Epidemiology of Schistosomiasis in Egypt: Travel through Time: Review.

Schistosomiasis is a parasitic disease caused by blood flukes (Trematodes) of the genus Schistosoma (S.). It is well documented that schistosomiasis haematobium was endemic in Ancient Egypt. Infection was diagnosed in mummies 3000, 4000 and 5000 years old. Scott was the first to describe the pattern of schistosomiasis infection in Egypt. Schistosomiasis haematobium was highly prevalent (60%) both in the Nile Delta and Nile Valley South of Cairo in districts of perennial irrigation while it was low (6%) in districts of basin irrigation. Schistosoma mansoni infected 60% of the population in the Northern and Eastern parts of the Nile Delta and only 6% in the Southern part. Neither S. mansoni cases nor its snail intermediate host were found in the Nile Valley South of Cairo. The building of the Aswan High Dam -which was completed in 1967 - did not cause any increase in schistosomiasis prevalence. In 1990, a study conducted in nine governorates of Egypt confirmed the change in the pattern of schistosomiasis transmission in the Delta. There was an overall reduction in S. mansoni prevalence while Schistosoma haematobium had continued to disappear. In Middle and Upper Egypt there was consistent reduction in the prevalence of S. haematobium except in Sohag, Qena, and Aswan governorates. However, foci of S. mansoni were detected in Giza, Fayoum, Menya and Assiut. All schistosomiasis control projects implemented in Egypt from 1953 to 1985 adopted the strategy of transmission control and were based mainly on snail control supplemented by anti-bilharzial chemotherapy. In 1997, the National Schistosomiasis Control Program (NSCP) was launched in the Nile Delta. It adopted morbidity control strategy with Praziquantel mass treatment as the main component. In 1996, before the NSCP, 168 villages had S. mansoni prevalence >30%, 324 villages 20-30% and 654 villages 10-20%. By the end of 2010, in the whole country only 29 villages had prevalence >3% and none had more than 10%.

Introduction

Schistosomiasis is a parasitic disease caused by the digenetic trematodes of the genus Schistosoma members which are commonly known as blood flukes. Schistosoma haematobium was discovered by Theodore Bilharz in 1851 during autopsy at Kasr El Ainy hospital [1]. In 1915, the life cycle of the Schistosome parasite was first described by Leiper [2]. Schistosomiasis comes after malaria among parasitic diseases as regards the number of people infected and those at risk of infection [3].

There are two major forms of schistosomiasis –intestinal and urogenital-caused by five species of the parasite. Intestinal schistosomiasis is caused by four species namely: Schistosoma mansoni (S.mansoni), S. japonicum, S. mekongi and S. intercalatum. S. mansoni is the most prevalent species being endemic in 55 countries e.g. Arab peninsula, Egypt, Libya, Sudan, Sub-saharan Africa, Brazil, some Caribbean islands, Suriname and Venzuela [3]. S. japonicum is endemic in China, Indonesia and the Philippines while S. mekongi prevails in several districts of Cambodia and the Lao Peoples’ Democratic Republic and S. intercalatum prevails in rain forest areas of Central Africa. On the other hand, S. haematobium –which is the causative agent of urogenital schistosomiasis – is endemic in 53 countries in Africa and the Middle East [4].

In the absence of accurate epidemiological data, estimates must still be used to determine the possible burden of infection due to schistosomiasis. On the basis of extrapolating the national prevalence data obtained from the world atlas of schistosomiasis and applying it to 1995 population estimates, it was calculated that about 625 million people would be at risk and 193 million would be infected. Based on these calculations, 85% of the estimated number of infected people are in the African continent [3].

Although successful control projects have been implemented in the last 50 years, yet neither the number of endemic countries nor the estimated number of people infected or at risk of infection were reduced [5].

History of schistosomiasis in Egypt

It is well documented that schistosomiasis haematobium was endemic in Ancient Egypt. Ruffer in 1910, was the first to diagnose S. haematobium infections in mummies. He recovered calcified schistosome eggs from two Egyptian mummies of the 20th Dynasty [6]. Calcified schistosome ova were identified radiologically in several mummies from later periods [7]. The radiological examination also strongly suggested that the calcified bladders in two other mummies were due to S. haematobium infection [8].

The use of an immunodiagnostic test, the ELISA, led to the diagnosis of the earliest case of human schistosomiasis (S. haematobium) which occurred more than 5000 years ago in an Egyptian adolescent [9]. ELISA also identified S. haematobium infection in two mummies aged 3000 and 4000 years [7].

Trend of schistosomiasis in Egypt

Scott, 1937, was the first to describe the pattern of S. haematobium and S. mansoni infection throughout Egypt [10] Fig. 1. His conclusions were based on two series of data which seemed to harmonize fairly well. The first were data obtained from 2 million samples collected by the Endemic Diseases section of the Public Health departments. The second were the results of examination of samples from 40,000 persons in a house to house survey done under Scott’s direct supervision. On the basis of the distribution of the human schistosomes, Scott divided Egypt into four regions (the first region is the Northern and Eastern parts of the Delta, the second is the Southern part of the Delta while the third and fourth parts are in the Nile Valley South of Cairo. The third part is areas with perennial irrigation system and fourth part is areas with basin irrigation). In the first three regions about 60% of the rural population was infected with S. haematobium. In the first region, the Northern and Eastern parts of the Delta, S. mansoni also infected 60% of the population, while about 85% had either one or both species. In the second region, the Southern part of the Delta, S. mansoni infected not more than 6%, although the intermediate host, the Biomphlaria alexandrina snail, seemed to be as abundant as in the first region. No topographic, hydrographic or demographic differences between these regions could be noted, although the line of demarcation was very sharp as far as the prevalence of the parasite was concerned. The third and fourth regions were in the Nile Valley South of Cairo and there, the snail intermediate host of S. mansoni had never been found. In the third region – where perennial irrigation was used – S. haematobium only was found and its prevalence was 60%. In the fourth region -areas with basin irrigation – S. haematobium prevalence was less than 5%. Scott observed that the snails were much more abundant where perennial irrigation furnished canals and ditches containing water throughout the year while with basin irrigation most of the breeding places were alternatively swept by the annual flood and desiccated in the hot summer. This led him to conclude that the change in the system of irrigation in Upper Egypt was responsible for the increase in S. haematobium infection rate from 5% to 60% [10]. Furthermore, Azim, 1935 and Khalil and Azim, 1938 demonstrated the impact of converting the basin irrigation system to perennial irrigation in Upper Egypt on the transmission of schistosomiasis haematobium [11,12]. Similarly, El Zawahry reported that the construction of the perennial irrigation system in old Nubia led to a remarkable increase in S. haematobium infection rate [13].

Fig. 1

Map of Egypt governorates.

After Scott’s survey, several studies were conducted to estimate the pattern of schistosomiasis transmission in one or more governorates of Egypt [14-19]. Twenty years after Scott’s study, Wright reported the distribution of both species of schistosomes based on data which originated from another survey carried out by the Egyptian Ministry of Health using the same methods employed by Scott and in the same villages which he had surveyed, but involving 124,253 persons taken by random sampling [14]. Comparing the data obtained with those of Scott’s, changes in the pattern of the two species were observed. Both S. mansoni and S. haematobium had decreased in the Nile Delta. However, S. mansoni had increased in Giza and S. haematobium had decreased in Upper Egypt except in Sohag, Qena and Aswan where there was a dramatic increase in these three governorates due to conversion to the perennial irrigation system. Furthermore, in 1977, studies conducted in eight villages in Qalubeia governorate reported obvious changes in the pattern of transmission of schistosomiasis prevalence during the previous two decades. The prevalence of S. haematobium showed a marked decrease contrary to S. mansoni which showed a relative increase [15]. These findings led to the design of two cross-sectional surveys of the population of the Nile Delta in 1983 and 1990 [17,19]. The two surveys included the study of 71 villages, one village from each of the 71 districts comprising the eight governorates of the Nile Delta. When the data of 1983 survey was compared with Scott’s data, a slight increase in the overall S. mansoni prevalence from 33% in 1935 to 39% in 1983 was observed. One governorate, Menofeia, had a sharp increase in the prevalence (from 3% to 20%) which accounted for most of the change. The authors attributed the overall increase in prevalence in 1983 to the use of more sensitive diagnostic tests. The relative sensitivity of the diagnostic techniques used in the two surveys must be considered. Scott used the Stoll and Hausheer dilution technique [20] while the Kato technique [21] was used in the 1983 study. The effective amount of stools examined was 5 mg in 1935 study as compared to the 43 mg in 1983 study. The Kato technique proved to be more sensitive than the dilution technique. The authors (Cline et al.) of 1983 study believed that if the dilution technique was used in their study, the infection rate would have been much lower than 39%. On the other hand, there was a striking decrease in S. haematobium prevalence from 56% to 5% in all governorates of the Delta which could not be attributed to diagnostic sensitivity [17,19].

The results of the survey in1990 demonstrated a 38% decrease in the overall prevalence of S. mansoni infections in the Nile Delta governorates since the 1983 study. On the other hand, S. haematobium infections have continued to disappear from the Delta showing a 40% decrease in prevalence during the same period. Mickelson et al., 1993 attributed the changes in prevalence of both species of schistosomal infections to the advent and the increasing availability of the safe and effective anti-schistosomal drug, praziquantel in addition to the dissemination of information about schistosomiasis through the mass media [17,19].

As regards the Middle and Upper Egypt governorates, it is obvious that there was consistent reduction in the prevalence of S. haematobium except in the most Southern three governorates, Sohag, Qena and Aswan [16,22]. In addition a number of communities with high prevalence of S. mansoni had emerged. Foci of S. mansoni were described in Fayoum [18], Menya [23] and Assiut [24]. Consequently, The National Schistosomiasis Control Program has formulated objectives to prevent the further spread of S. mansoni in Upper Egypt [25].

In 1990, an extensive national house to house survey similar to the one conducted by Scott in 1935 was conducted to investigate the prevalence and intensity of infection with schistosome species, the prevalence and magnitude of morbidity caused by schistosomiasis, the changing pattern of distribution of S. mansoni and S. haematobium and the determinants of infection and morbidity. A random sample of the rural inhabitants of nine governorates selected as representative of each area (Upper and Lower Egypt) and of governorates with both high and low infection rates [26]. Although the study was conducted over a period beginning in 1990 and ending in 1994 [26], yet the results were published in 2000 [22,26] except for Kafr El Sheikh (KES) governorate which was published in 1995 [27].

As regards Lower Egypt, the five governorates; KES, Gharbeia, Menoufeia, Qalubia and Ismailia, where S. mansoni is endemic, showed a prevalence rate ranging from 17.5% to 42.9% with an average of 36.45% [22,27-31]. S. haematobium on the other hand, was rare in these governorates; Ismailia had the highest infection rate of 1.8% while Qalubia had the lowest (0.08%) [22].

In Upper Egypt governorates, where S. haematobium is endemic, the prevalence rate ranged from 4.8% to 13.7% with an average of 7.8%. S. mansoni was rare being consequential in Fayoum only, which had a prevalence of 4.3% [23,32,33]. Although this survey did not include Giza, yet another study carried out in one of the villages of this governorate indicated that the estimated prevalence of S. haematobium was 7.4% which was in accord with the results of other areas of Middle and Upper Egypt [34-36]. On the other hand, the prevalence of S. mansoni was unusually high amongst the villagers (33.7%) and exceptionally high amongst the primary school children (57.7%) of the same village [36]. In conclusion, the study of the nine governorates of Egypt confirmed the already documented change in the pattern of transmission of both species of schistosome infection in Upper and Lower Egypt [22]. The detailed data are presented in Tables 1 and 2 and Fig. 1.

In general, two main factors were responsible for the pattern of schistosomiasis infection in Egypt. The first factor is the irrigation used whether basin or perennial. The change from basin to perennial irrigation was the result of the construction of the Aswan High Dam. The second was the control programs implemented by the Egyptian Ministry of Health. Since both these factors played a vital role regarding the situation of schistosomiasis infection, they will be discussed in details under separate topics later.

Effect of Aswan High Dam on schistosomiasis transmission in Egypt

The Aswan High Dam was constructed on the River Nile, 7 km South of Aswan. The designs of the Dam were completed in 1959 and its construction began in 1960. Temporary closure of the Nile in 1965 was instituted till the building of the Dam was completed in 1967 and by 1970 all 12 turbines were in operation [37,38].

The effect of Aswan High Dam on the prevalence of schistosomiasis has aroused a lot of controversy. Some scientists expected that schistosomiasis prevalence would increase after the closure of the Dam. A tremendous increase in bilharziasis was predicted by Van Der Schalie [39]. He even stated that” there is evidence that the high incidence of the human blood fluke (schistosomiasis or bilharziasis) in the area may well cancel out the benefits the construction of the Dam may yield”. This increase in schistosomiasis prevalence was attributed to reclamation of new land and conversion of the basin irrigation system to the perennial. Furthermore, in 1977, Malek stated other reasons for the increase in schistosomiasis transmission [40]. His study indicated that, at least in some sections of the lower Nile, ecological changes as a consequence of the Dam were enhancing the transmission of the disease. In addition to the abundance of the snail intermediate host in the Nile, the absence of silt and decrease in water current velocity in the lower Nile would have given higher chance for the miracidia to come in contact with the snails and for the cercaria to infect humans. The same author reported that human activities in and near the Nile water had increased considerably throughout the year because of the low, clean and slow water [40]. There was more fishing, swimming and washing of domestic utensils and clothes. Such activities used to be done only in irrigation canals. Another significant factor in the ecology of the snail host living in the irrigation canals in the Nile Delta is the elimination of the winter closure because clearance of the canals -during this period-from the flood silt deposited in their beds was not needed anymore. Such an adverse and disastrous factor which used to affect the snail population will be absent after the High Dam construction, thus leading to flourishing of the breeding of snails in the Nile Delta canals with increase in schistosomiasis transmission [40]. Another study conducted in some villages in Upper Egypt close to the Dam reported an increase in the prevalence rate of schistosomiasis haematobium among some inhabitants of these villages [41].

In contradiction to the previous conclusions, other scientists reported that the construction of the Aswan High Dam did not cause an increase in schistosomiasis prevalence [16,38,42]. In 1978, Miller et al. [42] conducted an environmental and epidemiological survey on 15,329 rural Egyptians who were selected from three major geographical regions of Egypt (Nile Delta, Middle Egypt and Upper Egypt) in addition to the resettled Nubian population. Prevalence of either or both species of schistosomiasis was 42.1% in the North Central Delta region. In Middle Egypt which spreads from Beni Suef and Assiut governorate, S. haematobium was the only prevalent species with a rate of 26.7%. In addition, sporadic cases of S. mansoni were noted. Prevalence in the study sites of Upper Egypt varied according to the location of the village. In desert villages, the prevalence of S. haematobium was very low (4.1%) compared to the prevalence in agricultural villages (24.8%). The same authors [42] concluded that: “there was sufficient historical and current data to firmly disregard any role of the Aswan High Dam in causing an increase in schistosomiasis in rural Egypt” [42]. They also reported that all available data pointed to an overall decrease of both S. haematobium and S. mansoni rather than increase and this included the resettled Nubia. However, there was an indication that the distribution of S. mansoni is expanding southwards [15,42]. They attributed the reported reduction in the prevalence to the improvement in the domestic water supply to villages, the development and delivery of proper health care in addition to the increase in the general awareness among the population at risk of how to avoid infection and how to get treatment.

As regards transmission of schistosomiasis in Lake Nasser, infection with S. haematobium was prevalent among the fishermen working there. The prevalence of detected S. haematobium infection at entry to the lake declined from 67% in both 1974 and 1975 to 18% in 1980 and 20% in 1981. This was attributed to the widespread use of metrifonate in Upper Egypt beginning in 1975. As regards S. mansoni infection, no cases were reported among fishermen examined [32].

The snail intermediate host of S. haematobium (Bulinus truncatus) was present in abundance; some of them were infected with S. haematobium. On the other hand, Biomphlaria alexandrina, (the intermediate host of S. mansoni) was detected in only one site at the Northern tip of the lake, but none of them was infected [32].

Schistosomiasis control in Egypt

The first attempt to control schistosomiasis in Egypt started in 1920 [43]. The Egyptian Ministry of Health installed mobile units to examine and treat all pupils at a large number of elementary, primary and secondary schools thus stopping the disease in early childhood. The number of these units increased from 6 in 1924 to 56 in 1933 with the number of annual treatments increasing from nearly 47000 to 311000 [44].

In 1926, the first planned control scheme was started at Dakhla oasis. It comprised treatment of about a third of the population with tarter emetic in addition to application of copper sulfate for 96 h to all irrigation canals. All Bulinus snails were killed and none was found 6 months later [45]. Regular surveys in the early 1930s failed to detect snails and none of the 70 children born after the last mollusciciding in 1929 was infected at 1936 survey [46].

Until mid eighties of the last century, the strategy for schistosomiasis control -recommended by the WHO – aimed at reducing transmission by diminishing the snail population. As this method became effective, morbidity in the human population was slowly reduced and in the long term, the complete eradication of the parasite might have been achieved [47].

In 1984, a major change in the strategy became possible with: (a) Recognition that morbidity of schistosomiasis was directly related to the prevalence and intensity of infection, both being high in the 10–14 years age group. (b) The development of simple quantitative diagnostic techniques suitable for field studies. (c) The development of new antischistosomal drug (praziquantel) which is safe and effective against the three important human schistosome species. At present, the main objective of control is to reduce or eliminate morbidity or at least serious disease [47].

All schistosomiasis control projects carried out in Egypt followed the same strategy recommended by the WHO. All programs conducted before 1984 aimed at transmission control and the main activity was based on snail control which might be supplemented by antibilharzial treatment. The following control projects were implemented during the period from 1953 to 1985 [35].

Qaliub project (1953–1954): Snail control using copper sulfate.

Qalubeya project (1953–1959): Mass treatment using tartar emetic.

Warrak El Arab project (1953–1959): Snail control using sodium pentachlorophenate.

Egypt 049 project (1961–1969): Snail control using niclosamide.

Iflaka project (1962–1966): Mass treatment using Astiban.

Giza project (Shimbari 1970): mass treatment using hycanthone.

Fayoum project (1969): Chemotherapy and snail control using niclosamide [48].

Middle Egypt control program: started in 1977, implemented in Beni Suif, Menia and Assiut North of Dairut).

Upper Egypt control program: started in 1980, implemented in Assiut South of Dairut, Sohag, Qena and Aswan.

Middle and Upper Egypt control projects [24,36] were the largest of those conducted in Egypt. They covered about two million irrigated Feddans and a total population of more than 12 million people. Extensive land reclamation with installation of the drainage has been carried out in different parts of the area. The project was divided into three phases: (1) Intensive phase; 3 years. (2) Consolidation phase; 3 years. (3) Maintenance phase. The intensive phase involved: (a) Area wide application of niclosamide for three times/year. (b) Chemotherapy for infected individuals, metrifonate (bilarcil), three doses with 14 days apart. In 1988 praziquantil was used for treatment.

Impact of the projects: an international evaluation team, in 1985, showed that since the initiation of control intervention, the overall prevalence of schistosomiasis haematobium [35] of about 30% in the Middle Egypt project area, determined in 1977, had been reduced to approximately 8.5% [34]. Furthermore, the detailed data reported by the Ministry of Health showed a continuous downward trend in prevalence rates. But significant re-infections were reported among school children particularly in young age groups during the summer season indicating that appreciable transmission was continuing in the project area [35].

Although it was apparent that a large measure of control had been achieved since intervention began, the results showed some upward trends in prevalence of infection during the maintenance phase [35].

National Schistosomiasis Control Program (NSCP) in the Nile Delta: This project started in 1997 and was based on the morbidity control strategy adopted by the WHO in 1984 [47]. The main activity of the project was praziquantel chemotherapy. Mass treatment was offered without prior diagnosis to all school children 6–18 years old and to all inhabitants of villages where S. mansoni prevalence among outpatients of rural health units was ⩾20%. Otherwise, treatment was given to infected persons only. Furthermore, focal mollusciciding using niclosamide was applied on water courses with high snail density or harboring infected snails. In addition, health education campaigns and capacity building through training of personnel working in rural health units or involved in snail control activities were applied [49].

As the program progressed, and S. mansoni prevalence decreased, the threshold for mass chemotherapy was changed to ⩾10% in 1999, ⩾5% in 2000 and ⩾3.5% in 2002 to 3% in 2003. The records of Ministry of Health reported that in 1996 before the application of mass chemotherapy campaign; 168 villages had prevalence >30%, 324 villages had prevalence 20–30% and 654 villages had prevalence 10–20%. By the end of 2010, in the whole country only 20 villages had prevalence more than 3.5% and none had prevalence more than 10% [49].

At present, a multi-sectoral approach is adopted. This aims at interrupting transmission and achieving elimination through wider integration of the present strategy with other interventions such as mass chemotherapy campaigns for school-age children and populations in hot spot areas together with improvement of health awareness, social mobilization, snail control within the activities of the primary healthcare system, and environmental sanitation. The strategy adopted for control differs according to the epidemiological setting. In newly developed areas with no transmission and no autocthonous cases, surveillance and routine screening is done. In villages where schistosomiasis prevalence is <3%, active population screening, monitoring after treatment, snail control and water and sanitation are done. On the other hand, in villages with active transmission and a prevalence >3%, mass treatment, snail control and clean water and sanitation are stressed [49].

Conclusion

Although it is well documented that schistosomiasis haematobium was endemic in Ancient Egypt, yet the first detailed study describing its pattern of prevalence was carried out in 1937 by Scott. He reported that schistosomiasis was highly prevalent in both the Nile Delta and the Nile Valley South of Cairo in districts where the perennial irrigation system was used. The highest prevalence was recorded in the Northern and Eastern parts of the Delta where 85% of the population was infected with either one or both species of the parasite. On the other hand, S. mansoni infection was very low in the Southern part of the Delta and completely absent from the Nile Valley South of Cairo whether basin or perennial irrigation system were used.

After Scott’s study, several large scale surveys were conducted to estimate the pattern of schistosomiasis infection. In general, in the Nile Delta governorates there was a gradual reduction in the overall prevalence of S. mansoni infection while S. haematobium prevalence continued to decrease till it disappeared completely. In Middle and Upper Egypt governorates, there was a consistent reduction in the prevalence of S. haematobium infection except in Sohag, Qena and Aswan following the construction of the High Dam where basin irrigation was converted to perennial irrigation system.

At present, the Ministry of Health and Population records indicate that by the end of 2010 only 20 villages in the whole country showed prevalence more than 3.5% and none had more than 10%. The great success in controlling schistosomiasis in Egypt is achieved through the implementation of several control programs which adopted the same strategy recommended by the WHO.

Table 1

Prevalence of S. haematobium and S. mansoni infection in Lower Egypt governorates, 1935–2000.

Governorate year	Source	S. haematobium (%)	S. mansoni (%)
Beheira
1935	Scott [10]	53.1	50.7
1955	Wright [14] (MoH)	46	31
1966	Farouq et al. [50]	29.7	28.5
1983	Cline et al. [17]	8	53
1990	Mickelson et al. [19]	1	24
Gharbeya
1935	Scott [10]	58.3	38.0
1955	Wright [14]	51	17
1983	Cline et al. [17]	4	43
1990	Mickelson et al. [19]	3	23
2000	El Khoby et al. [22]	0.26	37.7
Kafr El Sheikha
1935	Scott [10]	44	44.7
1955	Wright [14]	43.4	25.5
1981	Miller et al. [16]	30	20
1983	Cline et al. [17]	4	51
1990	Mickelson et al. [19]	2	17
2000	El Khoby et al. [22]	0.45	39.2
Menoufeya
1935	Scott [10]	70	10.3
1955	Wright [14]	45	1
1983	Cline et al. [17]	5	20
1990	Mickelson et al. [19]	3	19
2000	El Khoby et al. [22]	0.44	28.5
Dakahleya
1935	Scott [10]	53.4	37.8
1955	Wright [14]	50	9
1983	Cline et al. [17]	6	45
1990	Mickelson et al. [19]	2	31
Sharkeya
1935	Scott [10]	65.8	27.8
1955	Wright [14]	52	5
1983	Cline et al. [17]	5	26
1990	Mickelson et al. [19]	6	22
Qalubeya
1935	Scott [10]	61.5	25.9
1955	Wright [14]	31	3
1983	Cline et al. [17]	6	29.0
1990	Mickelson et al. [19]	7	19.0
2000	El Khoby et al. [22]	0.08	17.5

Previously part of Gharbeya governorate.

Table 2

Prevalence S. haematobium and of S. mansoni infection in Middle and Upper Egypt governorates, 1935–2000.

Governorate year	Source	S. haematobium (%)	S. mansoni (%)
Giza
1935	Scott [10]	53.7	0.6
1955	Wright [14]	34	3
1999	Talaat et al. [36]	7.4(10.6)a	33.7(57.7)
Fayoum
1935	Scott [10]	74.2	0.4
1955	Wright [14]	38	One case
2000	El Khoby et al. [22]	13.7	4.3
Beni Suef
1935	Scott [10]	75.2	0.0
1955	Wright [14]	36	ND
1981	Miller et al. [16]	27	ND
Menia
1935	Scott [10]	41.6	Two cases
1955	Wright [14]	39	ND
1987	Kessler [34]	5.0
2000	El Khoby et al. [22]	8.9	1.04
Assiut
1935	Scott [10]	29.7	0.0
1955	Wright [14]	16.0	ND
2000	El Khoby et al. [22]	5.21	0.42
Sohag
1935	Scott [10]	3.1	Two cases
1955	Wright [14]	42.0	ND
Qena
1935	Scott [10]	3.8	0.0
1955	Wright [14]	4
2000	El Khoby et al. [22]	4.78	0.44
Aswan
1935	Scott [10]	13.2	0.0
1955	Wright [14]	23	ND
1981	Miller et al. [16]	4 (25)b	0.0

ND: not done.

Prevalence in village population (prevalence in primary school children).

4% In desert villages, 25% in agriculture villages.