Environmental factors influencing the distribution hookworm infection in KwaZulu-Natal, South Africa [sic].

Abstract

The aim of this study was to investigate the occurrence of the soil transmitted parasitic nematode Necator americanus ("Old World" hookworm) in soils of different texture in KwaZulu-Natal. The key questions being asked were: (i) Is hookworm infection in KwaZulu-Natal confined to the coastal plain? (ii) Is there any association between hookworm prevalence and the different soil types in the province? (iii) Since several examples exist in the province of soil types on which hookworm is transmitted on the coastal plain, occurring inland, what is the status of infection in communities situated in these areas? (iv) What properties of soil are important in the transmission ecology of hookworm larvae? All available hookworm prevalence data of KwaZulu-Natal were mapped on Land Type maps of the province (Land Type Survey Staff, 1986). Several additional surveys were carried out to supplement this database. Faecal egg counts were obtained by the Formal-Ether Concentration Method and positive infections were confirmed as N. americanus by larval morphology after coproculture using the Harada-Mori Technique. Univariate analysis was carried out for significant associations between hookworm prevalence, altitude, climatic variables (rainfall and temperature) and soil type. The results showed that areas ≤ 150m above sea level (i.e. the coastal plain) support high prevalences (x ‾ = 45 %), and are characterised by low-clay textured soils, warm temperatures and relatively high rainfall. Areas > 150 m (i.e. inland) have low hookworm prevalences (x ‾ = 6 %), and are characterised by high-clay textured soils, cool temperatures and moderate rainfall. Hookworm prevalence also decreased southwards as climatic conditions (rainfall and temperature) become unfavourable, and the coastal plain also narrows in this direction. Multivariate analysis was done to determine which environmental factors combine best to provide favourable conditions for hookworm transmission. From the variables used, prevalence of infection was most significantly correlated with the mean daily minimum temperature for January followed by the mean number of rainy days for January. This points to the importance of summer conditions in the transmission of hookworm infection in KwaZulu-Natal. Moderate hookworm prevalences (x ‾ = 17.3 %) were found in the inland sandy areas, dropping to low prevalences (x ‾ = 5.3 %) in the surrounding non-sandy areas. The intensity-related data could not be significantly correlated with the environmental variables used in this study. The Spearman Correlation Coefficient was used to test for relationships between hookworm prevalence and soil variables. In the results, only the fine and medium sand fractions showed positive correlations with hookworm prevalence. Clay showed a significant negative correlation with hookworm prevalence. No significant correlations were found between soil pH or its organic matter content and hookworm prevalence. Age and sex related infection data could not be drawn into the analysis due to the small sample size of study localities.