Epidemiological Basis of Tuberculosis Eradication in an Advanced Country

Bull. Org. mond. Sante 1959, 21, 549 Bull. Wid Hith Org. Epidemiological Basis of Tuberculosis Eradication in an Advanced Country E. GROTH-PETERSEN, J0RGEN KNUDSEN & ERIK WILBEK' The first section of the report provides a background for the long-range epidemiological studies being conducted by the Danish Tuberculosis Index. An outline is given of the main indices of tuberculosis and the changing tuberculosis situation in Denmark during the past several decades with respect to prevalence of infection, morbidity and mortality. Difficulties encountered in international comparisons are briefly discussed. The prevalence of tuberculous infection in children and the prevalence of bacillary cases of pulmonary tuberculosis in adults are suggested as the most valuable indices of eradication. The second section describes the nation-wide mass campaign of 1950-52, which was planned and conducted as a combined service and research programme under the direction of the Danish Tuberculosis Index. With a view to the follow-up studies, details are given ofthe results oftuberculin tests ofthe unvaccinated, the documented vaccinated and mixedgroups in the population examined. A survey is made of the results of X-ray examination and of the relation between vaccination status, tuberculin reaction and X-ray findings. The results of the firstfollow-up period are analysed in the final section of the report in terms of new cases ofpulmonary tuberculosis appearing in the adult population. Seventy- five per cent of the new cases appeared among the unvaccinated tuberculin-positive population, and among this population groups ofpersons with widely different risks ofdeveloping tuberculosis could be identified on the basis of X-ray findings and reactions to the intra- dermal 10 TU tuberculin test. A proposal is made to reduce the number ofroutine repetitive examinations for adults and to concentrate efforts on preventive measures for the high- risk groups. INTRODUCTION In view of recent achievements the time has come to review critically our present tuberculosis Until recently, effective control of tuberculosis programme and to determine how to utilize the -a major health problem of the world-could discoveries of the past decade to the greatest hardly be called promising. Prevention of the disease advantage of both the individual and the community. was more dependent upon improvement in living In countries which already have a very low tubercu- standards than upon direct medical and public losis morbidity and a low tuberculous infection-rate health procedures. In the course of the last ten years, it should now be possible to visualize complete however, the situation has changed radically with eradication of the disease. A certain reorientation the dramatic discovery and rapid development of of the existing tuberculosis programme may be potent antituberculous drugs. Now, for the first necessary and must be based on descriptive as well time, we have the means for a direct attack upon as on experimental epidemiological studies. Further- the tubercle bacilli in the human reservoir, and the more, the events taking place during the phase curative and prophylactic values of these chemo- of eradication in some countries must be followed, therapeutic agents are high. described and measured very carefully, so that the experience gained may help to guide other countries 1 Danish Tuberculosis Index, Copenhagen in a reorientation towards eradication. 807 -5- 6 DANISH TUBERCULOSIS INDEX Denmark 1 is some years ahead of most other enormous number of routine examinations of the countries in the progress towards the eradication population. In the same way, it may be possible to of tuberculous disease; the organization of the define high-risk groups on which frequent observa- tuberculosis service, and the field work, as well tion and preventive efforts must be concentrated. as the centralized laboratory service and registration, The report is divided into three sections. The offer particularly favourable facilities for carrying first discusses briefly the main indices of tuber- out the required studies; and a research project, the culosis and outlines the tuberculosis situation in Danish Tuberculosis Index, established by the Danish Denmark during the past several decades as a back- National Health Service in collaboration with ground to the work of the Danish Tuberculosis the WHO Tuberculosis Research Office, has been Index. The purpose of the second section is twofold: (1) to describe how the Danish tuberculosis mass in operation over a number of years. Among campaign of 1950-52 was planned both as a commun- the problems it has taken up for study is the possi- ity health service and in the interests of research; bility of dividing the general population, on the and (2) to set forth the findings of the campaign basis of a single or a few examinations, into groups as far as they may be of interest in the follow-up with markedly different risks of developing tuber- studies. The final section is concerned solely with culosis. The study demonstrates that even such the new cases of tuberculosis appearing over the simple examinations as tuberculin testing and subsequent four-year period in the adult population chest radiography may result in a definition of large who were deemed healthy with regard to tuberculosis low-risk groups and permit a reduction of the at the time of the campaign. THE BACKGROUND FOR THE STUDIES: TUBERCULOSIS IN DENMARK Tuberculosis morbidity and mortality rates mortality and morbidity rates and the prevalence in Denmark have been among the lowest in the of tuberculous infection (WHO Study Group on world for some decades, and the goal of eradication Tuberculosis Control, 1956). is nearer than in most other countries. Yet despite For many years tuberculosis mortality has been such a favourable situation, Danish tuberculosis the only reasonably good measure for international specialists strongly advocated a nation-wide mass comparisons and for some parts of the world it is BCG vaccination campaign; and late in 1949 plans still the only available index. However, in an in- were being made for a programme which would creasing number of countries deaths from tuber- include tuberculin testing and X-raying as well culosis are now so few, particularly in the younger as vaccination of large segments of the population. age-groups, that mortality rates are no longer It was recognized that the results of examinations satisfactory. They do not give a proper picture of the conducted carefully and with uniform procedures morbidity trends and are affected too much by would provide basic material for prospective random fluctuations. epidemiological studies. The use of morbidity statistics for international Information which would characterize the popula- comparisons is complicated by differences in the tion as regards pertinent criteria for the campaign definitfon of the term " tuberculous disease ", in the against tuberculosis and pinpoint the sources of case-finding activities and in the systems of reporting infection in the present phase of the campaign could, cases. Through the years the conception " tuber- it was believed, be of great value not only to Den- culous disease" has been constantly widened to mark but to other countries soon to enter the keep abreast of advances in diagnosis, therapy and eradication phase of tuberculosis control. public health facilities. As tuberculosis is generally a chronic disease some patients will live for many years with such slight or bearable symptoms that INDICES OF TUBERCULOSIS medical attention is not sought and the disease The indices which are used to describe the tuber- remains unrecognized. Nevertheless such chronic culosis situation in a population are tuberculosis cases are of public health significance because, inter alia, they may be periodic or constant sources of ' Total population on 1 July 1955: 4 440 000 infection; but they will only be discovered ifexamina- EPIDEMIOLOGY OF TUBERCULOSIS IN DENMARK 7 tion, including adequate bacteriological examination, tubercle bacilli. Thus, neither the prevalence nor the is required by the public health service. Morbidity incidence of tuberculous infection can be determined statistics will therefore be influenced by the propor- in the vaccinated age-groups from the results of tion of the population examined without respect to tuberculin testing surveys. symptoms. Research studies of the WHO Tuberculosis Undoubtedly the diversity in the systems of Research Office and the Danish Tuberculosis Index reporting and registration is the major difficulty in have shown that with the 5 TU or 10 TU dose of compiling comparative morbidity statistics for purified tuberculin the problem of non-specific European countries. As an example may be men- tuberculin sensitivity from sources other than BCG tioned a comparative investigation in the Netherlands is of little significance in Denmark (Danish Tuber- and Denmark made in 1955 on Dutch initiative in culosis Index, 1955c; WHO Tuberculosis Research co-operation with WHO. Although through several Office, 1955a, 1955b). And, as indicated in the decades (apart from the war years) the two countries following, the difficulties created by BCG vaccina- have had the same tuberculosis mortality and com- tion in estimating the prevalence of tuberculous parable facilities for examination, diagnosis and infection cannot have been substantial in Denmark treatment, the figures for the incidence of pulmonary until recent years. tuberculosis in the Netherlands

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