USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 413 



more examples could be given of the reliability and value of the 

 method (for references see Anderson and Williams, 1956). 

 There are now about 60 laboratories in over 30 different coun- 

 tries where this work is pursued and among which information is 

 freely exchanged. The type distribution of S. typhi has been 

 determined over most of the civilized world, and the routine use 

 of Vi-phage typing has contributed materially to the control of 

 typhoid fever. 



6. Phage Typing of Salmonella paratyphi B and S. typhimurium 



Phage-typing schemes exist for two further Salmonella species 

 of major importance: S. paratyphi B and S. typhimurium (see 

 Felix and Callow, 1943, 1951; Felix, 1956). In contrast to 

 typhoid phage typing, these methods depend to only a very 

 limited extent on phage adaptation, most of the typing phages 

 being distinct from each other serologically and in other proper- 

 ties. The principle of using the routine test dilution, as des- 

 cribed earlier, is also employed in these methods. 



As the typing phages of these schemes are not all adaptations 

 of a single phage, few types exist for which there is a single comple- 

 mentary typing preparation. Instead, the differentiation of 

 types is by "pattern reactions." The paratyphoid B typing 

 scheme in current international use is given in Table XXII 

 to illustrate this. A comparison of this table with the typhoid 

 Vi-typing scheme shown in Table XIX will emphasize the 

 difference between the two methods. 



In addition to the 10 main types and subtypes shown in Table 

 XXII, a number of constant variant patterns of S. paratyphi 

 B with the typing phages have been established. These have 

 shown to be epidemiologically stable and the total number of 

 distinguishable types, subtypes, and variations now recognized 

 is over 30. 



The paratyphoid B and typhimurium schemes were originally 

 developed in the belief that the final battery of typing phages for 

 each organism were adaptations of the single starting phage 



