Background: Geographical differences in leprosy risk are not understood, but may provide clues about the natural history of the disease. We report an analysis of the geographical distribution of leprosy in Karonga District, a rural area of Northern Malawi, between 1979 and 1989.
Methods: Cohort study of the incidence of leprosy based on two total population surveys. Area of residence was determined using aerial photographs, which allowed identification of households, as well as location of roads, rivers and the lake shore.
Results: Incidence rates were between two and three times higher in the north compared to the south of the district, and lowest in the semi-urban district capital. The most obvious environmental difference between these regions is the north's higher rainfall and more fertile soil. There was no overall association between leprosy incidence and population density, although highest rates were observed in the least densely populated areas. Looking at the entire district, incidence rates increased with increasing distance from a main road, but declined with increasing distance from a river or from the shore of Lake Malawi. The negative association with proximity to rivers may reflect the larger number of rivers in the north of the district. Apparent differences in incidence rates between groups speaking different languages reflected confounding by area of residence.
Conclusions: There is a marked variation, not explained by socioeconomic or cultural factors, in the incidence of leprosy within Karonga District. Our results are consistent with a theme in the literature associating the environment, particularly proximity to water, with leprosy.
PIP: Researchers do not understand how geographical differences relate to the risk of contracting leprosy. The study of such differences, however, may provide clues about the natural history of the disease. The authors report findings from an analysis of the geographical distribution of leprosy in Karonga District, a rural area in Northern Malawi, between 1979 and 1989. Findings are based data from two total population surveys. The areas of residence were determined using aerial photographs, which allowed the identification of households, roads, rivers, and the lake shore. Analysis revealed that incidence rates were 2-3 times higher in the north compared to the south of the district, and lowest in the semi-urban district capital. The north has higher rainfall and more fertile soil. There was no overall association between the incidence of leprosy and population density, although the highest rates were observed in the least densely populated areas. Considering the entire district, incidence rates increased with increasing distance from a main road, but declined with increasing distance from a river or from the shore of Lake Malawi. This negative association with proximity to rivers may reflect the larger number of rivers in the north of the district. Apparent differences in incidence rates between groups speaking different languages reflected confounding by area of residence. The authors conclude that there is a marked variation, not explained by socioeconomic or cultural factors, in the incidence of leprosy within Karonga District. These results are consistent with literature which associates the environment, especially proximity to water, with leprosy.