Preventable Diseases

by Woods Hutchinson

"But," we fancy we hear some one inquire impatiently, "what do those academic, technical distinctions matter to us? Whether the avian tuberculosis germ is a variety or a true species may be left to the taxonomists, but it is of no earthly importance to us."

On the contrary, it is of the greatest importance. For the distinctive feature about a particular species of parasite is that it will live and flourish where another species will die, and, vice versa, will die in surroundings where its sister species might live and thrive.

One of the first differences found to exist among these three types of bacteria was the extraordinary variation in their power of attacking different animals. For instance, while the guinea-pig and the rabbit could be readily inoculated with human bacilli, they could only be infected with difficulty by cultures of the bovine bacillus; while the only animal that could be inoculated at all with the avian or bird bacillus was the rabbit, and he only occasionally. In fact, bacteriologists soon came to the consoling conclusion that the avian bacillus might be practically disregarded as a source of danger to human beings, so widely different were the conditions in their moist and moderately warm tissues to those of the dry and superheated tissues of the bird to which it had adjusted itself for so many generations.

And next came the bold pronunciamento of no less an authority than Koch himself, that the bovine bacillus also was so feebly infective to human beings that it might be practically disregarded as a source of danger. This promptly split the bacteriologists of the world into two opposing camps, and started a warfare which is still being waged with great vigor. As the question is still under hot dispute by even the highest authorities, it is, of course, impossible to pronounce any definite conclusions. But the net result to date appears to be that while Koch made a serious error of judgment in declaring that meat and milk as a source of danger to human beings of tuberculosis might be disregarded, yet, for practical purposes, his position is, in the main, correct: the actual danger from the bovine bacillus to human beings is relatively small.

There was nothing whatever improbable, in the first place, in the correctness of Koch's position.

It is one of the few consoling facts, well known to all students of comparative pathology or the diseases of the different species of animals, how peculiarly specialized they are in the choice of their diseases, or, perhaps, to put it more accurately, how particular and restricted disease-germs are in their choice of a host. For instance, out of twenty-eight actually infectious diseases which are most common among the domestic animals and man, other than tuberculosis, only one—rabies—is readily communicable to more than three species; only three—anthrax, tetanus, and foot-and-mouth disease—are communicable to two species; while the remainder are almost absolutely confined to one species, even though this be thrown into closest contact with half a dozen others.

Again, we have half a dozen similar instances in the case of tuberculosis itself. The horse and the sheep, for instance, are both most intimately associated with cattle, pastured in the same fields, fed upon the same food, and yet tuberculosis is almost unknown in sheep and decidedly uncommon in horses, and when it does occur in them is from a human source. The goat is almost equally immune from both human and bovine forms, while the cat and the dog, although developing the infection with a low degree of frequency, almost invariably trace that infection to a human source.

There is, therefore, no a priori reason whatever why we should be any more susceptible to bovine tuberculosis than the remainder of the domestic animals. It is only fair to say, however, that the animal whose diet—and appetite—most closely resembles ours, the hog, is quite fairly susceptible to bovine tuberculosis if fed upon the milk or meat of tuberculous cattle.

Next came the particularly consoling fact that although nothing has been more striking than the great increase in the amounts of meat and milk consumed by the mass of the community during our last twenty years' progress in civilization, this has been accompanied not by any increase of tuberculosis, but by a diminution of from thirty-five to forty-five per cent. The allegation so frequently made that there has been an increase in the amount of infantile tuberculosis has been shown, upon careful investigation by Shennan of Edinburgh, Guthrie of London, Kossel in Germany, Comby in France, Bovaird in New York, and others, to be practically without foundation.

Then, while repetitions of Koch's experiment, upon which his announcement was based, of inoculating calves and young cattle with human bacilli have proved that a certain number of them can be, under appropriate circumstances, made to develop tuberculosis, that number has never been a large percentage of the animals tested, and in many cases the infection has been a local one, or of a mild type, which has resulted in recovery. Lastly, while a number of bacilli, with bovine culture and other characteristics, have been recovered from the bodies of children dying of tuberculosis, and these bacilli have proved virulent to calves when injected into them, yet, as a matter of historical fact, the actual number of instances in which children or other human beings have been definitely proved to have contracted the disease from the milk of a tuberculous cow is still exceedingly and encouragingly small. A careful study of the entire literature of the past twenty years, some three years ago, revealed only thirty-seven cases; and of these thirty-seven Koch's careful investigations have since disproved the validity of nine.