Bilharziasis 
52 
(late autumn) the livers of the infested snails harboured full grown cercariae, 
while in the early spring, immature cercariae and sporocysts were the rule 
rather than the exception. 
Table IV. 
Seasonal incidence in infestation. 
B ULLIN US PLA NORTHS 
Number of 
Percentage 
Number of 
Percentage 
Month 
snails dissected 
infested 
snails dissected 
infested 
1917 
May 
51 
1-9 
280 
37 
June 
27 
0-0 
572 
19 
July 
140 
0-7 
288 
29 
August 
57 
1-7 
567 
8 
September 
93 
0-0 
689 
10 
October 
No record 
No record 
920 
6 
November 
50 
4-0 
1468 
30 
December 
362 
9-0 
379 
54 
1918 
January 
140 
0-0 
150 
24 
February 
72 
3-0 
100 
10 
March 
69 
0-0 
252 
32 
April 
41 
2-5 
124 
28 
Total* 
1102 
1-9 
5789 
18 
* These percentages have been worked out from the total number of specimens found infested 
each month. 
During the period of maximum infestation the canals are full of irrigation 
water resulting from the annual rise of the Nile—this would appear to be one 
factor. 
During the months of January and February, according to our observations, 
a large number of the adult snails die off, and, in fact, the survivors are very 
difficult to find as they remain inactive at the bottom of the pools. In April 
and May these canals are periodically hushed with a large addition of stored 
water, and it is at this time that the maximum breeding of Planorbis takes 
place. For then large floating masses of young Planorbis adhering to the weeds 
collect in stagnant backwaters in the native villages, and, owing to the prox¬ 
imity of the native houses, they stand a very good chance of becoming invaded 
by miracidia. 
The breeding season of Bullinus, on the other hand, from observations we 
were able to make in the sedimentation tanks at Kantara, would appear to 
be the months of July and August, when the water in most of the smaller 
canals is at low ebb. At this period also, this snail is most active and appears 
on the surface of the water, where it is scarcely ever found at other seasons. 
Owing to the small volume of water in the canals it is legitimate to assume 
that this is the season wherein the maximum opportunity of being brought 
into close contact with the miracidia of S. haematobium occurs. 
