43 8 Diphtheria 



gives an abundant supply of blood which clots readily and yields a 

 beautiful clear amber serum. 



The horse selected should be in perfect health, and should be tested 

 with mallein and tuberculin to avoid obscure glanders and tuberculosis. 



A small dose of the toxic bouillon say 0.1 c.c. should be given 

 in the beginning, as one occasionally finds exceptionally susceptible 

 animals that will succumb to larger doses. If a marked local and 

 general reaction follows, it may be better to try another animal. If 

 no reaction is brought about, the immunization is carried on as rapidly 

 as possible. The toxin is injected hypodermatically into the tissues 

 of the neck, the skin being thoroughly cleaned and disinfected before 

 each injection. The doses are cautiously increased and may often be 

 doubled each day. If any unfavorable symptoms arise, treatment 

 must be interrupted for a day or two. The animal yields good anti- 

 toxic serum when it can endure several doses of 500 c.c. of the strong 

 toxin mentioned above. 



III. Bleeding. When the withdrawal of a small quantity of 

 blood by a hypodermic needle introduced into the jugular vein shows 

 that the serum contains a maximum antitoxic strength (300 to 1000 

 units per cubic centimeter), the horse is ready to bleed. Some horses 

 can be bled without resistance, but most of them require to be fastened 

 in appropriate stocks. The blood is taken from the jugular vein, 

 which is superficial, of large size, and easily accessible. The skin is 

 carefully shaved over an area about 9 square inches in extent, thor- 

 oughly disinfected. A small incision is made over the center of the 

 vein, which is made prominent by pressure at the base of the neck, 

 and the point of a small sterile trocar being inserted in the incision 

 through the skin, it is directed obliquely upward into the vein. The 

 blood is allowed to flow through a sterile tube attached to the cannula 

 into sterile bottles prepared to receive it. A large horse may furnish 

 7 to 9 liters ; small horses 5 to 7 liters. 



IV. Preparation of the Serum. The blood is stood away in a 

 cool place until the clot retracts after coagulation and the clear serum 

 separates. The serum is then withdrawn under strict aseptic pre- 

 cautions. It is variously prepared for the market. Some manufac- 

 turers bottle it without any added preservative; some add a crystal 

 of thymol; some Pasteurize it; some add carbolic acid; some add 

 trikresol. 



The plain serum would be ideal, but the danger of subsequent con- 

 tamination through careless treatment makes it rather better to have 

 an antiseptic added. Trikresol is probably the most satisfactory of 

 these, though it throws down a precipitate that necessitates the fil- 

 tration of the product, and leaves the serum slightly opalescent. 



V. Determining the Potency of the Serum. The potency of the 

 serum is expressed as so many "immunizing units." Only one method 

 of testing is produced at the present time, though to understand it, 

 it seems wise to mention the original method from which it was derived. 



(A) Behring's Method. Behring's unit was an arbitrary standard 

 chosen in consequence of certain conditions existing at the time it 

 was devised. It is difficult to understand apart from the circum- 

 stances governing its creation, but may be denned as "Ten times the 

 least quantity of antitoxin serum that -will protect a standard (300 gram) 

 guinea-pig against ten times the least certainly fatal dose of toxic bouillon." 



The method of determining it is not difficult to those skilled in 

 laboratory technic, and is as follows: 



1. Determine accurately the least certainly fatal dose of a sterile 

 diphtheria toxic bouillon for a standard guinea-pig. 



2. Determine accurately the least quantity of the serum that will 



