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Volume: 24S2 - June 1998 Proceedings of the National Consensus Conference on
Tuberculosis EPIDEMIOLOGY OF TUBERCULOSISGlobally, tuberculosis (TB) continues to be a major cause of disability and death. It has been estimated that one third of the world's population is infected with the TB bacillus and that the disease is responsible for 3 million deaths annually. In 1993, the World Health Organization (WHO) declared TB to be a global emergency, the first disease so classified in the history of that organization. In developing countries the impact of TB has intensified with the spread of HIV infection/AIDS, and the disease has reached epidemic proportions; in industrialized countries, changes in immigration patterns, greater rates of homelessness, reduced funding for public health programs and frequent overseas travel have led to a reversal in previously well-established declines in TB notification rates. According to the WHO figures released in 1997(1), the reported regional incidence rates of TB in 1995 ranged from 33.0 per 100,000 in the Americas to 98.5 per 100,000 in Southeast Asia. HIV infection weakens the immune system and thus makes active TB disease more likely in an individual infected with the TB bacillus. As a result of the spread of HIV, an estimated 1.4 million worldwide cases of active TB are expected to occur annually in people co-infected with HIV by the end of the century. Already, TB is the leading worldwide cause of death among individuals who are HIV positive. Of great concern to the WHO is the emergence of drug-resistant TB strains, particularly in developing countries. These strains, especially those resistant to more than one of the usual first-line drugs used to treat the disease, will pose a very serious threat if they spread rapidly around the world. Unfortunately, this phenomenon is primarily the result of incomplete or improper treatment regimens. Directly observed therapy, which consists of closely supervised treatment to make sure that every dose is taken to the end of the treatment period, is a highly recommended method of ensuring patient compliance and cure as well as preventing the development of drug resistance. The recently released report of the WHO/IUATLD (International Union Against Tuberculosis and Lung Disease) Global Project on Anti-Tuberculosis Drug Resistance Surveillance(2) highlights this growing problem (Figure 1). From 1994 to 1997, the prevalence of combined resistance (primary and acquired) to any of the commonly used anti-TB drugs among cases studied in the countries participating in this project ranged from 2.3% in the Czech Republic to 42.4% in the Dominican Republic. The prevalence of multidrug resistance (MDR-TB), defined as resistance to at least isoniazid and rifampin, ranged from 0% in Kenya to 22.1% in Latvia. The weighted mean for overall resistance among the participating countries was 16.7%, and that for MDR-TB was 4.3%. Within the global picture, TB in Canada currently has not reached epidemic proportions. Historically, TB was a major cause of morbidity and mortality early in this century. The mortality rate was 84 per 100,000 in 1924, and the reported incidence rate reached a high of 119 per 100,000 in 1946 (Figure 2). With improvements in living conditions, public health programs and the advent of antibiotic therapy, the rates of disease and resulting death have decreased sharply since the mid 1940s. The reported incidence and mortality rates in 1995 were 6.5 and 0.4 per 100,000 respectively. However, the decades-long trend of declining incidence has levelled off since 1987 (Figure 3). Certain groups in Canada are at increased risk of the disease, including Aboriginal peoples (Status Indians, Non-Status Indians, Metis, Inuit and Innu), foreign-born residents from countries with a high prevalence of the disease, disadvantaged inner city populations and HIV-infected individuals. In 1995, the reported TB incidence rate was 1.9 per 100,000 for Non-Aboriginal Canadian born individuals, whereas those for Status Indians, all Aboriginal peoples and foreign-born residents were 44.5 per 100,000, 29.4 per 100,000 and 20.4 per 100,000 respectively (Figure 4). The reported incidence rate among Status Indians on reserves has declined since 1992, while the rate for those off reserve has been increasing during the 1990s (Figure 5). Although the overall incidence rate for Aboriginal peoples has remained fairly stable over the past several years, there is wide variation in the incidence rates between the various provinces/territories (Figure 6). Over time, the proportion of reported TB cases born outside the country has increased (Figure 7). In 1980, 35% of all reported cases were foreign-born, and in 1995 the proportion had increased to 58%. Not surprisingly, most of the countries of origin reflect areas of the world with a high prevalence of the disease (Figure 8). A significant proportion of these cases are diagnosed within a few years after arrival (Figure 9). The impact of HIV infection on the epidemiology of TB in Canada has not been well ascertained, although so far it does not appear to have been significant. Groups such as Aboriginal peoples and injection drug users are particularly at risk. By the end of 1996, a total of 606 individuals reported to have AIDS also had a diagnosis of TB, representing 4.2% of the total number of AIDS cases reported. However, this figure most likely represents an underestimate and further investigation is required. With regard to drug resistance, there are limited national data to date regarding the extent of the problem. A national study conducted in 1993-94 showed that 8.7% of the TB isolates studied were resistant to at least one of the commonly used anti-TB drugs, and 0.6% were MDR-TB (Figure 10). Figure 1: Prevalence of combined drug resistance to any drug and MDR TB, 1994-1997 Figure 2: Reported Tuberculosis Incidence and Deaths in Canada, 1924-1995 Figure 3: Reported Tuberculosis in Canada, 1980-1995 Figure 4: Crude Rates of Reported Tuberculosis Cases in Canada by Population Subgroup, 1985 Figure 5: Tuberculosis Incidence for Status Indians by Residence Status, 1991-1995 Figure 7: Tuberculosis Cases in Foreign-Born Persons, 1980-1995 Figure 8: Reported Foreign-Born Tuberculosis Cases in Canada, 1990-1995 Figure 9: Reported Foreign-Born Tuberculosis Cases in 1995 by Year of Arrival Figure 10: Prevalence of Drug Resistance Among Tuberculosis Cases in Canada, 1993-1994
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