Changing pattern of schistosomiasis in Egypt 1935--79

Area of Science:

• Epidemiology
• Parasitology
• Public Health

Background:

• Schistosomiasis is a significant parasitic disease affecting tropical and subtropical regions.
• Two main species, Schistosoma mansoni and Schistosoma haematobium, cause distinct forms of the disease with varying public health impacts.
• Environmental and infrastructural changes can influence the prevalence and distribution of schistosomiasis vectors and infections.

Purpose of the Study:

• To compare the prevalence of Schistosoma mansoni and Schistosoma haematobium infections in a Nile village between 1935 and 1979.
• To investigate potential environmental factors, such as changes in snail intermediate host populations and water flow, influencing the observed shifts in infection rates.
• To assess the public health implications of the changing epidemiology of schistosomiasis in the studied area.

Main Methods:

• A longitudinal study comparing parasitological survey data from 1935 and 1979 in the same village.
• Random selection of participants using identical parasitological techniques in both survey periods.
• Analysis of hospital records for S. haematobium ova in urine and S. mansoni ova in stool specimens from 1972 onwards.
• Assessment of snail intermediate host populations in local irrigation canals over a 7-year period preceding the 1979 survey.

Main Results:

• Prevalence of Schistosoma mansoni infection increased from 3.2% in 1935 to 73% in 1979.
• Prevalence of Schistosoma haematobium infection decreased from 74% in 1935 to 2.2% in 1979.
• Hospital data showed a decline in S. haematobium ova in urine (30% to 9%) and an increase in S. mansoni ova in stool (2% to 22%) since 1972.
• Snail intermediate hosts for S. mansoni outnumbered those for S. haematobium by a factor of 5-40 in recent years.
• Changes in snail vector proportions were linked to the Aswan High Dam construction and altered Nile water flow patterns.

Conclusions:

• The study documented a significant epidemiological shift in schistosomiasis in the Nile village, with a dramatic rise in S. mansoni and a near disappearance of S. haematobium.
• Environmental changes, particularly those related to the Aswan High Dam and Nile water management, are strongly implicated in altering the balance of snail vectors and, consequently, the dominant Schistosoma species.
• The observed shift has critical public health implications, as hepatosplenic schistosomiasis (S. mansoni) is more severe and challenging to treat than urinary schistosomiasis (S. haematobium).