Comparative intradermal tests between concentrated P.P.D. and diluted P.P.D. tuberculins in relation to non-specific tuberculin reactors

1. In a herd of 760 animals which has for years been more or less regularly tuberculin-tested and in which for years no case of tuberculosis was found quite a large number of animals showed sensitization at an annual test, when using 3 mgm. per c.c. P.P.D. Onderstepoort tuberculin. When testing half strength against full strength tuberculin in this herd, the number of reactors with the half strength were statistically significantly smaller than those with the full strength tuberculin. 2. In testing full strength against half strength tuberculin on a positive herd of 62 animals, the results were: - 39 positives were found with 3 mgm. and 31 with 1•5 mgm. 10 retests were found with 3 mgm. and 16 with 1•5 mgm. 13 negatives were found with 3 mgm. and 15 with 1•5 mgm. If one can make a statistical analysis of only 62 animals, then the difference in the number of positives (39 and 31) even though some of these are false positives, is not highly significant but nevertheless significant. However, when one considers the total number of reactors (positives and retests) to full strength and half strength which are 49 and 47 respectively, the difference is statistically insignificant. Since some of these so-called positives were believed to be actually free from the disease and others in a state of generalization, this herd was not considered to be very suitable for testing out the half strength against the full strength tuberculin. 3. In the case of 18 animals showing what is regarded as normal sensitization to tuberculin, full strength, half strength and 20 per cent full strength tuberculin gave for all practical purposes comparable end results. The size of the reacting swelling seems to be closely related to the potency of the tuberculin used. 4. Taking the above results into consideration, it would seem that a very much less potent tuberculin than is ordinarily being used, would be suitable for the single intradermal test, with the advantage that a smaller number of animals having a tuberculin sensitization to non-specific factors, will be condemned for tuberculosis, when they are in fact false positives. 5. Owing to the small numbers of animals dealt with, one is not justified in laying down what the optimum concentration of tuberculin, for the single intradermal test should be, but a case seems nevertheless to have been made out for a critical examination of the whole question, on a sufficiently large number of animals, using at the same time also sufficient animals, infected with conditions such as skin lesions, Johne's Disease, Actinomycosis etc., showing tuberculin non-specific sensitization.