P. Manson-Bahr and N. H. Fairley 67 
urgently require solution. On this knowledge must be based any rational 
prophylactic measures for the eradication of these potential molluscan inter¬ 
mediaries in Australia and in India as well. 
Purification of Water Supplies. 
In a country infested with endemic bilharziasis, the presence in a water 
supply of infested Planorbis or Bullinus, or even uninfested snails of these 
species, is sufficient evidence on which to condemn that supply, and to put it 
out of bounds for troops for drinking, or bathing purposes. 
Even when the molluscs have been removed, the water may still harbour 
cercariae for 36-48 hours. Storage over this period is the most satisfactory 
method of ensuring its safety. Boiling, naturally, is efficient but impracticable. 
Certain chemical agents are advocated but they appear less lethal for cercariae 
than for bacteria. 
The commonest bactericide utilized m the Army is bleaching powder_ 
1 part of available chlorine per 1,000,000 parts of water, being considered 
efficient. In the small book published by the War Office (Memorandum on some 
Medical Diseases in the Mediterranean War Area), it is stated that one part 
per 1,000,000 of available chlorine is efficient in killing cercariae. In numerous 
experiments we have proved the fallaciousness of this statement. Thus, after 
2 i hours ’ immersion in water containing 4 parts per 1,000,000 available 
chlorine, we have found cercariae alive and very motile. The cercariae were 
obtained from Planorbis boissyi, and the bleaching powder used contained 
28 per cent, available chlorine. 
One part per million of available chlorine is the maximum that can be added 
without making the water quite unpalatable. Therefore, unless much greater 
quantities be added, and some method of dechlorinating afterwards employed, 
this method of purification of water infested with bilharzial cercariae must 
prove both unsafe, and unsatisfactory. 
Personal Prophylaxis. 
This applies especially to Europeans visiting those parts of Egypt in which 
bilharziasis is endemic. 
In the case of troops it is advisable to explain to the men, by means of 
lectures and demonstrations, the dangers they incur by not obeying the rules 
drawn up for their guidance. After an ocular demonstration of cercariae in the 
iving Planorbis, for instance, they will be much more impressed than by any 
number of printed notices or orders. 
To the sportsman a knowledge of this disease is of the greatest importance. 
It has been suggested that there is little danger of contracting the disease 
n the winter season when the snipe and duck most abound, as owing to the low 
temperatures the snails will not be infested. 
From actual observations this does not appear to be the case. The snails 
>nce infested remain so for life and attain the maximum degree of infectivitv 
^ «/ 
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