Editor -- The relationship between the intensity of malaria transmission and the degree of malaria risk was discussed recently in the Bulletin's theme section on malaria (1). In their study in the Lancet of hospital admissions in Kenya and the Gambia, Snow et al. (2) found that the cumulative risk of severe malaria by 5 years of age was higher in areas of low-to-moderate transmission (mesoendemic) than in hyperendemic and holoendemic areas (i.e. medium high and high transmission, respectively). They concluded that in highly endemic areas, interventions that reduce transmission of malaria, and thus immunity, might lead to changes in both the clinical spectrum of severe disease and the overall burden of severe malaria morbidity. Recent results from the United Republic of Tanzania do not support this assertion.
Since 1992, the Tanzanian Ministry of Health, through the Adult Morbidity and Mortality Project (AMMP), has assessed age and cause-specific mortality in three districts using a demographic surveillance system (DSS). Key informants record all deaths occurring and inform project clinical officers who conduct verbal autopsy interviews (3). The AMMP DSS populations in rural areas live in Morogoro Rural District, a highly malaria endemic area, and Hai District, on the slopes of Mount Kilimanjaro, where the level of transmission of malaria varies between different altitude zones.
We estimated the cumulative risk of death from acute febrile illness (AFI) between July 1992 and June 1998 from infancy to the age of 10 years in the two rural surveillance areas. Villages in Hai were grouped into four transmission zones based on altitude levels measured using a global positioning system: moderately high or "mesohyperendemic" (750-1000 m), moderate or "mesoendemic" (1001-1250 m), low moderate or "hypoendemic" (1251-1500 m), and non-endemic ([is greater than] 1500 m). Mortality from AFI was compared between all areas and was found to decrease progressively with increasing altitude; older children did not show an increase in mortality as might have been anticipated (Fig. 1). Similarly, in the Muheza District of the United Republic of Tanzania, Ellman et al. (4) compared the prevalence of febrile malaria and anaemia among six groups of villages at different altitudes and with varying parasite prevalence (33-84%). These results did not show the inverse relationship between the level of exposure and malaria morbidity observed by Snow et al.
Interpretation of our observations in relation to the long-term effects of control measures that reduce malaria transmission needs caution. Firstly, we do not have data on intensity of transmission of malaria in the different altitude zones of Hai. Since several other factors such as rainfall, vegetation, vector dynamics, and human population density, migration, and behaviour (5) can alter the intensity of transmission, the classification of intensity of transmission based on attitude alone is necessarily crude. Secondly, we assume that the majority of AFI deaths are attributable to malaria. Because of the nature of the verbal autopsy technique, the exact proportion cannot be determined; it is likely to change, depending upon malaria endemicity. However, this implies that our findings underestimate the gap in mortality and risk of death among the transmission zones. …