418 A MANUAL OF BACTERIOLOGY 



Ley ton) probably first observed the agglutination re- 

 action in typhoid fever, but it was brought into prominence 

 through the work of Widal, and is frequently referred to 

 as the " Widal reaction " (Plate XIII, a). Normal serum 

 will generally agglutinate the typhoid bacillus in a dilution 

 up to 1 in 3 or 4, occasionally up to 1 in 10. 



Typhoid blood-serum commonly agglutinates in dilu- 

 tions of from 1-30 to 1-100, and sometimes in dilutions 

 up to 1-500. Dilutions should not be less than 1-25. 

 Zones of no reaction occasionally occur. (For method, 

 see p. 430.) Cases of mixed infection with typhoid and 

 paratyphoid bacilli occasionally occur, and give agglutina- 

 tion with both organisms. 



The reaction is not obtained before the sixth or seventh 

 day of fever, occasionally not until much later. Very 

 rarely the reaction seems to be intermittent. The blood 

 may retain its agglutinating power for two to three years 

 after an attack, and anti-typhoid vaccine also confers 

 marked agglutinative properties up to nine months after 

 inoculation. Cases do occur in which agglutination is 

 absent throughout the attack, but they are rare and often 

 tend to be severe and to terminate fatally. Usually, if 

 the blood during the course of a typhoid-like attack fails to 

 give a reaction when tested on three occasions at intervals 

 of three to four days, it is improbable that the case is one 

 of typhoid fever ; it may be a case of paratyphoid fever 

 due to infection with one of the paratyphoid bacilli (see 

 p. 435). If a positive reaction be obtained, yet the case 

 does not seem to be one of typhoid, a previous attack or 

 inoculation with typhoid vaccine must be excluded. The 

 previous injection of a typhoid anti-serum into a patient 

 might induce the reaction for a short time, three to four 

 weeks. 



Typhoid serum outside the body retains its agglutinative 

 power for weeks or months, slowly weakening, and typhoid 



