Transmission of Urinary Schistosomiasis in The Rahad Irrigation Scheme , Sudan

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Elias EL Ghayeb Elias
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University of Khartoum
Abstract Human and snail related aspects of the transmission of urinary schistosomiasis (Schistosoma haematobium) were studied in four village areas in the Rahad Irrigation Scheme in the Sudan. The study on human aspects of transmission took its starting point in cross-sectional surveys comprising - a total of 4,725 individuals, supplemented by an incidence study and by non-quantitative observations on human water contact behavioural patterns. The study on snail related aspects of transmission was based on longitudinal snail sampling over a period of 12 months in all the major human water contact sites in the four study areas. The overall prevalence of S. haematobium infection in the total study population was 30.3%. The age-prevalence curve showed a rapid increase to a maximum of 50.5% in the age group 11-15 years, followed by a decline in older age groups. The age-intensity curve paralleled that of the prevalence curve, with maximum eggs/l0 ml urine figures of 86.2 and 82.9 in the 6-10 and 11-15 age groups, respectively, followed by a decline to 8.8 and 4.7 eggs/l0 ml urine in the 36-40 and >40 year age groups, respectively. The overall prevalence of infection among males and females was statistically comparable. However, the peak prevalence among females was reached already in the 6-10 year age group, whereas that of males was first reached in the 11-15 year age group. On the contrary, prevalence of infection in males exceeded that in females in the 16-20 and 21-25 year age groups at a statistically significant extent. The pattern of intensity of infection in males and females was rather similar, the only difference being in the 16-20 year age group where the male figures exceeded those of females again to a statistically significant extent. The percentage distribution of negative, light, moderate, heavy and very heavy infections in the total study population was 70, 14, 11, 3 and 2%, respectively. Moderate, heavy and very heavy infections were found mostly in the 6-10 and 11-15 year age groups, which were also the age groups primarily responsible for contaminating the environment with S. haematobium eggs. Marked differences in prevalence and intensity of S. haematobium infection were recorded among the four village areas and also between camp and village inhabitants within each of the age areas. Thus, the overall prevalence of infection among the camp populations was 30.7% as compared with only 22.8% among the village populations. Similarly, the intensity of infection among the camp populations exceeded markedly that among the village populations, the geometric mean egg counts being 119.9 and 43.8 eggs/10 ml urine, respectively. Marked differences in infection status were recorded in relation to occupational activities. Thus, a low infection level was recorded among the farmers, whereas a high infection level was recorded among the farm workers. An incidence of infection study implemented to supplement the cross-sectional studies clearly showed intensive transmission of S. haematobium in the hot and rainy (April-September) and post rainy (October-December) seasons, whereas no transmission took place in the cool (January-March) season. While a marked variation in number of eggs of S. haematobium excreted in urine was recorded during the day, an only limited day to day variation was recorded. Maximum numbers of eggs/10 ml urine was excreted around noon. Human water contact behaviour was highly focal, and spetific sites were used for specific purposes. Site use was determined by accessibility and by suitability for a particular activity. Sites with sandy bottoms were thus found most suitable for bathing and swimming. A clear seasonal trend in the frequency of water contacts was seen. Generally water contact was relatively intensive during the hot and rainy seasons (April-June and July-September) and during the cool season (January-March). During the post-rainy season (October-December), water contact varied in intensity, ranging from moderate to occasionally high. A marked seasonal variation in the population density of Bulinus truncatus , the snail host for S. haematobium, was recorded. The pattern of variation in snail numbers was rather irregular, without any clear-cut seasonal trend. A maximum mean number of 130.5 snails collected per site visit was recorded in November, whereas the minimum mean number of 24.7 snails was recorded in February. The seasonal variation in snail numbers showed no similarity among the four village areas studied. S. haematobium infected B. truncatus were recorded throughout the year with the exception of February, March and July. Peak numbers of infected snails were recorded in December and June, and the presence of infected snails during May and June and during October to December was a rather general finding in all village areas. Judging from the number of human water contact sites where S. haematobium infected B. truncatus were found, transmission of urinary schistosomiasis is very focal in the Rahad Irrigation Scheme. Grouping of the different sites as either swimming or non-swimming, revealed that by far most of the infected snails were found in sites categorized as swimming sites. . The present study has provided a valuable insight into the epidemiology of urinary schistosomiasis in the Rahad Irrigation Scheme. In addition, the study has also clearly revealed that the present schistosomiasis control programme in the Scheme, comprising annual treatment of infected schoolchildren and focal mollusciciding, is relatively unsuccessful
Urinary Schistosomiasis Rahad Irrigation Scheme Zoology University of Khartoum
Elias EL Ghayeb Elias, Transmission of Urinary Schistosomiasis in The Rahad Irrigation Scheme,Sudan .- Khartoum : University of Khartoum, 1992 .- illus., 28cm., Ph.D