10 Report of the BiJharzia Mission in Egypt, 1915 



Versuche mit Crustaceen und Insektenlarven (Daphnia, Cyclops, Chiro- 

 nomus, Culex, Ceratopogoii, Ephemera, u.a.). 



" Nicht gliicklicher verliefen, nachdem so auch Crustaceen und 

 Insektenlarven als mutmaasliche Zwischentriiger des Wurmes hatten 

 von der Lisfce gestrichen werden miissen, entsprechende Versuche niit 

 kleinen Wurmern und Fisclien." 



" Das eben betonte Verhalten der Embryonen anderen Tieren 

 get^eniiber war es nun auch wesentlich, welches micb schliesslich zu der 

 definitiven Ueberzeugung brachte dass die Uebertragung der Embryonen 

 mit Hilfe eines Zwischentriigers aus der Klasse der niederen Tiere nicht 

 vor sich gehen konne. Es blieb desbalb nur noch die Moglichkeit ilbrig, 

 dass der Embryo direkt in den Menschen gelange und dort zu einer 

 Sporosyste auswachse, die ihre Brut dann an ihren Triiger abgebe." 



Naturally, the most likely course and that which one expected a priori was for the 

 embryo after escaping from the cgg-casc to penetrate, in a manner similar to that 

 observed in other species of distoma, into some intermediate host appertaining to the 

 class of moUusca. In my experiments in this connexion I repeated those of Cobbold 

 and Sonsino, but with the like absolutely negative results. Neither in the most 

 frequently occurring gastropods in the Nile Delta (Cleopatra bulimoides, Melania 

 tuberculata, Vlviparn jinicolor, Lanistes carinatus, Pliysa Alexandrina) nor in the 

 lamellibranchiatcs [Corhicula Gaillaudi) was any sign of infection to be detected, 

 whether the experiment was carried out in the day-time, in direct sunlight, or at 

 night, whether at high temperatures or normal ones, whether in large or in small 

 troughs. Equally negative were the results of attempts to find cercaria forms, which 

 could with any probability be referred to Bilharzia, in samples of the same mollusca 

 collected on excursions, sometimes extending over several days, from notorious foci of 

 infection in the Delta. It is more particularly these latter negative experiences that 

 induce me now definitely to exclude mollusca as the intermediate hosts of our parasite. 

 The latter is of such frequent occurrence in Egypt, far more so than would appear from 

 the published statistics— though I admit that these refer to both urban and rural 

 populations, whereas my experiments refer naainly to the latter — that the cercaria form 

 of the Bilharzia, if such exists in a free state, ought to be met with frequently and 

 with certainty under such conditions. And this should be so all the more, considering 

 that the mollusca found in the waters of the Nile harbour cercariie with remarkable 

 frequency ; nearly everywhere from fifty to sixty per cent, are found to be infected, and 

 in some localities only two out of every hundred proved to be free from parasites. 



Experiments conducted in a similar manner with Crustacea and insect larvic 

 (daphnia, cyclops, chironomus, culex, ceratopogon, and others) yielded the same result. 



Crustacea and insect larvte having had to be struck off the list of likely hosts of 

 Bilharzia, analogous experiments were carried out with small worms and fish ; the 

 results were just as unsatisfactory. 



The above described behaviour of the embryos towards other animals was in the 

 main what led me finally to the definite conviction that probably no transference of 

 the embryos by means of an intermediate host appertaining to the classes of the lower 

 animals takes place. Hence the only possible solution that remained was that the 

 embryo reaches man directly, and develops into a sporocyst in the human host, the 

 offspring of the cyst being subsequently distributed to its host. 



In 1896 Looss published, in " Kecbercbes sur la Fauue para- 

 sitaire de I'Egypte " [290], a detailed account of the larval forms 



