most notably Afro-Americans and the residents of the urban core of the larger cities. Subsequently, the impact has been demonstrated in a number of countries of sub-Saharan Africa. In many countries of Africa, the notification rate of active tuberculosis has been increasing steadily since the beginning of the HIV epidemic [55].
It is possible to identify regions in Africa with an early introduction of HIV and a subsequent high level of the problem and to compare them with regions in which HIV entered later and has not yet reached the same high levels. HIV infection affects groups within any community that are at high risk for the disease and, once these groups become 'saturated' with the infection, the rise in prevalence of cases may stabilize. If this is true, it might be possible that the HIV epidemic, like tuberculosis before it, will reach a peak and then either stabilize or decline following large-scale fatality among the high-risk groups in the community [56]. Infection with both tuberculosis and HIV occurs in young people (between the ages of 15 and 44 years), so that, where tuberculous infection is still common (as in tropical Africa), the two infections exactly coincide and the increase in tuberculosis cases due to HIV is seen in this age range. Where tuberculosu infection is now very rare (as in Europe and North America), there is very little overlap between the two infections. Because the mechanism by which HIV first affects the tuberculosis problem in the community is by increasing the likelihood of development of tuberculosis after infection with M. tuberculosis, the net impact within different countries may vary considerably. In order to plan for the control of tuberculosis, it is mandatory to determine the level and trend of both HIV and tuberculous infection, and their coincidence, in the community.
Tuberculosis is an early form of opportunistic infection accompanying the immuno-suppression resulting from HIV infection [57], as many of the cases of tuberculosis occurring in patients who are diagnosed with AIDS occur prior to any other opportunistic infection. The survival of tuberculosis patients, during the course of treatment, is much better among those who are HIV-negative (about 95%) than among those who are HIV-positive (about 67%). Follow-up of patients in Tanzania who were HIV-positive indicates that 18 months after the beginning of treatment, only 50% remain alive. Of those who survive, however, the response to treatment is not different either between areas with high and low levels of HIV infection, or over time in the regions with a high level of HIV infection. Treatment of infectious cases is as effective in rendering them non-infectious, regardless of HIV infection, and is the most important activity in containing the transmission of tuberculosis caused by the increased number of cases in the community due to HIV infection. Patients who have HIV infection, however, may not tolerate medications as well as those without HIV [58].
It is not possible to know what effect HIV infection will have on tuberculosis in the community. It may have a progressive effect (especially in countries without good treatment programmes) in increasing the number of cases and therefore the transmission of tuberculous infection. On the other hand, it is possible that HIV infection will primarily affect high-risk groups, destroying these groups and then stabilizing within the community, with major impact on the tuberculosis situation only in those high-risk groups in the community. The only effective means of containing the long-term effects of HIV on the tuberculosis situation (the increase in transmission of tuberculous infection) is by means of efficient and extensive programmes of tuberculosis control.
2.5 THE ELIMINATION OF TUBERCULOSIS
There are some very good reasons to support the notion that tuberculosis can be eliminated. The most important reason is the inefficiency of tuberculosis as an infectious disease. Tuberculosis is spread not from an environmental source but from one person (an individual who is sick) to another. Moreover, it is possible to detect such a source by means of sputum microscopy and rapidly to render the patient non-infectious through chemotherapy. Thus it should be possible continuously to reduce the problem of tuberculosis if control measures are applied. Moreover, tuberculosis control activities have what has been termed a 'ratchet' effect, that is, any reduction in the level of the problem, under normal circumstances, can be sustained for some time, even if those activities cease or are interrupted. This is in striking contrast to the situation with such diseases as polio, malaria and tetanus. Theses arguments make it reasonable to discuss the possibility of elimination of tuberculosis.
On the other hand, with the tools currently available, the elimination of tuberculosis will take place only slowly. This is because the disease, once transmission of tuberculosis infection has ceased, continues to arise for the remainder of the lives of infected individuals (although at an ever-decreasing rate). When transmission is disrupted, there is a 'step-down', a point at which the succeeding birth cohort is very much less likely to be infected. In spite of the cessation of transmission, cases continue to occur unless some from of secondary prevention is applied.
It seems theoretically possible, using the tools that are currently available, to increase the rate of decline of tuberculosis over the 10% seen after the extensive application of effective treatment programmes. This has been demonstrated in the Inuit of the Northwest Territories of Canada among whom the rate was increased to 20% in the 1970s, possibly as a result of extensive use of preventive chemotherapy. The advent of HIV infection threatens this possibility. However, the basis of the fight against tuberculosis (diagnosis and cure of infectious cases) is unchanged. Surprisingly, even in the presence of extensive HIV infection, tuberculosis
in Africa today is orders of magnitude less common than it was among the Inuit 30 years ago or even in Europe 70 years ago (Fig. 2.6). Some of the negative thinking concerning the value and effectiveness of contributions to the fight against tuberculosis in Africa [59] (and other developing nations) is clearly not supportable. Nevertheless, the routine application of the types of interventions currently available are fraught with difficulties. It is very hard, among the large number of people infected with tuberculosis, to predict who is at risk of developing disease. The current method of preventive therapy has major limitations, chief of which is the duration of therapy required. Thus, while it is necessary to continue to apply the current tools available, it is important to further refine these tools to improve their administration and performance. With such improvements, it should be possible to hasten the elimination of this disease.
REFERENCE
1. Kochi, A. (1991) The global tuberculosis situation and the new control strategy of the World Health Organization. Tubercle, 72; 1-6.
