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To quantify the magnitude of TB in any given area, a functional surveillance system for newly diagnosed cases must be in place. The public health interest in reporting cases was often met with fierce opposition from physicians who were reluctant to disclose to the government who among their patients had TB [31], an issue that persists in certain countries even today [32, 33]. At the British Congress on Tuberculosis in London in 1901, Koch made a strong plea for mandatory notification of TB [34]; in his presentation he referred to the success of notification, first voluntary, then mandatory in New York City. Beginning in 1899, Biggs of the New York City Health Department proposed a systematic approach to the control of TB [35]. The first and most important element was mandatory notification of TB. In 1907, Biggs defined the strategy more precisely in what he called “the administrative control of TB” and presented data about TB mortality in New York City [31]. There were no morbidity data in his report, but he proposed practical ways of approaching morbidity surveillance. One reason why it was easier to obtain information on death than disease was the attitude of practicing clinicians who suspected hardship for their patients when disclosing their condition to the authorities, while after death this obstacle was obviated. Interestingly, Biggs recounts a talk with Koch who told him that in Germany a notification system similar to the one implemented in New York City could only be possible when the current generation of professionals now in control would have passed away. Elsewhere in Germany, however, Saxony claims to have been the first to introduce legislation mandating notification of TB cases in 1900 [36], while Prussia prepared legislation for mandatory notification of TB cases considered to be transmissible (defined as pulmonary or laryngeal) in 1903 [37]. In 1901, Norway became another of the early countries in Europe to introduce national mandatory notification of TB cases [38, 39]. In Denmark, legislation for mandatory notification of transmissible cases came into parliament in 1904 [40, 41]. Other European countries and jurisdictions followed suit in the first decade of the 20th century, but it remained voluntary for many years in some administrations [42].

Fig. 4. Notification of tuberculosis in 2 hospitals in London by medical specialty in the period 1985–1989 compared to 1992–1993. Data reproduced from [49], with permission from BMJ Publishing Group Ltd.

Reliable notification of TB cases remained difficult to implement. In the United States, uniform national reporting was introduced only in 1953 [43]. A 1993 analysis of case notification systems in 14 Western European countries showed the dismal state of surveillance, in some as recently as between 1974 and 1991 [44]. As a disease with a slowly changing epidemiology, it is contrary to its nature to exhibit large annual changes at the country level. Changes of more than 10% in either direction from 1 year to another are epidemiologically decidedly unusual, and more likely reflect inconsistencies, inaccuracies, and blatant errors in the surveillance system. While in most of the examined 14 countries, the amplitude was constrained to realistic values, it was definitively not so in some others. Of course, even where there is consistency (smaller year-to-year amplitudes) in reporting, this view of the data cannot reveal the extent of underreporting. Nevertheless, it underlines the fact that where there is not at least a minimum consistency and instead huge year-to-year amplitudes are reported, the data are not trustworthy. We note that these data are not from the times of war but in a peaceful period when Western Europe was moving to ever-greater prosperity. Subsequent to this rather embarrassing assessment, a consensus was reached in Europe to base its TB case surveillance system on a uniform and more rigorous base by taking recourse at its core to mandatory notification, not just by physicians but notably as well by laboratories that isolate and identify M. tuberculosis complex [45]. In 1996, a Europe-wide “EuroTB” program for the surveillance of TB was set up to collect, analyze, and disseminate data on TB cases notified in the World Health Organization (WHO) European region, which began to produce annual reports from 1998 onwards [46]. The project was funded by the European Union through 1997, and the last annual report appeared in 2008 [47]. Subsequently, the responsibility was transferred to the European Centre for Disease Prevention and Control and the WHO Regional Office for Europe [48].

The impediment to early attempts to implement surveillance resulting from the reluctance of physicians to report cases of TB among their patients has already been mentioned. That such reluctance was nevertheless not necessarily the only or even main problem in case notifications are exemplified in a study from 2 London hospitals (Fig. 4) [49]. The same 2 hospitals were visited in 2 periods and their records examined for newly diagnosed TB cases, and the results were then compared to the actually notified cases. Chest physicians reported only four of five diagnosed cases, surgeons only 3 of 5, and other specialties only half of the known cases. Reluctance to report is likely to have been a minor issue in hospitals in this period in the United Kingdom. More likely is forgetfulness or preoccupation with care of patients rather than administrative public health tasks. That this is actually the case is suggested by the improvement in notifications when the same hospitals were visited again a few years later. Apparently, the first visit had sensitized the hospital staff about the necessity to report. Based on these considerations, the European recommendations for surveillance emphasized a certain need for both physicians and microbiologic laboratories to notify bacteriologically confirmed cases of TB [45]: there are fewer laboratories than physicians and for them the administrative procedures for reporting can easily be automated.

Most countries in the world probably have at least a mandatory legal notification system of cases. Nevertheless, it is quite remarkable that India – the highest burden country in the world – legally mandated notification of TB only as recently as May 2012. This long overdue edict resulted in about a 30% increase of notified cases from 2013 to 2014 [50], which rose to 34% from 2013 to 2015 [51].

Thus, in the last quarter of the 20th century, even in prosperous Western Europe, surveillance of TB cases was in a deplorable state in some countries and has only slowly been improving. Globally, some of the highest burden countries have started only recently to address issues with their sometimes inefficient surveillance systems [52, 53]. For these reasons, one must be prepared to have a critical mind when judging morbidity data from countries at earlier times and when their situation was in turmoil due to war, such as often prevailed during WWII.

So far, this chapter has addressed only numerator data, that is, actual TB case counts. To allow comparison across populations or over time, the magnitude of a problem is commonly expressed in rates; in other words, case counts are divided by the population from which the cases arose and by the observation time, usually 1 year of surveillance. Population data are obtained by a census that is repeated ever so often and then interpolated for intercensal years. Demography has a long history, and methods have been developed on whom to count and how to avoid losing targeted people in the count, which is a non-trivial task. The problem is compounded when jurisdictions change through political decisions, but may also be heavily affected by population movements. In wartime, movements are introduced through mobilization of population segments, most notably young men, into the military, imposing challenges on how and where to count such people. This might be relatively easily accomplished during a census, but the difficulties for health departments can be substantial between census years. It must be feasible to define the jurisdiction in which the cases occur and the correct population count for that jurisdiction, else correct rates cannot be calculated. This may require requesting information from one of more other authorities which may or may not be willing to share population information.

As an example, a fairly complex picture emerges for the population of Berlin from 1910 through 1946, as shown in Figure 5. Between 1910 and 1920, there were 4 censuses, an unusually high frequency for taking a census. The extrapolations from one census to the next have thus high credibility. The first complexity starts after the census in October 1919 and the next census taken in June 1925. On October 1, 1920, the population size doubled when the surface of the newly designated city of Greater Berlin increased more than tenfold. This raises the question of how one might appropriately calculate a rate for the year 1920 when both the numerator and denominator changed during the year in a complex manner. Even if both the numerator (cases) and denominator (population) were correct, the appropriate technical approach is not easy to decide upon. The population characteristics of the old and new parts of the city are likely to be substantially different, probably more urban versus relatively less rural, judging from the population density (i.e., the same population size in less than a tenth of the surface in the old compared to more than 90% of the greatly expanded new city surface). The changed city definition was also likely to affect TB case rates, which often disproportionally affected urban dwellings. Thus, even if all numbers and calculations were correct, a decrease in the rates in the same city might result from a calculation from 1 year to the next, even if the problems remain unchanged. During the entire period of WWII, no census was taken in Berlin. Whether the population really increased during the war years, as graphically suggested here, and then actually dropped by more than one third at some time in 1945 before the August 1945 census may be anyone’s best guess. While there are good reasons to construct the estimates in this way, they remain estimates rather than measures. If the postulated mass exodus that led to this drop occurred, then how would the TB case rate have been determined; in other words, how would the numerator data (case numbers) have been obtained? The entire war period for Berlin poses almost insurmountable problems in obtaining both credible numerator and denominator data. While in other parts of Germany the changes might have been less abrupt than for the capital, a correct enumeration of both the incident TB cases and the population from which they arose likely created substantial problems throughout most of Germany. A large proportion of the male population was in the military service, and the military is often assumed or known to have a tendency to be rather discreet about the magnitude of the TB problem among its rank and file [54].

Fig. 5. Population data for Berlin, 1910–1946, obtained from census data (indicated by hollow circles) and extrapolated for intercensal years. The dashed line shows the political decision to expand the city of Berlin to “Greater Berlin” in 1920 [81].

Heaf reports on pulmonary TB cases that were “notified or otherwise known to local health authorities” for England and Wales from 1938 to 1941 [55]; he showed a decrease of 6.1% from 1938 to 1939 that was followed by an increase of 4.9% the following year, and of 8.7% from 1940 to 1941, which coincided with attacks by Germany towards the end of June 1940 and onwards. Otherwise reported morbidity data from the United Kingdom remain scarce in Tubercle, its main easily accessible publication. Heaf himself does not put much trust in notifications of TB cases: “until the incidence is found out by a comprehensive survey in various groups of the community, we can only rely on mortality figures as a guide to the rise or fall of the incidence in the general population” [55].

Morbidity data are scarce during much of WWII, but these considerations about the calculation of rates also apply to mortality. This will be addressed in more detail in the next chapter on ascertainment of TB deaths.