BACILLUS OF TYPHOID FEVER 423 



with the typhoid culture, of course, each time doubles the dilutions 

 so that, for instance, a drop of serum dilution 1 : 10, plus a drop of 

 the typhoid culture, gives the final dilution of 1 : 20. The prepara- 

 tions may be examined with a high power dry lens or an oil im- 

 mersion lens. In a positive reaction, the bacilli, which at first swim 

 about actively, singly or in short chains, soon begin to gather in small 

 groups and lose much of their activity. Within one-half to one hour, 

 they will be gathered in dense clumps between which the fluid is clear 

 and free from bacteria, and only upon the edges of the agglutinated 

 masses may slight motility be observed. The degree of dilution and 

 the time of exposure at which such a reaction may be regarded as of 

 specific diagnostic value, have been largely a matter of empirical de- 

 termination. It is generally accepted at present that complete agglu- 

 tination within one hour in dilutions from 1 : 40 to 1 : 60 is definite 

 proof of the existence of typhoid infection. Exceptions, however, to 

 this rule may occur. Agglutinations of typhoid bacilli in dilutions of 

 1 : 40, and over, have occasionally been observed in cases of jaundice 

 and of tuberculosis, and these conditions must occasionally be consid- 

 ered, though their importance was formerly exaggerated. 



The method of making the Widal test from a drop of whole blood, 

 dried upon a slide, is not to be recommended, as accuracy in dilution 

 by this method is practically impossible. 



As stated above, the agglutinin reaction rarely appears in typhoid 

 fever before the beginning of the second week. It may continue during 

 convalescence for as long as six to eight weeks and occasionally, in cases 

 where there is a chronic infection of the gall-bladder, a Widal reac- 

 tion may be present for years after an attack. 



For very exact work, even in clinical cases, the microscopic agglu- 

 tination method may be replaced by macroscopic agglutination, ac- 

 cording to the technique described in another section (page 229) . 



In order to avoid both the necessity of keeping alive typhoid cultures 

 for routine agglutination tests and also to preclude the danger of in- 

 fection by the use of living culture, Ficker ' has recommended the use 

 of typhoid bacilli killed by formalin. This method has no advan- 

 tages for practical purposes and in scientific bacteriological work it is, 

 of course, not to be considered in comparison with the other exact 

 methods. 



Precipitins. — The investigations of Kraus ^ in 1897, by which the 



■ Ficker, Berl. klin. Wooh., xlviii, 1903. ^ /Craws, Wien. klin. Woch., xxxii, 1897. 



