If the United States were threatened with invasion by a foreign power, even if we knew that the invasion would be only temporary and that only a few thousand of our citizens would be killed, the whole country would be aroused in an effort to prevent that invasion. If necessary, millions of men would be drafted and trained to meet the invaders and billions of dollars would be expended to protect those few thousand people from the death that must otherwise overtake them. In such a case, every real man and every real woman in the country would be doing something to insure the defeat of that invading army. Yet such an army is like a box of tin soldiers compared with armies that threaten us all the time, but which cause scarcely an extra beat of the nation’s pulse. I refer to the armies of disease. The army of bubonic plague alone, if permitted to effect a foothold on our shores, might at any time ravage our cities as it once ravaged the cities of Europe and Asia, leaving scarcely enough living to bury the dead. We read in DeFoe’s “History of the Plague” in London in 1665 of “people in the rage of their distemper or in the torment of their swellings, which were indeed intolerable, running out of their own government, raving and distracted, and often times laying violent hands upon themselves, throwing themselves out of windows, shooting themselves, mothers murdering their own children in their lunacy.” Indeed, we do not have to go back so far to realize what the plague can do. In 1905 in India alone there were 1,040,429 deaths from this one disease.
The Conquest of Bubonic Plague
In this country no layman loses any sleep on account of bubonic plague. Is that because it does not exist? Not at all. It comes to our waters, even effects a landing sometimes. But we have a small garrison of vigilant medical men on our coasts watching day and night for that enemy, ready to give him instant combat if he comes. We sleep in peace because we trust that garrison. Thirty years ago we did not know what caused this terrible plague, but in 1894 the germ (Bacillus pestis bubonicae) was discovered. Even then it was not known how the disease was carried or what caused it to spread so rapidly—and before it could be combated successfully, that must be known. A series of experiments on living animals, chiefly rats, guinea-pigs and monkeys, yielded the desired information and through these experiments we have been delivered from this terrible scourge. It was known that rats were subject to plague; consequently attempts were made to find out how it was transmitted from one rat to another. The idea that it might be carried by parasites occurred to several investigators. Accordingly, healthy rats were placed in cages close to diseased rats; they remained perfectly well until a few fleas were introduced. Then, almost immediately, the hitherto healthy rats were stricken with plague. Cages containing healthy monkeys were suspended over cages occupied by diseased and flea-infested rats. At regular intervals the monkeys were lowered nearer to the stricken rodents. The monkeys were all right until they were brought within jumping distance of a flea, when they at once contracted the plague. These and other experiments left no doubt that rat fleas were the carriers among animals, and since rat fleas also feed on man when their natural prey is not available, it was an easy matter to show that the plague is spread by means of rat fleas. This led to a definite program for checking the spread of the disease, by relentless warfare on fleas and the rats that carried them. The rats were trapped, their breeding places destroyed, and diseased rats from infested ports were prevented from entering the country. For example, when it was found that rats frequently come ashore along the cables stretched between the ships and the wharves, metal cones similar to those used to prevent rodents from climbing into corn cribs were placed on the cables. The fact that I wish to emphasize is that it is due to experiments on living mammals that this black death is no longer a terror to us.
Experimental Study of Health and Disease
Until the middle of the last century very little had been done in the way of experimental study of physiology and pathology. Physicians depended almost entirely on bedside observations. Some of these physicians were wonderful men, and often their observations were remarkably shrewd. But the human body is a complex machine, the organs are so interdependent, that in the presence of any given set of symptoms and signs of disease, it was almost impossible to be sure just what caused them, and, consequently, what was best to do for the patient. When the experimental method was adopted disease could be observed systematically, conditions could be controlled, and the phenomena that resulted could be studied intelligently because the experimenter knew exactly what had produced them. In such experiments mammals are the animals chiefly used, because in most respects they most nearly resemble man, himself a mammal. Practically all the domestic mammals have been used, horses, cattle, sheep, goats, swine, dogs, cats, rabbits, guinea-pigs, and rats and mice; monkeys are also used. And all have made wonderful contributions to medicine or surgery or both.
Types of Experiments on Animals
I
There are several classes of experiments. Some are in the field of pure research, not having for their object any immediate benefit to man or animals. Experiments of this nature were carried on some years ago in work on bubonic plague among rodents in California. It was discovered that ground squirrels have a disease similar to plague and yet distinctly different. By a long series of experiments it was found that monkeys are susceptible to this disease, and it was predicted that eventually cases would be found in man. As a result of this work a bacteriologist in Cincinnati was able to identify the disease in persons in his own vicinity. Another investigator found it among persons in Utah, and showed that it is carried from infected rabbits and ground squirrels by biting insects. It also was shown that the disease is widespread over the United States. With this knowledge of the means of transmission of the disease it is comparatively easy to prevent the infection of man.