‘W-Beijing’ strains of Mycobacterium tuberculosis have spread quickly through one of the world’s most TB-stricken areas over the past 12 years, report researchers online this month in Clinical Infectious Diseases. 

“These are the first data to clearly document the emergence and rapid spread of W-Beijing strains of tuberculosis in a region where tuberculosis is endemic over an extended period,” write Donna Cowley and colleagues. “Our findings suggest that these strains possess a significant advantage in their ability to disseminate within a community.” 

W-Beijing strains were first described as a cause of TB in eastern Asia in 1995. Since then they have been found across the world, often associated with multidrug resistance and HIV infection. Tests in animals have shown that the strains are particularly virulent and can infect even those animals that have been vaccinated against TB. 

Recent evidence also suggests that more people around the world are being infected with W-Beijing strains. Scientists working in the Western Cape province of South Africa have noted the upward trend in the number of infections seen in the area, the TB burden of which ranks among the highest globally. 

“We found that W-Beijing strains have recently emerged and are rapidly becoming more prevalent in Cape Town,” write Cowley et al. These strains accounted for 23% of samples taken from TB-infected children between 2000 and 2003 at a paediatric hospital in Cape Town, the capital of the Western Cape. 

The proportion of disease caused by W-Beijing strains rose by 20% over the four-year period, say the group. By looking back over archives of tissue samples taken from patients who died of TB at a nearby hospital, they were able to determine that the growing number of infections is a recent phenomenon. 

“W-Beijing strains were rare or absent in post-mortem specimens from Cape Town from 1930 through the mid 1960s,” write the authors. “It was only during the most recent decade (1996–2005) that these strains emerged as a major cause of death due to tuberculosis.”

Testing of 352 randomly selected archived samples of tissue determined that the first known cases of W-Beijing infection in Cape Town appeared in 1966. For the next three decades, the number of cases remained steady and low. But by 1996–2005, one in five tissues analysed was infected with W-Beijing, a rise of more than 17%. In comparison, case counts of infection with other types of M. tuberculosis fell over the same period.

Cowley and colleagues argue that these trends represent the bigger picture of W-Beijing transmission in the Western Cape. Because children’s immune systems are not fully developed, they explain, TB infections almost always progress into disease; this means that their infections reflect strains circulating at the time. This idea is supported by recent research in adult patients, they add. 

Genetic analysis of the W-Beijing strains behind cases of illness revealed considerable variation, say Cowley et al. This suggests that subfamilies of the bacteria are changing and expanding, and that the growing number of infections is not down to an outbreak. 

“The rapid expansion of W-Beijing strains in a region with a very high background incidence of tuberculosis suggests that these strains have a significant selective advantage,” write the authors. An understanding of the biological mechanisms that explain why these strains have the edge over other kinds of M. tuberculosis is needed to prevent the spread of the emerging strains, they add.

Most of the W-Beijing strains infecting children in Cape Town were susceptible to the first-line TB treatments, isoniazid and rifampicin. Even so, the virulent strains are likely to pose additional challenges for tuberculosis control, conclude the authors.