THE TUBERCULIN TEST.

Otago Witness, Issue 3853, 17 January 1928, Page 12

THE TUBERCULIN TEST.

Some information regarding the tuberculin test may prove interesting at the present time, seeing that our veterinarians are testing dairy herds and, generally, making efforts to eliminate tuberculosis.

Tuberculin (says an exchange) is a product prepared by sterilising, filtering, and concentrating the liquid upon which tubercle bacilli have been allowed to grow. “It contains the cooked produce of the growth of the tubercle bacilli, but not the bacilli themselves. The injection of tuberculin has no ill effects upon the animal, but where the animal .is tubercular there are certain reactions that make it possible to determine that the animal does have tuberculosis. In making the interadermal or tail test a few drops of a special kind of tuberculin is injected into the skin of one of the folds beneath the tail. The tuberculin makes a little nodule or swelling at tlie point of the injection, but this soon disappears. A positive reaction is indicated by a swelling which is evident at the seventy-second hour after the injection, or, in a few eases, at a later hour, as the 96th or 120th. A swelling which may be noted at the twenty-fourth or thirty-sixth hour, and which soon disappears, is not classified as a specific reaction. The nature and size of the swelling must be considered in interpreting the test. Practice with an experienced operator is essential for efficient work. It is extremely difficult to define understandingly in words just how the tests should be interpreted.” The United States Bureau of Animal Industry gives instructions as to how the interadermal test should be read. Suffice it to say’ here that it takes an expert to make correct deductions from the swellings, which vary in size and character according to the degree of infection. The tuberculin test will not detect all animals that have the disease, and occasionally’ a health; 7 aitimal may show a typical reaction. This latter error is the least important, for there is not more than 1 .or 2 per cent, of all the animals that have been condemned that fail on careful post-mortem examination to actually show that they have tuberculosis. The most serious limitation of the tuberculin test is that it will not always discover every animal in the herd that has tuberculosis. It will occasionally 7, according to an authority, “ fail to show a reaction in those animals in which the disease is very’ far advanced, but such animals usually show physical signs of the disease that are apparent to the well informed. An animal ordinarily will not react to the test when she has been recently infected with the disease. It takes some time for the disease to gain sufficient foothold in the body’ of the animal, so that it is possible to diagnose its presence by the use of the tuberculin test. This period of incubation or growth of the disease may be several months in length. It is for this r ason that it is desirable to retest an infected herd within six months after the first test has been made. It also sometimes happens that when the disease has ceased to progress in the animal it will fail to ’■eact.”

Even if a cow has tuberculosis she does not necessarily expel the tuberculosis germ in her milk. It is probable that those cases in which tuberculosis is located in the udder are the more dangerous. Still the germs may get into the milk from a .number of sources it there are affected animals in the herd, and it is as well to know them. Tuberculosis of the udder may not be frequent, but there are other sources of contamination, and in any case if it is possible to detect the cow* or cows which are affected surely it is a wise precaution to do so. The only sure measure is to condemn all cows that react to the tuberculin test.