F. G. Cawston 
87 
Date 
Place 
River 
Feb. 14 
Verulam 
Umhloti 
„ 16 
Avoca 
Little Umhlanga 
„ 16 
Toll Gate 
Brickfields 
(Durban) 
„ 17 
Hill Crest 
Umhlatazana 
„ 18 
Umlaas Bridge 
Umlaas 
Maritzburg 
Quarry 
Mar. 10 
99 
Boshoff St. pool 
„ 31 
99 
Umsindusi 
- 
Grey town 
River 
Feb. 16 
Durban 
Umgeni 
May 1 
99 
99 
„ 13 
Henley 
Umsindusi (near 
source) 
June 23 
Maritzburg 
Brickfields 
Observations 
» 
Reeds clear after rains 
Physopsis plentiful in infected pools 
Physopsis, Planorbis, Limnaea and Isidore 
plentiful in infected pools 
Reeds clear in river, bathing not allowed 
Physopsis plentiful in infected river 
Only Limnaea, doubtful infection 
Bathing forbidden. No Bilharzia cercariae. 
Physopsis, Limnaea, Planorbinae and Ancyli 
plentiful 
Physopsis and Limnaea plentiful. Bilharzia 
cercariae in Physopsis. Infected river 
Reeds clear and no infection known 
Reeds clear 
Physopsis and Planorbis. An infected place 
Reeds clear in river. No infection known 
Reeds clear. No bathing 
THE FRESH WATER SNAILS OF NATAL. 
I have encountered the following species of snail in Natal rivers and fresh¬ 
water pools: 
Limnaea natalensis is a common light brown snail with a semi-transparent 
dextral shell. Some specimens are infested with “tadpole'’ cercariae. 
Physopsis africana is a common black snail with a blunt-pointed sinistral 
shell, the shell is characterised by a truncate columella. This snail harbours both 
the “tadpole” and furcocercous cercariae; but 1 have only found the latter forms 
of cercariae in bathing-places which are known to give rise to Bilharziasis. 
Planorbis pfeifferi is similar to the intermediary host of S. mansoni in 
Egypt. It is a common brown snail with a round, flat shell. 
Planorbis leucocheilus is a light brown, less common snail. The shell is 
small and flat. Planorbis pfeifferi harbours many “tadpole” cercariae. 
Isidora tropica is a fairly common, dark brown snail with a blunt-pointed, 
sinistral shell. I have never found cercariae in this species. 
Isidora forskali is a rare, brown snail with a conical shell, I have only 
seen about three specimens of it. 
In one brickfield, I found a large number of a small oval snail; which is a 
new species of Ancylus not yet identified. 
The following table shows the percentage of snails harbouring Bilharzia 
cercariae between the months of April and July, 1916. 
Month 
Source 
No. infected 
Percentage 
Form of cercaria 
April 
Toll Gate (Durban) (bathers infected) 
1 out of 
7 
14 
human 
May 
• 
99 99 99 99 
13 
99 
85 
15 
12 human (1 eye- 
spotted) 
June 
99 99 99 99 
2 
99 
13 
15 
human 
July 
99 99 99 99 
8 
99 
131 
6 
99 
April 
Umsindusi (bathing not allowed) 
30 
99 
197 
15 
1 or 2 human 
May 
99 99 99 99 
38 
99 
170 
22 
99 
June 
99 9T 99 99 
7 
99 
30 
23 
almost all C. secobii 
July 
99 99 99 99 
0 
99 
6 
0 
June 
Boshoff St. (no bathing) 
0 
99 
20 
0 
July 
o 
G 
0 
9 9 
