When a fatal dose of diphtheria toxin is injected into a susceptible animal, such as a guinea pig, there is a period of incubation dur ing which the animal shows no marked de parture from the normal. This incubation period varies somewhat with the size of the dose, but is never less than about eight hours, even when many times the fatal dose has been used. However it should not be inferred that nothing happens during this period of incuba tion. The disturbance simply does not rise to the plane of gross clinical observation. The toxin begins to 'act soon after its introduction into the body. Within an hour or two the clinical thermometer, which is not a highly delicate indicator of changes in temperature, shows an elevation which proceeds slowly un til a short time before death, when there is a progressive and rather rapid fall in tempera ture which may reach some degrees below nor mal before death. The skin about the point of injection becomes edematous and later necrotic. The interval between injection of the toxin and death is the same as after inoculation with the living organism. When sublethal doses are given there is often paralysis, beginning in the posterior extremities and gradually extending over the body. The internal organs are hy peremic, with minute hemorrhages in the adrenals, stomach and intestines. Occasionally a gastric ulcer is produced. Diphtheria toxin apparently has a special avidity for nervous tissue. Whether this action is primarily cen tral or peripheral has not been satisfactorily determined, though it is most probably the lat ter. When the toxin is injected into a sus ceptible animal it soon disappears from the blood current and manifests its activity on certain organs and tissues. It has not been found in the urine except when massive doses have been given. In unsusceptible animals it remains for a long time in the blood stream and is of course without action on the tissues, and it is for this reason that the animal is re fractory. Animals differ widely in their suc ceptibility to diphtheria toxin. This is true of individuals, and still more markedly true of species. The most susceptible animal is the guinea pig. Horses, sheep and goats are suc ceptible, while rabbits are less so, and white mice practically refractory.
The toxin is a secretion of the living bacillus and different strains of the organism vary widely in the amount of toxin which they elaborate. There is apparently no constant and fixed relation between the virulence of a given strain of the bacillus and its toxin pro duction. A strain isolated by Park of New York, and generally Icnown as Park No. 8, has proved in both American and European laboratories to be a most efficient toxin pro ducer. It was obtained from a relatively mild case of diphtheria and it has shown no great vi.ulence as tested by the experimental inocu lation of guinea pigs. With this strain filtered products have been obtained with as low a minimum lethal dose as 0.0015 cubic centimeter. However this strain sometimes fails to produce a satisfactory toxin. Through another strain Madsen once reported a toxin whose minimum lethal dose was as low as 0.0005 cubic centi
meter. This is the most powerful diphtheria toxin yet reported. The cellular substance of the diphtheria bacillus contains a protein poison, as was first shown in the writer's laboratory. Against this poison diphtheria antitoxin has no neutralizing effect and it must be regarded as a poison, but not a true toxin. It will be seen that the word toxin has come to have a distinct and specific meaning. All toxins are poisons, but all poisons are not toxins. Those poisons which when injected into animals in gradually augmented doses produce antibodies are known as toxins. The scrum of horses immunized to diphtheria toxin, and known as diphtheria anti toxin, is usually preserved by the addition of 0.5 per cent phenol or 0.3 per cent tricresol. Filtration of the serum through porcelain re moves the antitoxin on account of the large size of its molecules. Heating antitoxin to from 60° to 70° C. destroys its value, but the dried antitoxin will bear a temperature of 110° C. for half an hour without injury. Diph theria antoxin is quite certainly a protein. It is true that it bears some resemblance to fer ments. It is in and of itself perfectly harm less to the animal body. All disturbances which may result from the administration of diphtheria antitoxin are due to other constitu ents of the serum and may be induced by the use of the serum of a normal horse. The anti toxin may be precipitated from the serum by metallic salts as other protein constituents are, but it is not carried down mechanically as hap pens to the toxins and the ferments. It is more than probable that the antitoxin is a globulin. It is precipitated with this protein fraction, and the concentrated antitoxin has been prepared by this method. Unfortunately the immunity induced by diphtheria antitoxin is only temporary,. lasting from three to four weeks, while one attack of diphtheria does not give immunity to another. For this reason it freqttently happens that a child who has once had an immunizing dose may, a few months or possibly a few years later, need a curative dose. The fear of anaphylactic shock has led many physicians to hesitate about a °reinjec tion)) of horse serum after an interval of 10 days or longer, but with proper care one may do this with safety. In all cases of °reinjection') after an interval of 10 days or longer one drop of the antitoxin should be administered and if no untoward symptoms follow within an hour, any amount of the serum may be injected with safety. This procedure should be adopted not only in all cases of greinjection" but when the patient has shown at any time asthmatic symp toms. With these precautions no physician need fear to use diphtheria antitoxin in the treat ment of this disease. When antitoxin is used early in the disease the extension of the mem brane in the throat usually stops in a few hours. For this reason laryngeal diphtheria now rarely seen except in neglected cases. Not only does the extension of the membrane stop on the administration of the antitoicin, but as a rule that already formed begins to recede, becomes detached and fades away.