It is therefore apparent that tuberculin should be applied only by or under the direction of a competent veterinarian, capable not only of injecting the tuberculin but also of interpreting the results, and particularly of picking out all clinical cases by physical examination. The latter observation is extremely important and should always be made on every animal tested.
In the second cla.s.s, where the temperature test is used, errors are avoided by eliminating from the test those cases that are nearing parturition or are in heat or show evidence of the previously mentioned diseases or exhibit temperatures sufficiently high to make them unreliable for use as normal. Where other methods of test are used these conditions do not have an important bearing on the results. In addition, a satisfactory tuberculin must be used; also an accurate thermometer and a reliable syringe, in order that a sufficient dose of tuberculin may be given. Finally, the number of apparent errors of the tuberculin test will be greatly diminished if a careful post-mortem examination is made, giving especial attention to the lymph glands. This low percentage of failures being the case, cattle owners should welcome the tuberculin test, not only for their own interest but for the welfare of the public as well. Where this method of diagnosing the disease has been adopted tuberculosis is gradually being eradicated.
Without its use the disease can not be controlled and the owner is confronted with serious and continuous losses; with its use the disease can be eradicated from the herd, a clean herd established in a few years without very serious loss or hardship, and the danger of its spread to man removed. Tuberculin may therefore be considered a most beneficial discovery for the stock raiser.
Law has clearly stated the question when he says--
Many stock owners still entertain an ignorant and unwarranted dread of the tuberculin test. It is true that when recklessly used by ignorant and careless people it may be made a root of evil, yet as employed by the intelligent and careful expert it is not only perfectly safe, but it is the only known means of ascertaining approximately the actual number affected in a given herd. In most infected herds living under what are in other respects good hygienic conditions two-thirds or three-fourths are not to be detected without its aid, so that in clearing a herd from tuberculosis and placing both herd and products above suspicion the test becomes essential. * * * In skilled hands the tuberculin test will show at least nine-tenths of all cases of tuberculosis when other methods of diagnosis will not detect one-tenth.
Probably the most popular objection to tuberculin is that it is too searching, since it discovers cases in which the lesions are small and obscure. While this fact is admitted, it should also be remembered that such a small lesion to-day may break down and become widely disseminated in a relatively short period. Therefore any cow affected with tuberculosis, even to a slight degree, must be considered as dangerous not only to the other animals in the herd but also to the consumer of her products.
In 1898 Bang, of Copenhagen, one of the highest European authorities, in his paper presented to the Congress for the Study of Human and Animal Tuberculosis, at Paris, said:
Numerous tests made in almost every civilized country have demonstrated that in the majority of cases tuberculin is an excellent means for diagnosing the existence or nonexistence of the disease, but giving us no positive information as to the extent to which the disease has progressed. When tuberculin produces a typical reaction we may be almost sure that there exists in the body of the animal a tubercular process. The cases in which a careful examiner has not succeeded in finding it are very rare, and I am led to believe that when, notwithstanding all the pains taken, it has escaped discovery, the reason is that it is located in a portion of the body that is particularly inaccessible. Nevertheless, it is not to be denied that a fever, entirely accidental and of short duration, may in some rare cases have simulated a reaction. However this may be, the error committed in wrongly condemning an occasional animal for tuberculosis is of no practical consequence.
A worse aspect of the case is that there are some diseased animals in which tuberculin fails to discover the existence of tuberculosis. In most of these, no doubt, the deposits are old, insignificant, and generally calcified, or they are cases where the disease is arrested and perhaps in process of recovery, and which are possibly incapable of disseminating the contagion. But it is known that there are cases, not altogether rare, where tuberculin fails to cause a reaction in a highly tuberculous animal, and consequently one in which the disease exists in an extremely contagious form. For this reason a clinical examination should always be made of an animal which does not give a reaction but which shows symptoms indicating that, notwithstanding the test, it may suffer from tuberculosis.
Nocard, of Paris, wrote also in 1898 as follows:
The degree of certainty of the indications furnished may be stated in precise terms. _The observation of a clear reaction to tuberculin is unequivocal; the animal is tuberculous._ The pretended errors imputed to the method are explained by the extreme sensitiveness of the reagent, which is capable of detecting the smallest lesion. It often requires prolonged and minute researches in the depths of all the tissues to discover the few miliary centers, the presence of which has been revealed. The reaction is absolutely specific. In those cases where it is observed with animals which show lesions of another disease (actinomycosis, hydatid disease, verminous bronchitis, distomatosis), it may be affirmed that there exists, in addition to these conspicuous changes, a tuberculous center which alone has provoked the reaction.
_The failure to react does not necessarily imply absence of tuberculosis._ Such failures of tuberculin are very exceptional. They are seen most frequently with animals affected with tuberculosis in a very advanced stage and made evident by plain external signs. Sometimes, also, there are found at the post-mortem examination of animals which have not reacted small fibrous or calcified lesions in such a condition that one is tempted to believe them cured. Whether sterile or not, these lesions have no tendency to increase, and they are not very dangerous from the point of view of contagion.
These opinions of two eminent authorities, living in different countries, after long experience of their own and after studying the results of the many tests made in different parts of the world, should have great weight.
They are essentially the same throughout.
In 1897 Voges compiled statistics of tuberculin tests, the accuracy of which had been determined by post-mortem examination. Of 7,327 animals tested, it appeared that errors had been made with 204, or 2.78 per cent.
In the work of the Pennsylvania Live Stock Sanitary Board post-mortem examinations were made on about 4,400 reacting cattle and the disease was found in all but 8 of those which had given characteristic reactions.
The results of a much larger number of tests might be compiled at this time, but they would not materially change the average of those already mentioned. It is plain that tuberculin is a remarkably accurate test of tuberculosis, that the animals which react may be safely considered as tuberculous, and that when a careful clinical examination is practiced in addition to the test there are few animals in a dangerous condition which escape detection.
The first questions asked by those who oppose the adoption of the tuberculin tests are: Is this test infallible? and, if it is not infallible, why should it be forced upon the cattle owners of the country?
In answer to these questions it may be said that tuberculin is not absolutely infallible, and yet it is by far the best method of diagnosing tuberculosis that has been discovered. It is much better than any test known for pleuropneumonia when that disease was eradicated.
Practically all the animals that react are affected with tuberculosis and should be separated from the herd, not only in the interest of the public, but in the interest of the owner of the herd. The best authorities admit, after studying many thousands of tests, that there are few, if any, mistakes made in condemning cattle which show a typical tuberculin reaction. The errors are princ.i.p.ally in the other direction--that is, some tuberculous animals are not discovered by the tuberculin test, but as the most dangerous of these may be picked out by ordinary clinical examination this fault of tuberculin is not so serious as it at first sight appears.
This being the case, it should not be necessary to force the tuberculin test upon owners. They should be anxious to adopt it in their own interests and for the protection of their patrons. There is to-day no greater danger to the cattle and hog industries than that which confronts them in the form of tuberculosis, a disease already widespread and rapidly extending.
Furthermore, in view of the results revealed by numerous tests covering vast numbers of animals, tuberculin must be considered as harmless for healthy animals. It has also been clearly demonstrated that tuberculin interferes in no way with the milking function in healthy cattle; neither in the quant.i.ty of milk nor in b.u.t.ter-fat value has any variation been detected. The conclusions of some of the best authorities on the subject of its harmlessness to healthy animals are given below.
Nocard and Leclainche state:
Direct experiments and observations collected by thousands show that the tuberculin injections have no unfavorable effect. With healthy animals the system is indifferent to the inoculation; with tuberculous animals it causes slight changes which are not at all serious.
Bang has written as follows on this question:
We will now consider the following question, a very important one, in the application of tuberculin, viz: Can the reaction produce a worse condition in tuberculous animals than before existed? Hess emphatically states that it can, and on this account he earnestly warns against its application. My attention has been directed to this question from the beginning. In my first publication on tuberculin injection I reported two cases in which acute miliary tuberculosis was proved in two high-grade tuberculous cows several weeks after the tuberculin injection. I then stated my suspicion that perhaps the tuberculin injection had some connection with this, just as is often supposed to be the case in human practice. With my present very large amount of material for observation at hand I may express the following opinion: Such an acute development of tuberculosis as a result of tuberculin injection is to be feared only exceptionally, and then in cases of advanced tuberculosis. _It must not be forgotten that acute miliary tuberculosis by no means rarely accompanies an advanced tuberculosis of long standing._ It is therefore impossible to offer strict proof of the causal connection with the injection, and only oft-repeated observation could make this probable.
In support of my view I offer the following: In the course of the last three years I have made careful post-mortem examinations of 83 tuberculous animals, which have been removed from my experiment farm, Thurebylille. Among these were 18 (or, strictly speaking, 23) high-grade tuberculous animals. I have been able to prove miliary tuberculosis in only 4 of these. Among the others, which showed less developed tuberculosis, I have never found miliary tuberculosis, and with very many I have never found any sign of a more rapid development of the process.
On the contrary, it has been proved that the disease was restricted locally, often for years, in spite of yearly repeated injections.
Dissections were made at very different periods after the injections--in 17 cases from 4 to 12 days after the last test. In all of these cases earlier tests had been made months or years before. In 28 cases the injection took place from 19 days to 2 months before the butchering; in 3 of these cases earlier injections had been made. In 38 cases from two and one-half months to one year intervened between the last injection and the dissection. Dissection gives the best explanation of this question, but a clinical observation, continued for years, of a herd tested with tuberculin can render very essential aid. If Hess"s opinion is correct, it is to be a.s.sumed that tuberculosis must take an unusually vicious course in such herds, but this I have been unable to prove. At Thurebylille there has existed for three years a reacting division, consisting originally of 131 head and now 69. Although these animals are yearly tested, and although most of them react every year, the division certainly appears to be made up of healthy animals, and the farm inspector has expressed the decided opinion that the tuberculosis in this division is no more developed than at the beginning of the experiment.
The testimony of many owners of large herds of cattle which have long ago been injected is to the same effect. I will adduce statements from several. A farm tenant whose cattle were injected 20 months previously, when 82 per cent of the grown animals reacted, wrote me recently as follows: "Only 2 cows from the division of 100 head had been sold as decidedly tuberculous. The majority appeared afterwards, just as before, entirely healthy. The fat animals which had been slaughtered had been p.r.o.nounced healthy by the butchers." Another farm tenant with a herd injected in 1894 had not been obliged to remove a single animal from the tuberculosis division, numbering 70 head. A large farm owner in Jutland stated in September that he had traced no undesirable result from the injection. His herd of 350 had been injected in February and about 75 per cent reacted. Similar answers have been given by other owners and veterinarians.
A veterinarian who had injected 600 animals, among them a herd of a large farm, 18 months previously, expressed the belief that the injection had produced in no single case an unusually rapid or vicious course of tuberculosis. In spite of a demand made months ago, I have received thus far no report from any veterinarian of an undesirable result.
On a large farm, on which before the injection tuberculosis had appeared in a vicious form, the owner had the impression that the severe cases had afterwards become more numerous. He had, however, not suffered severe losses, and 8 months later the large reacting division by no means made a bad impression. Finally, it is to be noticed that tuberculin has been employed on a large scale in Denmark for years, and still the demand from farmers constantly increases. This could certainly not be the case if the injections were generally followed by bad results.
Paige said, after the tests of the herd of the Ma.s.sachusetts Agricultural College, that "its use is not followed by any ill effects of a serious or permanent nature."
Lamson, of the New Hampshire College Agricultural Experiment Station, said: "There is abundant testimony that its use is not in any way injurious to a healthy animal."
Conn, who made a special study of the present att.i.tude of European science toward tuberculosis in cattle, reached the following conclusions:
It has been, from the first, thought by some that the use of tuberculin produces a direct injury upon the inoculated animals. This, however, is undoubtedly a mistake, and there is no longer any belief anywhere on the part of scientists that the injury thus produced is worthy of note. In the first place, the idea that it may produce the disease in a perfectly healthy animal by the inoculation is absolutely fallacious. The tuberculin does not contain the tubercle bacillus, and it is absolutely certain that it is impossible to produce a case of tuberculosis in an animal unless the tubercle bacilli are present. The use of tuberculin, therefore, certainly can never produce the disease in the inoculated animal.
It has been more widely believed, however, that the inoculation of an animal with this material has a tendency to stimulate an incipient case of tuberculosis. It has been thought that an animal with a very slight case of the disease may, after inoculation, show a very rapid extension of this disease and be speedily brought to a condition where it is beyond any use. The reasons given for this have been the apparent activity of the tuberculosis infection in animals that have been slaughtered shortly after inoculation. This has been claimed, not only by agriculturists who have not understood the subject well, but also by veterinarians and bacteriologists. But here, too, we must recognize that the claim has been disproved, and that there is now a practical unanimity of opinion on the part of all who are best calculated to judge that such an injurious effect does not occur. Even those who have been most p.r.o.nounced in the claim that there is injury thus resulting from tuberculin have, little by little, modified their claim, until at the present time they say either that the injury which they formerly claimed does not occur or that the stimulus of the disease is so slight that it should be absolutely neglected in view of the great value which may arise from the use of tuberculin. Apart from two or three who hold this very moderate opinion, all bacteriologists and veterinarians unite in agreeing that there is no evidence for believing that any injury results. In Denmark, especially, many hundreds of thousands of animals have been inoculated, and the veterinarians say there is absolutely no reason in all their experience for believing that the tuberculin inoculation is followed by any injurious results.
In 1898 tuberculosis was found in the large Shorthorn herd belonging to W.
C. Edwards, of Canada, who with commendable promptness and public spirit had his animals tested, and at once proceeded to separate the diseased from the healthy animals. They were all finely bred animals, and of the very cla.s.s which we have been told are most susceptible to the injurious effects of tuberculin. After using this test regularly for two years, Mr. Edwards wrote as follows:
I have seen nothing to lead me to believe that the tuberculin test had any injurious influence on the course of the disease. It is by no means our opinion that the disease has been stimulated or aggravated by the application of the tuberculin test. All animals that we have tested two or three times continue as hale and hearty as they were previously, and not one animal in our herds has broken down or failed in any way since we began testing.
Mr. Edwards, in December, 1901, verbally stated that his views as to the harmlessness of tuberculin remained unchanged, and that he had not seen the least ill effect in any of his cattle from its use.
Those who have had most experience with tuberculin have failed to observe any injurious effects following its use upon healthy cattle. With tuberculous cattle it produces a fever of short duration, and in the great majority of cases all derangement of the system which it causes disappears within 48 hours after the tuberculin is administered. There appear to have been a very few cases in which the disease was aggravated, and a greater number in which it was benefited by the injection of tuberculin. The cases of abortion following the tuberculin test have not been numerous, even when cows were tested within a few weeks of the normal time of calving. The few cases of this kind which have occurred may be explained by the fact that abortion in cattle is a very common occurrence, and that it would inevitably happen sometimes after the tuberculin test as a mere coincidence and without any relation between the test and the loss of the calf. The cases of abortion which have been cited appear to be no more numerous than might be expected to have occurred among the same number of cattle within the same period if the test had not been applied.
At the present time there is ample evidence to show that tuberculin is the most reliable means of detecting tuberculosis in the living animal and that its use is not attended by any harmful aftereffects.
An act of Congress was approved July 24, 1919, for the purpose of controlling and eradicating tuberculosis of animals. The official means of detecting tuberculosis in the living animal is the tuberculin test, which may be applied by three different methods--the subcutaneous, the intradermic, and the ophthalmic. It is not necessary to discuss here the details of these three methods, which are made use of in the work of eradication of tuberculosis.
The plan adopted by the State and Federal authorities in eradication of the disease is known as "The Accredited-Herd Plan." Under this plan herds are tested under State and Federal supervision, the diseased animals are appraised, removed, and slaughtered under Federal inspection. Retests are then made after definite periods of time until two successive tests show all the animals to be free from the disease. At this time the herd owner is given a certificate of an accredited herd.
Details concerning the accredited-herd plan may be obtained by applying to the Chief of the Bureau of Animal Industry, Washington, D. C.
THE TUBERCULIN TESTS.
Testing animals with tuberculin is the process of introducing tuberculin into the animal and interpreting results according to well-known standards.
From the investigations and observations that have been mentioned, it may be safely concluded--
1. That the tuberculin test is a wonderfully accurate method of determining whether an animal is affected with tuberculosis.
2. That by its use the animals diseased with tuberculosis may be detected and removed from the herd, thereby eradicating the disease.
3. That it has no injurious effect upon healthy cattle.
4. That the comparatively small number of cattle which have aborted, suffered in health, or fallen off in condition after the test were either diseased before it was made or were affected by some cause other than the tuberculin.
THE SUBCUTANEOUS TEST (UNDER THE SKIN).