Kala-azar in Ethiopia I: Leishmanin skin test in Setit Humera, a kala-azar endemic area in northwestern Ethiopia

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Abstract
Leishmanin skin test surveys were carried out in 1057 farmers and non-farmers in the low-lying Setit Humera area of northwest Ethiopia, a known endemic region for kala-azar. The population of the Humera area is composed mainly of new migrants, and the mean time of residence for the entire population is only 4.4 yr. The corresponding periods for the urban and farm-owning population are 6.3 yr and for migrant farm laborers 2.6 yr. Most of the population comes from the highland provinces of Begemdir, Tigre and Wollo areas where kala-azar is virtually unknown except for 2 recent cases. Of farmers, 45.6% were skin test positive; 8.3% of non-farmers were positive. Women in this area are rarely involved in farming; there is a concomitant marked sex difference. The skin test results apparently are correlated with the results for sex, age and occupation of known kala-azar patients. The percentage of skin test positivity increases with age and, particularly among farmers, with the length of time spent in the Humera lowlands. This correlation is lacking among non-farmers. The annual rate of infection to Leishmania sufficient to result in skin test conversion is around 7% per annum in agricultural workers in the Humera area, but < 1% per annum in the non-farming population. The infection rate for farm workers to Leishmania prior to coming to Humera is around 2%, compared to < 1% for the non-farmers. A weak correlation (significant at 10%) is apparent between the induration of positive skin test and the length of time spent in Humera; the degree of induration may be related to the degree of exposure to Leishmania. The positive skin test in the kala-azar endemic areas is probably the result of non-pathogenic Leishmania spp., L. donovani of low virulence and/or human host resistance to L. donovani.

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