Kala-azar in Ethopia I: Leishmanin skin test in Setit Humera, a kala-azar endemic area in northwestern Ethopia.
Leishmannin skin test surveys were carried out on 1057 farmers and non-farmers in the low-lying Setit Humera area of northwest Ethiopia, a region known to be endemic 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 years. The corresponding periods for the urban and farm-owning population are 6-3 years and migrant farm labourers 2-6 years. Most of the population comes from the highland provinces of Begemdir, Tigre, and Wollo, areas where kala-azar is virtually unknown except for two recent cases (Ashford et al., 1973b). 45-6% of farmers were skin test positive, but only 8-3% of non-farmers. Women in this area are only rarely involved in farming, hence there is a concommitant marked sex difference. The skin test results seem to be 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 less than 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 less than 1% for the non-farmers. There appears (significant at 10%) to be a weak correlation between the induration of positive skin test and the length of time spent in Humera, suggesting that the degree of induration is related to the degree of exposure to Leishmania. The positive skin test in kala-azar endemic areas is probably the result of (a) nonpathogenic Leishmania species, (b) L. donovani of low virulence and/or (c) human host resistance to L. donovani.