Fig. 2.3 Relation of extrapulmonary tuberculosis to birthplace and ethnic group for cases reported in Canada from 1970 to 1981 (groups Canada, Aboriginal, Europe, Asia) and in Malawi in 1989.
a majority of cases the chest radiograph demonstrates either current or previous disease. The disease presents with non-specific fistula or abscess formation. Mesenteric lymphadenitis usually occurs in young people.
Tuberculosis of the genitourinary system is a late form of the disease, occurring many years after primary infection. For this reason, it preferentially affects individuals after mid life. For reasons that are not clear, it has a predilection for Europeans (Fig. 2.3). It is the most frequent cause of infertility in many developing countries.
Tuberculosis can affect virtually any organ in the body and may present in a large variety of ways; pericarditis, adrenal gland tuberculosis (which may cause Addison's disease), iridocyclitis, mastoid sinus involvement, a space-occupying lesion in the brain, soft-tissue abscess (especially of the breast), and other sites.
2.2.2 IN THE COMMUNITY
When the risk of tuberculous infection is relatively high, the likelihood of first becoming infected is greatest in childhood (before the age of 20 years), and the majority of the population has been infected by that age [11]. Even when the risk is lower and declining, it is likely that the greatest risk of infection is in childhood [16]. If the highest likelihood of developing disease is within the first 5 years after infection, the rate of disease would be greatest between the ages of 20 and 25 years, an observed fact when tuberculosis is common [17-23]. The reason for the higher rates observed in young women, as compared with young men, where tuberculosis is common, is unknown. Where tuberculosis rates are low, if the incidence of disease is calculated only for those who are infected (rather than for the whole community), the peak rates at age 25 in young women are once again observed [24].
A conceptual model of the course of tuberculosis in a human population has been proposed [25 26]. This model indicates that, after introduction of the disease into a group previously without experience of the disease, the incidence of active tuberculosis in the population rapidly rises to a peak. As an increasing proportion of the community becomes infected and their risk of developing disease declines with time, the incidence of disease declines and may stabilize or continue a steady decline [27]. The peak only rarely exceeds 1% [28] and does so only where other factors such as tremendous overcrowding, lack of nutrition or reduced immunity, occur. The rate of decline before the introduction of specific treatment was approximately 4% per annum. With the introduction of chemotherapy, the incidence of active tuberculosis declined much more rapidly (the average annual decline in many industrialized countries has been, until recently, approximately 10%). Recently, however, the rate of decline has either been reduced, or even reversed; the reasons for this development are predictable [25, 29].
When tuberculosis is very common, most cases result from recent infection and the disease is most common in young people (especially women). Rates are very high (greater than 100 par 100 000 par annum) and high-risk groups are absent. When tuberculosis is uncommon in the community, very few young people become infected and most cases result from remote infection, the disease affecting predominantly old men. Tuberculosis cases arise from high-risk groups whose risk is determined mainly by the likelihood of having been infected in the past.
The rise in rates immediately following the introduction of tuberculosis into the community, hypothesized by this model, has rarely been observed in human populations. In one example [28], mortality in aboriginal Indians in Canada rose from 1% per annum in 1881 to 9% per annum in 1886 and only slowly declined again to 1% per annum by 1900. Studies of Sudanese recruits to the Egyptian army [30],and Senegalese recruits to the French army in the early 1900s [31], illustrate a similar phenomenon. Young men, who had come from a low prevalence area and who had previously been uninfected, upon moving into a high prevalence area, were exposed to infectious cases, rapidly developed fulminant disease and died at a much higher rate than Europeans.
Detailed information on tuberculosis at the height of its occurrence in the community has been obtained for Inuit (Eskimous) in Canada, Greenland and Alaska [22, 23], precisely documented since 1962. In the Northwest Territories of Canada, resident nurses tested and recorded the status of every inhabitant, using the tuberculin skin test, a chest radiograph and bacteriology laboratory. Tuberculosis cases were promptly evacuated to hospitals for prolonged residential treatment. In the early 1960s the incidence of active tuberculosis in the entire community was between 1 and 2% per annum, highest among young people (particularly women); by the 1980s highest rates were much lower and in old men. Between 1964 and 1984, BCG vaccination coverage had increased from less than 50% to over 80% of all children. At the same time the prevalence of previous truberculosis (fibrotic lesions) in adults had increased from 35% to 42%. During this period, there was an aggressive programme of preventive therapy, which often consisted of 12-18 months of fully supervised, two-drug therapy. By 1984, 43% of those with fibrotic lesions and 34% of those without fibrotic lesions, but who had been infected with tuberculosis, had been treated.
In 1970-1972, the greatest number of cases and highest incidence rate of active tuberculosis was in the group with fibrotic lesions. Lower rates were seen in those who were previously uninfected and in those who had been vaccinated. By 1980-1982, incidence rates in all groups had fallen but the decline was most dramatic in the group with fibrotic lesions; no cases in this group occurred among those who had been previously given preventive chemotherapy.
The level of tuberculosis is very low in some areas. For example, the notification rate of active tuberculosis in Alberta, Canada in 1989, among non-aboriginal Canadian-born persons, was between 1 and 2 per 100 000 with a rate of smear-positive pulmonary tuberculosis near 0.5 per 100 000. Although the transmission of tuberculous infection has virtually ceased, the disease will remain in the community for the lifetime of the last heavily infected cohort and will disappear only when this cohort has completed its life span.
When tuberculosis incidence declines, high-risk groups emerge in the community. Theses groups, with rates of active tuberculosis at least ten times higher than the national average (with actual rates exceeding 100 per 100 000 per year) have been investigated in the USA [32] and in Canada [33]. In Canada, they included contacts of active cases, with a rate ratio of 62; previous case, both treated and untreated, 38; silicosis patients, 39; residents of urban slums, 20; Asian-born
