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15. Experimental Control of Malaria in West Africa
Pages 190-204

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From page 190...
... Although the project was carefully designed and executed in an integrated way over several years and data were gathered on the basis of a realistic mathematical model of malaria transmission, it failed to produce the predicted results, because incorrect assumptions were made about the biology of the mosquito vectors. In particular, it was assumed that all mosquitoes behave alike, whereas resting behavior actually varied in such a way that a large fraction of the mosquitoes did not land inside buildings, where they would have been exposed to the contact insecticide used.
From page 191...
... The intensity of malaria transmission is so high that control is very difficult, and many malariologists have suggested that humans should not interfere with the established natural immunity in the population, inasmuch as intervention could increase the severity of the clinical manifestations of malaria and the mortality caused by it in older children and adults. Largely for that reason- and because the public health infrastructure was inadequate theory predicted that available programs to control malaria would not work, and Africa was not included in the global malaria eradication program launched by the World Health Organization (WHO)
From page 192...
... Thus, the overall cycle of the malaria parasite involves transmission both from mosquito to human and from human to mosquito. One method of control or eradication of malaria, therefore, is to reduce the mosquito population sufficiently to keep the parasite cycle from maintaining itself.
From page 193...
... Such intervention included spraying with a residual insecticide, propoxur, alone and in combination with mass administration of drugs. · To undertake control measures in conjunction with the construction and testing of a mathematical model of malaria transmission.
From page 194...
... In the baseline phase, from October 1970 through March 1972 (which included a dry season, a wet season, and a second dry season) , baseline epidemiological and parasitological data were collected in the clusters of villages that had been selected for followup.
From page 195...
... . The vectorial capacity, defined as the rate of contact between human hosts via the mosquito vectors, reached a seasonal peak of about 40, which is some 2,000 times the critical value for maintaining endemic malaria.
From page 196...
... falciparum to a very low point in the dry season, but it did not significantly interrupt malaria transmission. Nor did it prevent an increase in prevalence in the wet season, when natural conditions favored mosquito breeding.
From page 197...
... (2) x 1S y 1S N is M is the proportion of the human population infected, the proportion of the female mosquito population the size of the human population, the size of the female mosquito population, a is the rate of biting on a human by a single mosquito (number of bites per unit time)
From page 198...
... Notwithstanding those shortcomings, the simple model defined by Equations 1 and 2 is useful in laying bare the essentials of the transmission process and in elucidating patterns in the diverse array of epidemiological data on different geographical regions. In particular, the model makes plain the significance of the "basic reproductive rate" of the parasite, in this context conventionally called z0 (MacDonald, 1952, 19571.
From page 199...
... . Members of both classes repeatedly are exposed, become infected, and recover, remaining within their own class except for a fixed rate of transition from the relatively susceptible class to the relatively immune class (the transition rate is determined by fitting the model to the data)
From page 200...
... equal 10 days for all mosquitoes. The correct way to calculate the effective index, a expt-/, which determines the transmission rate from infected mosquitoes to human hosts and is therefore relevant to evaluating the effects of intervention, is to take the appropriate arithmetic average of the separate indexes: peat exp(- ~l~l,u~ + p2a2 expf-~2T)
From page 201...
... This very high transmission rate put malaria in tropical Africa in a category of its own and constituted one reason that a control program like that tried in the Garki project was not attempted earlier. Against this background, however, it was not understood that heterogeneities in the environmental setting of the mosquitoes might invalidate simple estimates of the efficacy of insecticide application, which were based on assumptions that the mosquito population was effectively homogeneous and that most mosquitoes rested indoors much of the time.
From page 202...
... The cytogenetic investigations of Coluzzi suggest that resting behaviour and exposure to propoxur are related less to the relative abundance of the 2 species than to the intraspecific frequency of certain chromosomal inversions, some of which may be associated with a relatively stable behaviour pattern of the individual. In short, the essential conclusion of the Garki study, which was not anticipated in the early planning, was that, if the resting behavior of a mosquito species is genetically determined, exophily will be a stable characteristic of individual vectors, and the usual method of interpreting the impact of residual insecticides on longevity, which tacitly assumes uniform bet haviour, is overoptimistic.
From page 203...
... The Garki project is notable in that it was very thoughtfully designed, was maintained in an integrated way over several years, and carefully interdigitated the data gathered with a relatively realistic mathematical model. Moreover, the mathematical model, based on hard data and tested against data gathered in the course of the study, could be thought of as a
From page 204...
... It seems increasingly likely that spatial heterogeneity, with different dynamic processes going on in separate patches, will be the most important factor in the overall persistence of many natural preypredator or host-parasite associations (Hassell and May, 19851. The Garki project is certainly one example: behavioral heterogeneity in the mosquito population was, on the one hand, the most important factor in maintaining infection in the presence of an intervention program and, on the other hand, a factor not initially reckoned with (in a preliminary analysis drawn from the conventional traditions of ecological theory, which too often treat the world as homogeneous)


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