CHAPTER XXI.
ANIMAL INOCULATION.
Animal inoculation has been referred to (1) as a method of assisting in the preparation of pure cultures of pathogenic organisms; (2) as a means of testing the poisonous properties of substances produced in bacterial cultures; (3) in order to test the ability of an organism to cause a disease; (4) for the production of various antibodies; it may be added (5) that some bacteria produce in the smaller experimental animals lesions which do not occur in animals naturally infected, but which nevertheless are characteristic for the given organism. The best illustration is the testicular reaction of young male guinea-pigs to intraperitoneal injections of glanders bacilli. Experimental animals are also inoculated (6) to test the potency of various bacterial and other biological products, as toxins, antitoxins, etc.
Guinea-pigs are the most widely used experimental animals because they are easily kept and are susceptible to so many diseases on artificial inoculation. Rabbits are used very largely also, as are white mice. For special purposes white rats, pigeons, goats and swine are necessary. For commercial products horses (antitoxins) and cattle (smallpox vaccine) are employed. In the study of many human diseases the higher monkeys and even the anthropoid apes are necessary, since none of the lower animals are susceptible.
The commonest method of animal inoculation is undoubtedly the subcutaneous. This is accomplished most readily with the hypodermic needle. The skin at the point selected (usually in guinea-pigs the lateral posterior half of the abdominal surface, in mice the back near the root of the tail) is pinched up to avoid entering the muscles and the needle quickly inserted. Clipping the hairs and washing with an antiseptic solution should precede the inoculation as routine practice. Frequently a small “skin pocket” is all that is needed. The hair is clipped off, the skin pinched up with small forceps and a slight snip with sharp scissors is made. The material may be inserted into this pocket with a heavy platinum needle. Cutaneous inoculation is made by shaving the skin and rubbing the material onto the shaved surface or scratching with a scalpel or special scarifier, but without drawing blood, and then rubbing in the material to be inoculated.
Intravenous injections are made with larger animals. In rabbits the posterior external auricular is a convenient vein. In larger animals the external jugular is used.
Intraperitoneal, -thoracic, -cardiac, -ocular, -muscular injections, and injections into the parenchyma of internal organs are accomplished with the hypodermic needle. In the case of the first two, injury to contained organs should be carefully avoided. Intracardiac injection, or aspiration of the heart to secure blood, requires considerable practice to be successful without causing the death of the animal at once through internal hemorrhage. In subdural injections into the cranial cavity it is necessary to trephine the skull first, while such injections into the spinal canal may be accomplished between the vertebra with needles longer and stronger than the usual hypodermic needle. Occasionally animals are caused to inhale the organisms, or are fed cultures mixed with the feed.
SECURING AND TRANSPORTING MATERIAL FROM ANIMALS FOR BACTERIOLOGICAL EXAMINATION.
If the site of the lesion is readily accessible from the exterior, material from the living animal should be collected with sterile instruments and kept in sterile utensils until the necessary tests can be made. Testing should be done on material as soon after collection as possible, in all cases, to avoid the effects of “decomposition” bacteria.
If the blood is to be investigated it may be aspirated from a peripheral vein with a sterile hypodermic syringe of appropriate size or allowed to flow through a sterile canula into sterile receptacles. The site of the puncture should be shaved and disinfected before the instrument is introduced.