Sensitivity to avian and human Old Tuberculin in man in Great Britain

Abstract

A total of 1,119 men in the Royal Air Force, nearly all aged 18–20 years, was tested intracutaneously and simultaneously with human and avian Old Tuberculin. The tuberculins were standardized by bio-assay in two groups of guinea-pigs, sensitized with human and avian tubercle bacilli respectively, in order to determine the relative doses of the two preparations which would give the best practical differentiation between the two types of sensitization in simultaneous intracutaneous tests; by coincidence, the relative doses so determined were very nearly equal, and it was therefore decided to use the same dilutions of the two preparations. The young men examined included 605 not previously skin-tested and not given BCG vaccine,, and 354 who had received BCG vaccination, nearly all about five years previously. For the initial tests, a small dose (0·1 ml. of 1/3000 dilution) of each tuberculin was used. In those men who reacted to both small doses, the reactions to human tuberculin tended to be larger than those to avian, tuberculin both in the non-BCG group (a mean excess of 2·5 mm. induration) and in the BCG group (a mean excess of 1·3 mm.). Those who showed less than 5 mm. induration at 48 hr. to both small doses were tested with a large dose (0·1 ml. of 1/100 dilution) of each tuberculin. In the non-BCG group there was a tendency, in those reacting to both these large doses, for the reactions to avian tuberculin to be larger than those to human tuberculin; and there were also many more reactions to avian tuberculin only than to human tuberculin only. There was, therefore, a ‘cross-over’ in this group in passing from the small to the large doses. This cross-over was not shown in the BCG group, the relative reaction size (i.e. the mean difference between the human and avian reactions) with the large doses being similar to that with the small doses. These findings were independent of the tester-reader, the sites of the tests on the forearm, and the place of past residence of the participant. A total of 182 patients with active, bacteriologically positive pulmonary tuberculosis, of whom 28 were aged 15–24 years, was tested similarly with the same two tuberculins. The average excess of the human over the avian reactions with the small doses was substantially greater (5·8 mm. induration at 15–24 years) than in either the non-BCG or the BCG group. Nearly all showed 5 mm. or more induration to the 1/3000 dilutions, so that the possibility of a cross-over in passing from small to large doses was not examined. The cross-over in the non-BCG group and its absence in the BCG group (all of whom were known to have had an artificial infection with mammalian tubercle bacilli), suggest that the reactions to the large dose of human tuberculin in the non-BCG group were largely caused by organisms other than mammalian tubercle bacilli, antigenically related to the avian tubercle bacillus. However, the above-mentioned differences in relative reaction size with the small doses of human and avian tuberculin between the patients (all with active disease), the non-BCG group (infected at some time in the past 18–20 years), and the BCG group (all infected about five years previously), suggest that the relative reaction size may be influenced by the interval since infection or by its current activity; this could provide an alternative explanation for the cross-over in the non-BCG group. The prevalence of natural tuberculin sensitivity at ages 18–20 in Britain in 1961 can be judged from the findings with human Old Tuberculin in the non-BCG group, and is compared with findings of similar tests in 1954–56. Between these two dates there was a decrease in the proportion positive (5 mm. induration or more) to 1/3000 OT, from 56% to 29 %, and an increase in the proportion positive only to 1/100 OT, from 13 % to 25 %.