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APPENDIX 12.
An example of rules drawn up for the management of a herd in which an attempt is being made to eradicate bovine tuberculosis. Be sure that the tuberculin used is of assured potency and purity. Do not test the same cattle at shorter intervals than six months, except for a very special reason.
Avoid testing with Johnin, or vaccinating against epizootic abortion, within two months of doing the next test with tuberculin.
If using the subcutaneous method, guard against the cattle being frightened or excited prior to or during the test, in order to avoid getting a misleading rise in temperature.
When using the double intradermal test, inform the veterinary surgeon that your society insists on his using the following technique : ---
The test is carried out in two stages:-
1. A preliminary or sensitising injection.
2. The test dose or diagnostic injection.
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or
The tuberculin employed is "old" ("concentrated "
"undiluted "') tuberculin of known potency, or the new synthetic" tuberculin prepared at the Institute of Animal Pathology, Cambridge, and not the dilute tuberculin employed for the subcutaneous test.
Technique.
An area of from 2 to 6 square inches is prepared about the centre of the side of the neck by shaving or clipping. When many cows are being tested it is convenient to use the left side of one and the right of the other. A fold of skin is taken between the forefinger and thumb of the left hand, and a short strong steel dental needle, not less than 1 cm. in length, is introduced into the fold. It is desirable to insert the needle obliquely, so that the point reaches the deeper layers of the derma. A syringe of 10 cc. capacity, charged with tuberculin, is then attached to the needle, and 0-1 cc. of the tuberculin is injected. Considerable pressure is usually necessary, and if the injection is made easily it should be suspected that the needle has penetrated to the subcutaneous tissue. When the injection has been made correctly, the tuberculin is felt as a small pea-like enlargement when the fingers are passed lightly over the site. Failure to detect such an enlargement suggests that the tuberculin has been injected below the skin, and a second injection should be given in a fresh site. It is always advisable to ascertain that the syringe and needle are working efficiently. This may readily be done by charging the syringe with liquid, thrusting the needle into a piece of rubber and then forcibly compressing the piston. No fluid should escape either past the plunger or at the junction of the needle with the syringe.
The second or test dose is given 48 hours after the first or sensitising dose, and also consists of 0-1 cc. of tuberculin. It should be injected into exactly the same spot as the first or sensitising dose, that is, into about the centre of the swelling produced by that dose.
Observations and measurements.
After preparing the area, and before the first injection, the fold of skin is grasped between the forefinger and thumb of the left hand, and the thickness of the fold is measured by means of callipers. This measurement varies from 3 to 6 mm. in calves, from 4 to 10 mm. in cows and from 11 to 16 or more in bulls.
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After taking the preliminary measurement, the sensitising dose is injected.
Forty-eight hours later, the fold is again measured at the site of injection. In some tuberculous animals a definite reaction is obtained at this stage, so that a further dose is unnecessary, but, if the swelling is circumscribed, the second injection must be given.
Twenty-four hours after this second or test dose has been injected, the area is again examined and measured. Non-tuberculous animals usually show little increase in thickness of the area. The actual site of injection remains as a sharply circumscribed pea or bean-like area devoid of diffuse- ness, heat or tenderness. In tuberculous animals the swellings exhibit varying degrees of diffuseness which may be accompanied by heat and tenderness.
This examination is the most important of all, as at this time a definite conclusion can be reached. The necessity for the second injection is now apparent, for it is frequently found that a certain proportion of animals which failed to react to the sensitising dose are now definitely positive. Particular attention should be paid to the character of the swelling, and the site of the injection should be carefully palpated in order to detect slight degrees of diffuseness which constitute a positive reaction. While the difference in the measurements of a small positive reaction and a non- reactor may be trifling, the character of the swelling is of the utmost importance. In tuberculous animals, the site, whatever the measurement may be, is ill-defined, soft or
to the touch. fluffy
"
In the case of the Graded Herds, under the scheme of the Ministry of Health, it is necessary to record the measurement at each visit. It cannot be too clearly pointed out, however, that palpation of the area is of far greater diagnostic value than are measurements, and for ordinary purposes measurements may be omitted.
Summary of technique.
1-(a) Preliminary measurement of prepared fold of skin.
(b) Injection into the derma of 0-1 cc. concentrated tuberculin. Forty-eight hours later:
2. (a) Measurement of site of injection, palpation and observation of the
area.
(b) Injection of second dose of 0-1 cc. in the same spot as the first. Twenty-four hours later:-
3. Final measurements and observations of the site of inoculation. Abbreviations employed in recording the results; --
Clean land.
diffuse.
slightly diffuse.
D S.D
H
= heat.
T
= tenderness.
N.S.I no second injection.
Rreactor.
N
non-reactor.
Before beginning to segregate "clean" from infected cattle, insure that you have sufficient uninfected pasturage on to which to turn the "clean" cattle. Tubercle can remain infective on grass and in dung and soil for some time, how long is not yet known for certain. Probably safety can be assured by keeping all cattle off the land for three months in summer or six months in winter and, in the meantime, well harrowing the land, in order to break up the dung and expose all tubercle to the sun and air, and feeding off the grass closely with sheep. If possible, lime with unslaked lime. It is advisable to keep poultry off the land.
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