58 Bejyort of the Billiarzia Mission in Egypt, 1915 



Origin of Bil- The source of infection of the Nile water at 

 HARZiA Infec- Cairo is somewhat difficult to locate. Under the 

 TiON IN Cairo, miracidium infection theory it was attributed to 

 contamination of the water by infected urine from the crews on 

 the boats which daily congregate near the Kasr Nil bridge. On 

 the alternative hypothesis of a molluscan intermediary one must 

 look farther afield. The molluscs known to harbour bilharzia 

 cercaria congregate mostly in the smaller canals and ditches where 

 there is a good deal of vegetable substance for food. They are air- 

 breathers and require to seek the surface from time to time. In 

 the Delta, water does not return to the Nile when once it has been 

 used for irrigation purposes. The whole of the agricultural drainage 

 water is discharged either into the salt lakes near the Mediterranean 

 shore or directly into the sea. That of the valley or " Wadi Tumilat " 

 which runs from Zagazig to Ismailia discharges into Lake Timsah 

 on the Suez Canal. In Upper Egypt, however, the canals have 

 escapes at various points on their courses which allow surplus 

 water to return into the Nile. The agricultural drains also discharge 

 into the Nile at certain places. These escapes are indicated on the 

 accompanying map (fig. 25). It will be seen that from Minia to 

 Fashn the drainage is turned sometimes into the Nile, and at other 

 times into the Bahr Yusef, whence it makes its way through the 

 Fayum into Lake Kurun or continues to discharge at El Ayat by 

 the Giza Canal escape. Between Fashn and El Ayat all drains 

 escape into the Nile, while below El Ayat the drains discharge into 

 the large Moheit drain which enters the Bayah el Behera below 

 the Barrage north of Cairo. A few miles south of Cairo it will 

 be noticed that the Giza Canal has two escapes into the Nile. 



Effect of The velocity of the Nile varies from month to 



Varying Velo- month. Whereas the movement of water down 

 cities of the the Nile from the Assiut Barrage to the Delta 

 ^^^' Barrage occupies seven days in a mean year during 



March to August, in September the water travels the same distance 

 in three days, in October and November it takes four days, while 

 from December to February five days are necessary. Taking one and 

 a half days as the approximate duration of life of the free-swimming 

 cercaria, it is evident that at all times of the year water freshly 

 contaminated with cercaria at Assiut would become safe long before 

 it reached Cairo. During high flood in September the Nile has a 

 velocity of about one hundred and fifty kilometres per diem, that 

 is water containing freshly discharged cercaria entering the Nile 

 within about one hundred and fifty miles up-stream of Cairo would 



