C1IOLEK. \ 155 



recommended by Loftier for the destruction of field mice, and has already 

 been used with much success (138). 



7. CHOLERA. When Robert Koch returned in i<S83 from his famed 

 journey to India, the headquarters of cholera, having discovered the parasite 

 that causes the disease, it seemed as though the recognition of the organism 

 would in all cases be an easy task (139). It was the only pathogenic 

 organism of its kind then known, a motile, curved, wriggling rod. But the 

 occurrence of several European epidemics of cholera gave opportunity for 

 more extended researches, and evidence was soon forthcoming on all sides, 

 showing that micro-organisms similar to the cholera bacillus were to be 

 found in almost all kinds of water. This fact must strike every one who 

 makes a bacteriological examination of stagnant water (Fig. 22, b). Diligent 

 search for physiological differences was made, but many of these, the growth 

 in gelatine and the indol reaction, for instance, were soon found to be 

 unreliable. Whether the newest method, Pfeiffer's ' immunity reaction ' 

 (see p. 167), will fulfil the expectations it has raised, remains to be seen. 

 Experiments on animals too must be elaborated in other directions than 

 hitherto. Animals, even in India, are never attacked spontaneously by 

 cholera. Guinea-pigs can be infected per os with the germs only after 

 special preparation. If instead of passing through the natural portal of 

 infection, the mouth, the bacteria be injected into the peritoneum, the 

 animals die, it is true, but only with the symptoms of general bacterial 

 poisoning, and afford no clue which could serve to differentiate between the 

 cholera germ and other aquatic vibrios. Even the bacteriological analysis 

 of the excreta in suspicious cases is accompanied by great difficulties. 

 For this reason all public announcements of the discovery of the germ 

 should be regarded with a considerable amount of scepticism. 



In true Asiatic cholera the characteristic dejecta (rice-water stools) 

 contain enormous numbers of the Koch vibrios, the mucous flocculi 

 discharged being practically pure cultures of them. They are confined to 

 the intestine, the wall of which remains generally intact, although it is 

 occasionally invaded by the proliferating bacilli. They do not attack other 

 parts of the body, as a rule being able to exert their full toxic power in the 

 intestine, from which they disappear in non-fatal cases after one or two 

 weeks. Whilst the experience gained in epidemics is enough to establish 

 the causal connexion between the bacilli and the disease, the result of 

 laboratory infections and voluntary experiments afford conclusive proof. 

 Pettenkofer and Emmerich (140) swallowed pure cultures of vibrios from 

 the Hamburg epidemic and were attacked, the former slightly, the latter 

 very severely, with choleraic symptoms. That in cholera, as in all other 

 infectious diseases, a certain something, a ' predisposition,' must be present 

 before infection can take place, is self-evident. In the case of cholera an 

 important part is played by the diminution of the acidity of the gastric 



