Studies in the epidemiology of East African leishmaniasis: 2. The human distribution and its determinants

1. 1) The human distribution of leishmaniasis in Kenya has not been fully studied in the past. Population censuses and careful studies of the age and sex distribution of leishmaniasis were therefore carried out in two areas of Kitui district, an old-established focus in Tseikuru and a newly affected area in Voo. 2. 2) Marked differences in the population structures were present in these two areas; children accounted for a higher proportion of the population of Tseikuru than Voo; females made up a higher proportion of the population of Voo. 3. 3) There were considerable differences in the age and sex distribution of leishmaniasis in the two areas; children and females both accounted for higher proportions of cases in Tseikuru than in Voo. 4. 4) To eliminate the difficulties in comparing disease distribution in the two areas directly, an epidemiological parameter called the “proportional risk ratio” was calculated for the two areas. This eliminates from consideration time scales and exact knowledge of the size of the populations at risk. 5. 5) The distribution of the “proportional risk ratio” shows that in Voo, the newly affected area, young adult males constitute the population subgroup most affected by kalaazar, whereas in Tseikuru children of both sexes and adult females suffer a proportionally greater risk. 6. 6) The habits of life of the people and the distribution and behaviour of the suspected sandfly vectors are similar in the two areas. It is therefore suggested that the human distribution of leishmaniasis in Voo is determined solely by the relative exposure to infection experienced by the various population subgroups; in Tseikuru where there has been a high degree of endemicity for several years, the present human distribution is affected by immunity as well as by relative exposure to infection. 7. 7) In Voo where immunity is not a determinant of disease distribution the “proportional risk ratio” correlates very well with the relative amounts of time spent by the various population subgroups in close proximity to termite hills. The present study thus provides further confirmatory evidence in support of the “termite-hill sandfly” hypothesis of transmission.