XVIII. The Study of Experimental Infections During Life

The possession of pathogenetic properties by an organism under study is indicated by the "infection" of the experimental animal—a term which is employed to summarise the condition resulting from the successful invasion of the tissues of the experimental animal by the micro-organisms inoculated and by their multiplication therein. Infection is considered to have taken place:

1. When the death of the animal is produced as a direct consequence of the inoculation.

2. When without necessarily producing death the inoculation causes local or general changes of a pathological character.

3. When either with or without death, or local or general changes occurring, certain substances make their appearance in the body fluids, which can be shown (in vitro or in vivo) to exert some profound and specific effect when brought into contact with subcultivations of the organism originally inoculated.

The important factors in the production of infection are:

A. Seed. Virulence of organism.
Dose of organism.

B. Soil. Resistance offered by the cells of the experimental animal.

The first two factors, although variable, are to a certain extent under the control of the experimenter. Thus by suitable means the virulence of an organism can be exalted or attenuated, whilst the size of the dose may be increased or diminished. The third factor also varies, not only amongst different species of animals, but also amongst different individuals of the same species. The essential causes of this variation are not so obvious, so that beyond selecting the animals intended for similar experiments with regard to such points as age, size or sex, but little can be done to standardise cell resistance.

Immediately an animal has been inoculated a period of clinical observation must be entered upon, which should only terminate with the death of the animal. The general observations should at first and if the infection is an acute one, be made daily—later, and if the animal appears to be unaffected or if the infection is chronic, both general and special observations should be carried out at weekly intervals. If the animal appears to be still unaffected, it should be killed with chloroform vapour at the end of two or three months and a complete post-mortem carried out.

A. The general observations should take cognisance of:

1. General appearance. The experimental animal should be inspected daily, not only with a view to detecting symptoms due to the experimental infection, but also to prevent any intercurrent infection, naturally acquired, from escaping notice (vide page 337).

2. The weight of the inoculated animal should be observed and recorded each day during the course of an experimental infection at precisely the same hour, preferably just before the morning feed.

3. The temperature should similarly be recorded daily, if not more frequently, during the whole period the animal is under observation, and carefully charted—individual variations will at once become apparent. It should be borne in mind that the temperature regarded as normal for man (37.5° C.) is not the normal average temperature of any of the lower animals save the rat and mouse. The accompanying table of normal averages for the animals usually employed in bacteriological research may be of use in preventing the erroneous assumption that pyrexia is present in an animal, which merely shows its own normal temperature.

NORMAL AVERAGES.

Rectal Temp. °C. Pulse. Respirations.

Animal. Rate per minute.

Frog 8.9-17.2 80 12

Mouse 37.4 120 ...

Rat 37.5 ... 210

Guinea pig 38.6 150 80

Rabbit 38.7 135 55

Cat 38.7 130 24

Dog 38.6 95 15

Goat 40.0 75 16

Ox 38.8 45 ..

Horse 37.9 38 11

Monkey (Rhesus) 38.4 100 19

Pigeon 40.9 136 30

Fowl 41.6 140 12

B. Special observations comprise some or all of the following, according to the method of inoculation and the character of the virus.

1. The site of inoculation should be minutely examined at least at weekly intervals, and the neighbouring lymphatic glands palpated.

2. Any local reaction at the site of inoculation and any other readily accessible lesion should be carefully investigated. Any suppurative process which may occur, whether in the subcutaneous tissues or in joints, should be explored and the pus carefully examined both microscopically and culturally.

Fluid secretions and excretions, such as pus or serous exudates when accessible are collected direct from the body in sterile capillary pipettes (vide Fig. 13a,) in the following manner:

1. Open the case containing the pipettes, grasp one by the plugged end, remove it from the case, and replace the lid of the latter.

2. Attach a rubber teat (vide page 10) to the plugged end of the pipette and use the teat as the handle of the pipette.

3. Pass the entire length of the pipette twice or thrice through the flame of the Bunsen burner.

4. Snap off the sealed end of the pipette with a pair of sterile forceps.

5. Compress the india-rubber teat, thrust the point of the pipette into the secretion; now relax the pressure on the teat and allow the pipette to fill.

6. Remove the point of the pipette from the secretion, allow the fluid to run a short distance up the capillary stem and seal the point of the pipette in the flame. (If using a pipette with a constriction below the plugged mouthpiece (Fig 13b), this portion of the pipette may also be sealed in the flame.)

When ready to examine the morbid material snap off the sealed end of the pipette with sterile forceps and eject the contents of the pipette into a sterile capsule. The material can now be utilized for cover-slip preparations, cultivations and inoculation experiment.

3. The peripheral blood should be examined from time to time for from this tissue is often obtained the fullest information as to the course and progress of an infection.

a. The histological examination of the blood should be directed chiefly to observations on the number and kind of white cells; and since but few bacteriologists are at the same time expert comparative hæmatologists, some notes on the normal characters of the blood of the commoner laboratory animals, contrasted with those of man, are inserted for reference. These have been very kindly compiled for me by my friend and one time colleague Dr. Cecil Price Jones.