nutritional status and resulting labor productivity effects of helminth infections discussed previously all share this attribute. Infections with intensity levels greater than this threshold can be defined as helminth disease. Since only a proportion of total infections cause disease it is obvious that the distribution of helminth disease is even more highly clumped than the helminth distribution. If the ultimate objective is to maximize the benefits of helminth control policy, the straightforward implication of these considerations is that intervention should aim to minimize the prevalence of helminth disease rather than helminth infection. The greater the degree of aggregation of the disease distribution the less relevant is the prevalence of infection as an object of control policy. It is tempting to interpret this observation as suggesting that policy should aim simply at control rather than eradication. In the following section we show that this inference is not necessarily correct. HOW TO CONTROL
The two major techniques of helminth control are mass chemotherapy and improvements in water supply and sanitation. Of course there are many possible variations of these basic techniques. For example, the nature of mass chemotherapy can vary enormously with respect to the frequency, population coverage and selectivity of the intervention. These alternatives are, each associated with different costs, epidemiological effectiveness and resulting benefits. In a cost-benefit analysis the optimal choice of intervention would be that which maximizes the net present value of the benefits from control, that is, the excess of discounted benefits over discounted costs. In a cost-effectiveness analysis the optimal intervention could be defined as the one which minimizes the discounted cost per case of helminth disease averted. In either case the measurement of costs and the role of discounting are both important and we consider these aspects of the analysis here. First we examine the choice between techniques which aim at substantial control of disease and those which aim at eradication. At the outset we reject infrequent mass chemotherapy as a completely inefficient alternative. Empirical results [24] and theoretical transmission models [25,26] have established conclusively that this type of intervention is basically worthless because the rate of recovery to the pre-control equilibrium is so rapid under typical transmission conditions. Only programs of sustained mass chemotherapy will achieve significant benefits. We can easily imagine two types of sustained mass chemotherapy. Alternative A represents a combination of frequency and random coverage which does not result in a dramatic reduction in the prevalence rate but does yield a much greater proportionate reduction in the mean work burden per host to a level which approximates the elimination of helminth disease. However it does not reduce the
control
1059
effective reproductive rate below unity, the breakpoint required to eradicate the infection, and therefore requires infinite repetition of the intervention to maintain the controlled equilibrium. As a result the stream of control costs continues indefinitely. This corresponds to an Ascaris example modelled by Anderson and May [25] where chemotherapy is repeated each month with a random coverage of 20 ‘x of the population. Alternative B represents a more intensive combination of frequency and coverage but does achieve eradication after a set number of years. It is more costly per year than Alternative A but the stream of costs is finite. A simple comparison of the costs and benefits of the two alternatives obviously shows that eradication is preferable. The benefits of control and eradication are an infinite series as are the costs of control, but the costs oferadication are limited in time. Thus the excess of total benefit over cost is higher for the eradication alternative. However, the conventional introduction of discount rates into the economic analysis tends to switch the comparison in favor of control. The two alternatives can be considered to have the same discounted benefit because their infinite benefit series are the same. The solution depends entirely on how the discounted costs compare. With discounting, the costs of the control alternative converge to a finite number because the discount rate assigns a progressively lower value to costs the later they occur. The question is whether the discounted costs of eradication are lower or higher than for control. The lower the cost premium of the eradication policy, and the more quickly its costs are terminated by achieving eradication the more likely it is to be preferred, and conversely. We do not attempt an empirical solution here, but note that joint simulations of the costs and epidemiological outcomes of alternative interventions would be a valuable exercise. A second issue concerns the relative benefits of selective vs random coverage with mass chemotherapy, Anderson and May [25] have demonstrated clearly that increasing the selectivity of coverage has a significant, but diminishing, marginal impact on prevalence and intensity of infection at any given coverage level. However, a complete economic analysis would need to take into account the additional screening costs to assess whether greater selectivity would in fact be more cost-effective. Again, we do not attempt this here but note its potential importance. WITH
WHOSE
RESOURCES
The question of who should finance the provision of health services is a central health policy issue, especially in the low-income developing countries where the prevalence of helminthiasis is most widespread. Extremely scarce budgetary resources for health are being increasingly squeezed between declining rates of growth in overall government revenues and increasingly ambitious national objectives for health improvement. In these circumstances the effective implementation of new health interventions is increasingly dependent on minimizing the fiscal burden which they impose. Here we examine briefly the case for and against public subsidy of helminth control.
N. PRESCOTTand M. F. JANCLOES
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Two considerations argue in favor of a degree of public subsidy. One is that, like all communicable diseases, the benefits to society ofindividual treatment, especially on a reasonably large scale, are greater than are received by the treated individual. This is because individual treatment has at least a marginal externality effect on transmission of the parasite to others. The second is that infected individuals, who are the potential consumers of chemotherapy, do not demand treatment because they are ignorant of its potential benefits to themselves. These considerations suggest that the helminth problem really results from a failure of private demand to reach socially optimal levels, the appropriate policy response being to subsidize the price of anthelminthics in order to increase the demand for it. However, informal observation suggests that a complete government subsidy may not be required to stimulate demand for anthelminthics in many communities. A fairly strong demand already seems to exist, the major problem on the demand side being an inadequate appreciation of the need for continuous treatment to suppress reinfection. Difficulties also arise on the drug supply side both in the public and private sectors, a problem which reflects general inadequacies in the pharmaceuticals distribution system in many developing countries. Thus, even where a demand exists for anthelminthics it may remain unsatisfied simply because the drugs are not available or are priced excessively by private traders. The appropriate nature of government intervention in helminth control may therefore lie in a combination of partial subsidy of anthelminthic prices, improved health education and efforts to improve the functioning of the drug distribution system. REFERENCES
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