Drug-resistant Mycobacterium tuberculosis in Estonia

Abstract

Tuberculosis (TB) infection and disease patterns among different populations are extremely heterogeneous. This thesis explores mainly by microbiological methods the epidemiology of TB in Estonia.

Through the work high rates of drug resistance were found. The first countrywide study carried out in 1994 ascertained that drug resistant TB, and particularly multidrug-resistant TB (MDR-TB) is a serious problem for Estonia. Initial resistance to one or more of the drugs tested was 28%, with 10% being initially multidrug-resistant. Molecular typing with IS6110 RFLP has revealed that 29% of Estonian M. tuberculosis isolates belonged to the genetically closely related group of strains with a predominant IS6110 banding pattern. This isolates were classified by spoligotyping as Beijing genotype strains, widely found in Asia. The majority (87.5%) of all multidrug-resistant isolates and two thirds (67.2%) of all isolates with any drug resistance belonged to Beijing genotype family.

The incidence of TB among health care workers (HCW) in Estonia was 1.5 to 3 times higher (mean 91/100 000/year) than in the general population. In a chest hospital the incidence was 30 to 90 times higher. The highest rate was observed among physicians. In addition, this work shows that nosocomial isolates of M. tuberculosis can often be MDR after MDR-TB becomes more common in the general population. More than one third (38%) of all M. tuberculosis isolates obtained from HCW were multidrug-resistant.

The investigation of means by which drug resistance evolves among drug-susceptible M. tuberculosis strains during antiT13 treatment revealed that initially drug-susceptible M. tuberculosis does not always evolve drug resistance despite highly irregular and prolonged therapy. Yet, the remained susceptibility of M. tuberculosis does not grant treatment success. When advanced method of molecular typing is not employed, exogenous re-infection with drug resistant M. tuberculosis may be misinterpreted as creation of drug resistance.

This work also encompass of our experience and knowledge on testing drug susceptibility of MDR M. tuberculosis isolates to second-line and alternative drugs. In Estonia, a standardized treatment regimen with up to 6 second-line drugs can be suggested for 2/3 of MDR-TB patients; for the remaining cases additional testing of an extended panel of drugs is required.

The MDR M. tuberculosis clinical isolates with unusual kanamycin-resistant but arnikacin-susceptible phenotype were characterized by identification of mutations with-in the rrs gene. Mutation possible related to intermediate level of resistance to kanamycin, showing a thymine for cytosine substitution at the 16S rRNA position 516, has not previously been associated with kanamycin resistance in M. tuberculosis. To date, genetic methods fail to detect all clinically relevant levels of drug resistance to aminoglycosides. Consequently it is important to test antimicrobial susceptibility of resistant clinical isolates of M. tuberculosis by culture.