II. EPIDEMIOLOGY OF TB IN CANADA- Guidelines for Preventing the Transmission of Tuberculosis in Canadian Health Care Facilities and Other Institutional Settings

[Table of Contents]

Volume: 22S1 • April 1996

Guidelines for Preventing the Transmission of Tuberculosis in Canadian Health Care Facilities and Other Institutional Settings

II. EPIDEMIOLOGY OF TB IN CANADA

Canada is one of the few countries in the world reporting fewer than 10 TB cases per 100,000 population. (International TB rates are published by the World Health Organization and can be obtained using the Laboratory Centre for Disease Control (LCDC) electronic Bulletin Board system, which can be accessed on INTERNET. Since 1987, the Canadian TB rate has stabilized at 6.9 to 7.4 cases per 100,000 persons (1,947 to 2,108 cases per year) (8) . This plateau ends decades of decreasing rates and has prompted an intensive re-evaluation of the TB situation in Canada. The United States and several European countries reported increasing TB rates after their TB rates had reached a plateau in the early 1990s.

Approximately 57% of TB cases reported in Canada (1990-1993) are classified as pulmonary TB (infection in the lung tissue, respiratory airways or larynx) and 43% as extrapulmonary TB (infection in other tissues, such as cervical and intrathoracic nodes, pleura, meninges, bone, and other organs, or disseminated TB) (8) . Although not all TB cases are confirmed by culture, the clinical diagnosis of active TB should be supported by laboratory findings, whenever possible. The Canadian Thoracic Society proposes that 85% of cases of pulmonary TB should be confirmed by culture (9) . In Canada from 1990 to 1993, 77% of reported pulmonary TB cases had a positive culture. The remaining 23% were diagnosed by clinical criteria (8,10-12) ; 52% of the pulmonary TB cases reported were AFB-smear positive (8,10-12) .

Geographic Differences

There are marked differences in TB rates reported by the provinces and territories. In 1993, for example, the rates ranged from 1.8 to 53.6 per 100,000 population (Table 1) (8) . The focal nature of TB in Canada is further illustrated by the range of rates reported in federal electoral districts of each province (13) . Provincial and territorial trends in TB rates are illustrated in Figure 1.

Table 1 Canadian, Provincial and Territorial TB Rates per 100,000 Population, 1993
CA	NF	PE	NS	NB	QC	ON	MB	SK	AB	BC	YK	NT
6.9	10.6	3.8	1.8	2.0	4.9	7.1	9.6	15.1	5.8	9.4	9.8	53.6
Source: Statistics Canada, 1995 (8)

Risk Groups

In Canada, people at higher risk of having active TB than the general population include the following:

Aboriginal Canadians (13) ("Aboriginal" is used throughout this document to refer to the indigenous inhabitants of Canada and their descendants);
foreign-born individuals from countries reporting high TB rates (13-15) ;
poor (16,17) and homeless people;
inner-city residents prone to substance abuse (including alcoholics and injection drug users) (17) ;
elderly people (especially single men) (13) ; and
individuals co-infected with HIV and M. tuberculosis (18) .

People who work with individuals in these high-risk groups are also at increased risk of becoming infected with M. tuberculosis since they may be exposed to individuals who have active, infectious TB but who have not yet been diagnosed as having TB (3,4) .

The number and proportion of TB cases occurring in different Canadian risk groups have changed over the past 14 years. The number of cases identified in non-Aboriginal, Canadian-born individuals has progressively fallen, whereas the number of cases reported for Aboriginal peoples and foreign-born individuals has remained relatively stable (Figure 2). In 1993, 503 of 2,011 (25.0%) cases occurred in non-Aboriginal, Canadian-born individuals, 372 (18.5%) in Aboriginal Canadians, and 1,061 (52.8%) in foreign-born individuals (8) . There is a high variability in the proportion of TB cases reported in each risk group among the provinces and the territories (Figure 3).

The TB rate generally increases with increasing age. Figure 4 illustrates the trends according to age group. From 1990 to 1993, the median age of non-Aboriginal, Canadian-born individuals with TB was 54.9 years compared with 28.3 years for Aboriginal Canadians and 43.9 years for foreign-born individuals (unpublished data, Statistics Canada).

Drug-Resistant TB

Resistance to antibiotics has been noted since the introduction of antibiotic treatment of TB. Drug resistance is classified as either primary or secondary (acquired) resistance. Individuals with primary drug-resistant TB were infected by a person who had drug-resistant TB. Individuals classified as having secondary or acquired drug-resistant TB were infected by someone with drug-susceptible TB but drug-resistant TB developed because of inadequate, inappropriate or irregular treatment.

MDR-TB is defined as resistance to at least isoniazid (INH) and rifampin, the two most important drugs used in treating TB. The possibility of drug-resistant TB, including MDR-TB, must always be considered in patients born in countries reporting high levels of drug-resistant TB (e.g., developing countries) or in persons who have lived or visited areas that have high levels of drug-resistant TB or in persons who have previously been treated for TB.

The number of cases of drug-resistant TB reported in Canada to the end of 1993 has been small, and no outbreaks of MDR-TB have been reported. The experience is different in some parts of the United States. In New York City, for example, 26.3% of new TB cases in 1991 were resistant to one or more drugs, and 12.9% were resistant to at least INH and rifampin (19).

FIGURE 2
Who had Tuberculosis in Canada in 1980 and 1993?

FIGURE 3
Percentage of Canadian, Provincial and Territorial Tuberculosis Cases Occurring
in Specific Groups — 1993

FIGURE 4
Tuberculosis Rates by Age Group — 1980 and 1993

The percentage of cases reported to be drug-resistant may be used to evaluate the effectiveness of TB management programs. The level of drug resistance reported in Canada is low compared with most other countries. In 1975, 6% of cases were resistant to one or more drugs (20) , compared to 7% in 1993 (21) .

The percentage of patients who complete therapy can also be used to evaluate the effectiveness of TB management programs. Since national statistics do not document the rate of completion of therapy, the rate of reactivated (relapsed) TB may be used as a surrogate marker for failure to complete an appropriate course of therapy. Reactivated (relapsed) TB is said to occur in individuals who develop active TB after having been treated at a previous time for active TB, which was followed by a period of inactivity. Reactivated (relapsed) TB has been documented in 9% to 12% of individuals with TB reported each year in Canada since 1980 (8) .

HIV and TB

HIV infection may be a significant risk factor for acquisition of TB disease. Individuals infected with HIV and TB are known to have a markedly increased risk of developing active TB (14% risk over 2 years compared to a 5% to 10% lifetime risk for non-HIV-infected persons) (22) (see Section III). The diagnosis of TB may be problematic in HIV-infected individuals, either because other opportunistic pathogens have presentations similar to TB or because TB may have uncommon presentations in an immunocompromised person (23) .

The number of Canadians reported to have HIV infection and active TB is small; however, under-reporting is likely. Only individuals with disseminated or extrapulmonary TB were reported in the AIDS Case Reporting Surveillance System (ACRSS) between 1987 and July, 1993. The Canadian AIDS surveillance case definition was expanded in July, 1993, and individuals with pulmonary or extrapulmonary TB are currently reported in ACRSS. The Bureau of HIV/AIDS and STD, LCDC, reported that, as of December 31, 1995, 522 (4.1%) of the 12,670 reported Canadian AIDS cases had a diagnosis of TB (119 pulmonary TB, 300 extra-pulmonary TB, and 103 both).

The Canadian Tuberculosis Reporting System does not report HIV status. The level of co-infection with HIV and active TB can be examined, however, from data collected by provincial and territorial TB registries. Ontario (24) and British Columbia (17) report that 1.5% of individuals with active TB are co-infected with HIV, and Alberta reports 0.5% (25) .

At least one hospital-based study has shown that having HIV infection is a risk factor for the nosocomial acquisition of MDR-TB (4) . In studies reported in the United States, MDR-TB has been shown to frequently and rapidly lead to disease and death in HIV-infected individuals. In Canada, there are sporadic reports of individuals with HIV infection and MDR-TB.

Guidelines for the identification, investigation and treatment of individuals co-infected with M. tuberculosis and HIV have been published (26) . Effective TB management programs in health care facilities must take into consideration the important interaction of HIV and M. tuberculosis.

Risk of Nosocomial Transmission of TB

In 1994, LCDC, the Community and Hospital Infection Control Association (CHICA) and the University of Alberta conducted a "Hospital TB Readiness Study" to collect information about individuals with TB who were cared for in Canadian acute care facilities. Sixty percent (286 of 474) of the health care facilities responded to the questionnaire. The study revealed that 3,746 patients with pulmonary TB were treated in 191 health care facilities from 1989 to 1993. The average number of patients with pulmonary TB treated each year in these health care facilities over this 5-year period varied from 0.0 to 102.4. The number of patients with pulmonary TB treated in a health care facility ranged from 0 to 119 per year (mean 4.1, median 1.4). In any given year, over 50% of the responding health care facilities had not cared for a patient with pulmonary TB from 1989 to 1993 (27) .

The published literature documents that higher rates of infection with M. tuberculosis are found in HCWs than in the general public (28) . Estimates of a HCW's annual risk of infection have ranged from 0% to 10% in population studies although higher rates have been documented during outbreaks in the United States (28) . Annual rates of acquiring infection with M. tuberculosis are estimated to be less than 1 in 1,000 (0.1%) in the general public living in developed countries (29) .

Although advances in the care and treatment of persons with TB since World War II have dramatically reduced HCWs' exposure to TB, recent reports confirm that nosocomial transmission of TB to HCWs continues to occur (28,30) .

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	Last Updated: 1996-09-24		Important Notices