TUBERCULOSIS DRUG RESISTANCE: SUMMARY REPORT FOR 2003

A laboratory-based national surveillance system was established in 1998 to monitor tuberculosis (TB) drug resistance patterns in Canada. Those responsible for its establishment were Tuberculosis Prevention and Control (TBPC) at the Centre for Infectious Disease Prevention and Control, Health Canada, in collaboration with the Canadian Tuberculosis Laboratory Technical Network and participating laboratories (representing all provinces and territories) in the Canadian Tuberculosis Laboratory Surveillance System (CTBLSS).

Laboratories report their results on anti-tuberculous drug susceptibility testing to TBPC for every patient from whom they receive a specimen or an isolate in each calendar year. TBPC subsequently produces an annual report. A report presenting 2003 and adjusted 2002 data (to reflect duplicate removal and late reporting) on drug susceptibility of TB isolates across Canada as of 10 March, 2004, is available on Health Canada's Web site ( http:// www.phac-aspc.gc.ca/publicat/tbdrc03/ index. html). The following is a brief summary of this report.

Methods

TBPC maintains a computerized database containing drug susceptibility test results of Mycobacterium tuberculosis (MTB) and MTB complex (MTBC) isolates. Data are collected either through manual completion of a standard reporting form or by electronic transmission. Information collected includes sex, year of birth, province/ territory from which the report originates, province/ territory from which the specimen originates, and susceptibility results. TBPC makes every effort to eliminate duplicate specimens; only the most recent susceptibility results for a given patient in the current reporting year are included for analysis.

Laboratories generally perform routine susceptibility testing of MTB or MTBC to first-line anti-tuberculous drugs using the radiometric proportion method (Bactec®). Saskatchewan uses MGIT® 960, and all others use Bactec® 460. Table 1 lists the first-line anti-tuberculosis drugs and the concentrations in milli-grams per litre used by the participating laboratories.

Results

There were 1379 TB isolates included for analysis in 2003, of which 173 (12.5%) were resistant to one or more first-line anti-tuberculous drugs. Resistance to INH was the most common type of drug resistance (9.3%). Twenty isolates (1.5%) were multi-drug resistant tuberculosis (MDR-TB) strains (defined as resistance to at least INH and RMP), of which seven demonstrated resistance to four or five first-line anti-tuberculous drugs tested.

The number of reported TB isolates in 2003 was relatively unchanged from the previous year (1420 in 2002 and 1379 in 2003 isolates). In addition, the percentage of isolates demonstrating any type of drug resistance was also unchanged between the two reporting years (12.6% in 2002 and 12.5% in 2003), and the proportion of isolates classified as MDR-TB was identical (1.5%) in both years. Overall, levels of TB drug resistance have shown no significant difference since the inception of this reporting system in 1998. Figure 1 shows the overall pattern of TB drug resistance in Canada from 1998 to 2003.

Table 1. Minimal inhibitory concentrations (MIC) for routine testing of first-line anti-tuberculous drugs

Figure 1. Overall pattern of reported TB drug resistance in Canada, 1998-2003

The results observed to date are consistent with international data. In the latest report of the global TB drug resistance surveillance project jointly conducted by the World Health Organization and the International Union Against Tuberculosis and Lung Disease ⁽¹⁾ , the median prevalence of TB drug resistance in participating countries was 1. 1% for new cases and 7% for previously treated cases (as compared with 12.5% overall in Canada). The median prevalence of MDR-TB was 10.2% for new cases and 18.4% for previously treated cases (as compared with 1. 5% overall in Canada).

With growing worldwide concern regarding TB drug resistance, this surveillance system is vital in providing the necessary data in a timely fashion to monitor trends in TB drug resistance in Canada.

Reference

1. The WHO/ IUATLD Global Project on Anti-tuberculosis Drug Resistance Surveillance (1999-2002). Anti-tuberculosis drug resistance in the world. Report No. 3 (WHO/ CDS/ TB/ 2000.278). Geneva: World Health Organization, 2004.

Source: Tuberculosis Prevention and Control, Centre for Infectious Disease Prevention and Control, Health Canada.