New strains of drug-resistant tuberculosis (TB) may become widespread, Australian researchers from the University of New South Wales and the University of Western Sydney have warned.
Writing in the Proceedings of the National Academy of Sciences, the researchers say that their finding raises concern that even though TB incidence is falling in many regions, the emergence of antibiotic resistance could see virtually untreatable strains of the disease become widespread.
Considering that lab-based studies have suggested that antibiotic-resistant TB strains cause longer-lasting infections, but with a lower transmission rate, the researchers questioned whether drug-resistant TB strains are more likely than drug-sensitive strains to persist and spread - an important question for predicting the future impact of the disease.
Led by UNSW's Dr. Mark Tanaka, the research team used epidemiological and molecular data from Mycobacterium tuberculosis strains isolated from Cuba, Estonia and Venezuela to estimate the rate of evolution of drug resistance, and to compare the relative "reproductive fitness" of resistant and drug-sensitive strains.
"We found that the overall fitness of drug-resistant strains is comparable to drug-sensitive strains. This was especially so in Cuba and Estonia, where the there is a high prevalence of drug-resistant cases," says Dr. Tanaka of the Evolution and Ecology Research Centre.
The finding may reflect an inconsistency in drug treatment programs in these countries. Indeed, Estonia now has one of the highest rates of multi-drug resistance in the world.
According to the researchers, the intermittent presence of drugs and the resulting transmission of resistant strains would have let drug-resistant strains collectively spend more time within untreated hosts, allowing them to evolve ways to become more infectious and out-compete the drug-sensitive strains.
The researchers have also found that the contribution of transmission to the spread of drug resistance is very high - up to 99 per cent - compared with acquired resistance due to treatment failure.
"Our results imply that drug resistant strains of TB are likely to become highly prevalent in the next few decades. They also suggest that limiting further transmission of TB might be an effective approach to reducing the impact of drug resistance," says UNSW's Dr. Fabio Luciani, the study's lead author.
Research co-author, Dr. Andrew Francis from the University of Western Sydney, adds: "Mathematical and statistical methods can add a lot of value to empirical data by allowing us to account for the processes behind them. In this case, we use samples of TB genotypes, together with information about drug resistance, to make inferences and predictions that wouldn't have been possible just a few years ago."