398 INFESTATION OF FRESH-WATER SNAILS. 



has been recorded in man, I have seen in a patient ova 0.236 mm. 

 in length and 0.06 mm. in breadth, which is much longer and 

 relatively narrower than the ova of »$'. haematobium. These ova 

 closely resemble those I obtained from the adult female S. bovis. 

 The cercariae are possibly C. gladii, which is redia-produced. 

 Other cercariae in these pools are S. mansoni, and I have obtained 

 its lateral-spined ova from a patient who contracted the infection 

 from the Mayville pools last year. It is possible that the survival- 

 time of these schistosomes during their free aquatic existence 

 varies with the different species 



The mivival time of the free-swimming bilharzia cercaria in 

 South Africa is a short one, partly because it cannot encyst and 

 partly because its snail-host is not provided with an operculum to 

 resist drought. Whilst active in fresh water, and said to become 

 more active in water containing minute traces of salt, I have 

 observed that the bilharzia cercariae are readily destroyed by 

 strong solutions of salt. This point is of great importance because 

 the intermediary hosts of various trematode worms, particularly 

 the liver-fluke parasite, are continually being washed down-stream 

 into the lagoons along the Natal Coast, and I have recent lv 

 obtained several varieties of them at the mouth of the Umbogin- 

 twini and have found Limnaea natalensis attached to floating 

 wood where the river breaks through the sand-bank to empty into 

 the sea. When the water in these lagoons is fresh, as it commonly 

 is at low tide and during the floods, the cercariae which escape 

 into the lagoon water are likely to prove a danger to swimmers; 

 but, when itie tide is coming in and the water in the lagoon is 

 mixed with sea water, the free-swimming cercariae are rapidly 

 killed and the lagoon is rendered fit for bathing. Fortunately from 

 a public health point of view the current is often too strong when 

 the tide is receding, and visitors to these popular seaside resorts are 

 advised to bathe in the lagoons only when the tide is coming in. 

 The risk from disease from cercariae when carried up the river is 

 considered to be of less importance than the risk of death from 

 sharks when carried out into the open sea. 



The survival time of the miracidium which escapes from the 

 bilharzia egg is only a few hours, but should it reach Physopsis 

 africana, mature cercariae will escape in thirty-five days. These 

 cercariae last as free-swimming organisms only a few hours; but, 

 if they effect an entry into a suitable host, in three months they 

 develop into mature parasitic worms capable of laying eggs. The 

 complete cycle is therefore about four months. This is of import- 

 ance, as it indicates how the disease is introduced into new 

 countries. 



If a pool infested with Physopsis africana. but free from bil- 

 harzia infection becomes contaminated with the excreta of a 

 bilharzia patient, the water will swarm with bilharzia cercariae 

 capable of attacking man within five weeks, and three to four 

 months later one may expect an outbreak of the symptoms of 

 bilharzia infection amongst those who have bathed or drunk this 

 water. 



