346 TYPHOID FEVER. 



a complete reaction. Some typhoid sera have, however, 

 very powerful agglutinating properties, and may in a com- 

 paratively short time produce a reaction when diluted many 

 hundreds of times. The conditions giving rise to such 

 sera are not known, and the cases from which they are 

 derived are not necessarily of a severe type. With highly 

 diluted sera not only may the reaction be delayed but it 

 may be incomplete. Here, what is usually seen is that 

 the clumps formed are small, many bacilli being left free. 

 These latter may either have been rendered motionless 

 or they may still be motile. No diagnosis is conclusive 

 which is founded on the occurrence of such an in- 

 complete clumping alone. Seeing that low dilutions 

 sometimes give a reaction with non-typhoid sera, great 

 discussion has taken place as to what is the minimum 

 dilution at which, when complete clumping occurs, it 

 may safely be said that the reaction is due to the specific 

 action of a typhoid serum. The general consensus of 

 opinion, with which our own experience agrees, is that when 

 a serum in a dilution of i : 30 causes complete clumping in 

 half an hour, it may safely be said that it has been derived 

 from a case of typhoid fever. Suspicion should be enter- 

 tained as to the diagnosis if a lower dilution is required, or 

 if a longer time is required. 



The reaction given by the serum in typhoid fever 

 usually begins to be observed about the seventh day of the 

 disease, though occasionally it has been found as early as 

 the fifth day, and sometimes it does not appear till the 

 third week or later. Usually it gradually becomes more 

 marked as the disease advances, and it is still given by the 

 blood of convalescents from typhoid, but cases occur in 

 which it may permanently disappear before convalescence 

 sets in. How long it lasts after the end of the disease has 

 not yet been fully determined, but in many cases it has 

 been found after several months at least. As a rule the 

 reaction is more marked where the fever is of a pronounced 

 character. In the milder cases it is less pronounced. In 

 some cases which from the clinical symptoms were almost 



