FOUNDATIONS OF MODERN TROPICAL MEDICINE 23 



Still more important are the labours of Roux, Haffkine, Wright, 

 Strong, Lustig, Galeotti, and others, in perfecting and applying 

 vaccines to the prevention as well as to the treatment of disease. 



Those of greatest tropical importance are Haffkine's plague 

 vaccine, the same worker's cholera vaccine, and Wright's mono- 

 valent typhoid vaccine and his pyogenic vaccines. 



A further advance was the introduction of Castellani's multiple 

 vaccines for the prevention and treatment of disease. For long 

 it was thought that a vaccine must be monovalent, but Castellani's 

 triple vaccine for typhoid and the paratyphoid fevers has now been 

 used on a very large scale for the British and other armies with 

 success, and the tetravaccine typhoid, para A, para B, cholera has 

 been adopted by the Serbian Army since 1916. He has also pre- 

 pared tetravaccines, which include undulant or Malta fever, and 

 has prepared and advocated the use of penta- and hexa-valent 

 vaccines. 



Entomology. — It is obvious from the preceding sections that 

 insects play a great part in the spread of protozoal diseases, and the 

 same holds good for worms and bacteria. 



Filaria can be spread by Culex, Stegomyia, or Anopheles; malaria 

 by several species of the Anophelinae ; Piroplasma and spirochaetes by 

 ticks; Leishntania donovani perhaps by bugs; Leishmania infantum 

 probably by fleas; trypanosomes by tsetse-flies, and possibly by 

 some kind of flies. 



But apart from these diseases, of which we know the cause, 

 there are two infections the unknown agent of which is carried 

 by mosquitoes. Thus Finlay, in 1881, formulated definitely the 

 hypothesis that yellow fever was spread by a mosquito, which 

 in 1900 was proved by Reed, Carroll, Agramonte, and Lazear to 

 be a fact, the mosquito being Stegomyia fasciata, now Stegomyia 

 calopus. 



With regard to dengue fever, Graham of Beirut, in 1903, thought 

 that he had discovered a protozoon in the red corpuscles of persons 

 suffering from this disease, and that this parasite underwent develop- 

 ment in Culex fatigans (Wied). Doubt has been thrown upon 

 Graham's parasite, but the idea that Culex fatigans (Wied) is respon- 

 sible for the spread of dengue fever has been strongly supported by 

 Ashburn and Craig in 1907. 



Turning how to bacterial diseases, the work of the Indian Plague 

 Committee, published in 1906-1908, proves that the rat-flea (Xenop- 

 sylla cheopis) is the main means of the spread of plague. With 

 regard to typhoid, it was conclusively proved in the army concen- 

 tration camps of the United States in the Spanish-American War 

 of 1898 that flies were great spreaders of the disease, a fact already 

 emphasized by Celli and others ; and this has been further supported 

 by the work of Firth and Horrocks (1902), Chantemesse, and numer- 

 ous other observers. Dysentery may also be spread by flies. 



A knowledge, therefore, of ticks, biting flies, and other insects is 

 of the greatest importance to the doctor who is to practise in the 



