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Pathogenesis.

Notwithstanding the statements of a certain small class of unscientific persons the experimental inoculation of animals has undoubtedly led to great advances in medicine and surgery redounding with benefit to the human race, and has placed many new and valuable facts of treatment and diagnosis within the reach of those engaged in alleviating suffering and combating disease.

The animals chiefly used in experimental inoculation by bacteriologists licensed to perform such experiments are rabbits, guinea pigs, rats and mice, whilst in the production of antitoxine on a large scale horses are generally employed. The experimental inoculation may be made in various ways.

(a) Subcutaneous Injection, the material used being injected under the skin on the abdominal surface in guinea pigs and rabbits, and at the root of the tail in mice. Solid matter is thus introduced.

(b) Intra-peritoneal Inoculation.-The fluid is introduced into the peritoneal cavity direct by means of a sterilised hypodermic syringe.

(c) Intra-venous. Inoculation, the injection being made directly into the veins, generally the posterior vein of the rabbit's ear.

(d) Intra-muscular Inoculation.-The injection is made deeply into the muscles. Various other methods are also employed, such as mixing the infecting material with the animal's food, intercranial injection by removing a portion of the skull with a small trephine ; inoculation into the anterior chamber of the eye, &c. The animal is placed in a separate cage after inoculation and watched, the symptoms noted, and if death occurs a careful autopsy made. To avoid any contamination sterile instruments (well boiled) are used, and the surfaces of glands, heart, &c., well seared with a hot iron before cultivations are made. Cultivations and coverslip preparations are made from the site of inoculation, peritoneal fluid, heart blood and spleen, and from any other sites that the special case under consideration suggests.

For the full particulars of experimental inoculation the reader is referred to the laboratory text-book by Eyre; and it must be remembered that, owing to the stringent regulations in force, no one may perform inoculation experiments under pain of heavy penalty unless they hold a licence from the Home Secretary.

"Bacterological Technique."

71

CHAPTER VI.

Susceptibility and Immunity.

Immunity.

General. By immunity is meant the non-susceptibility to a given disease or to a given micro-organism. Such a resistance may be natural to a whole genus of animals or may be acquired by an animal as the outcome of experimental inoculation; immunity is therefore divided into two great divisions—natural immunity as possessed by certain animals naturally, and acquired immunity developed either as the result of passing through an attack of the disease, or as the result of experimental inoculation with the agents of the disease (bacteria or their toxines). All diseases do not produce a corresponding immunity, whilst others apparently confer a very large degree of protection. For instance in the case of smallpox, scarlet fever and typhoid fever (enteric), one attack generally produces protection lasting many years. On the other hand influenza, pneumonia and erysipelas may occur several times in the same individual, but even with these a certain transitory degree of immunity may be produced. All individuals do not exhibit the same degree of susceptibility to a given disease, as is seen in the percentages of those developing a disease after exposure to infection (cf. diphtheria).

The immunity shown by various races of man differs for certain diseases, and many diseases affecting man are unknown in the lower animals and vice versa.

As a general rule the greater the virulence of the organism producing a disease the more pronounced the protection on recovery; in the experimental production of immunity therefore organisms of a high degree of virulence are employed in the final inoculations.

Changes in the Animal organism associated with Immunity.-In acquired immunity, and to a small extent in natural immunity, the

blood of the immunized animal is found to contain certain bodies which were not present before the disease was contracted or the special inoculations commenced, and the development of immunity is coincident with their appearance in the blood. These anti-bodies (antikörper) may be of two varieties: (a) anti-toxic, which neutralize the toxines produced by the invading bacteria; (b) anti-bacterial, which attack and produce solution of the bacteria themselves. These anti-bodies are not always produced in equal quantities and vary within wide limits. The antitoxines used therapeutically as in treatment of diphtheria, are "antitoxic" rather than "antibacterial." The formation of anti-bodies is not limited to the infection by bacteria by their products, for instance the injection of the washed red blood corpuscles of one animal into another results in the formation of "hæmolysins" which produce solution of the red corpuscles of the blood injected, and so on with various other substances. Rennet ferment on injection produces "anti-rennin," which prevents rennet ferment from acting upon milk. Besides the production of these anti-bodies "agglutinins” are formed which cause the agglomeration of the bacteria against which the animal has been immunised. These bodies. are referred to in greater detail below.

Artificial Immunity.-Two varieties are recognised: (a) Active immunity; (b) Passive immunity (protective).

Active immunity is obtained by injecting an animal with a nonfatal dose of a given bacterium, the dose being so arranged that a considerable illness (reaction) with recovery ensues. After an interval the injection is repeated with a slight increase in the dose, and so on until the animal can withstand many times the initial fatal dose (determined by the injection of a control animal of the same size and weight) without reaction. The animal is now said to be "immunized." Instead of the bacteria themselves, their products, obtained by filtering broth cultures through porcelain filters, may be used, the animal developing immunity as the result of the injection of the toxines in the filtrate.

Active immunity may be produced:

(4) BY THE INJECTION OF LIVING ORGANISMS ATTENUATED IN VARIOUS WAYS.

(1) By growing in the presence of air or oxygen. All pathogenic organisms gradually lose their initial virulence when cultivated for some time outside the body.

(2) By the inoculation of another species of animal. Thus passing anthrax through a guinea-pig lessens its virulence for cattle.

(3) By cultivating the organisms at abnormal temperatures. Pasteur found that anthrax was so much attenuated that it no longer produced fatal illness in sheep, if the cultures were exposed for a certain time to a temperature of 55° C.

(4) By growing the organisms in weak antiseptic solutions, and sometimes by injecting such solutions with the

organism.

(5) By injection of non-fatal doses of virulent organisms. (B) BY THE INJECTION OF DEAD ORGANISMS.

(C) BY THE INJECTION OF FILTERED BACTERIAL CULTURES. (D) BY FEEDING WITH DEAD CULTURES OF BACTERIA. Sometimes it is necessary to increase the initial virulence of a given organism.

EXALTATION OF VIRULENCE.

(A) By the method of "passage," first described by Pasteur.
An animal is injected with an organism either intra-
venously or intraperitoneally, and another animal in-
jected with the blood of the first, containing the
organisms; or a culture may be made upon each occa-
sion. By this means the virulence of the organism may
be raised to an enormously high pitch.

(B) By the combined injection with other organisms.

Thus an attenuated diphtheria bacillus may be raised in virulence by injecting it into an animal together with the streptococcus pyogenes; an attenuated streptococcus by adding bacillus coli, &c.

(C) By artificially lowering an animal's resistance by the action of heat, cold or overwork, &c., or by exposure to general depressing conditions, i.e., guinea-pigs exposed to sewer air succumb to smaller doses of diphtheria than do control animals.

By keeping a frog at a temperature of 30° C. it is rendered susceptible to anthrax and by cooling the ordinary fowl by iced water it becomes less resistant to the cholera vibrio.

By neutralizing the stomach contents of an animal immune to cholera and paralyzing the peristaltic action of the intestines with opium Pasteur found choleraic symptoms were produced on feeding with the organisms. Passive Immunity is that form of resistance to a given disease conferred on a susceptible animal by injecting it with the serum of an immune animal.

The immune serum may be inoculated with the cultivation to be tested, or subsequently within certain time limits which differ for the different organisms used.

For instance, an animal A is injected with increasing doses of diphtheria toxine until it becomes immune. The animal is now bled under aseptic precautions and the clear serum separated from the clot.

An animal B, which has not undergone the process of immunization is injected with a fatal dose (determined by previous injection of other animals) of the diphtheria toxine, and at the same time with the serum obtained from A, B is protected and recovers from what would prove a fatal issue without the protection afforded by the serum of A. Such serum is termed antitoxic, as it apparently neutralizes the toxine. If the animal A is immunized by the injection of bacteria instead of their toxines the serum will protect against living organisms, that is, the immune serum is antibacterial.

Passive Immunity may therefore be divided into varieties.

(A) Antitoxic-when the anti-serum neutralizes the toxine.
(B) Antibacterial-when the anti-serum destroys the bacteria
themselves.

It follows, therefore, that an antitoxic serum is obtained from an animal immunized by toxine injections, and an antibacterial serum from an animal immunized by the injection of living bacteria. The injection of killed cultures also produces an antibacterial serum containing lysins. The serum of an animal immunized by injection of living bacteria which produce a toxine may be both antitoxic and antibacterial, the two phenomena depending on different reactions.

The production of antitoxine in immunized animals has been applied to the treatment of various diseases of a toxic nature, especially diphtheria. The antibacterial sera have so far not met with any great success in the treatment of disease, although attempts have been recently made to protect man against enteric

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