Getting to the bottom of TB transmission

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A meerkat – part of the mongoose family. CC0 Public Domain image.

A surprising new finding was released recently* that reports an entirely novel route of transmission for a well-known global health threat: tuberculosis (TB). As is often the case with scientific research, a line of seemingly straightforward inquiry yielded an unexpected discovery – in this case, researchers have identified a link between TB transmission and anal secretions of mongooses. Yep, you heard it here first.

Antibiotic resistance to standard anti-TB drugs is on the rise, with multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) threatening the use of our standard lines of defence for this pathogen. Although global health organisations have made great strides in combatting both the spread of TB and antibiotic resistance, this new insight will likely open new avenues of investigation and inquiry. This could help us to understand the bacterial pathogen further, hence hopefully enhancing treatment strategies.

Tuberculosis is an infectious disease caused by the bacteria Mycobacterium tuberculosis and its related species. TB typically infects the lungs and can be transmitted by aerosol droplets or via oral exposure from infected individuals. According to the World Health Organisation (WHO), over 9.6 million people – roughly the population of Sweden – became ill with TB in 2014, and 1.5 million people died from TB infection. It’s estimated that one-third of the world’s population has latent TB, which means they’ve been exposed and infected, but do not yet show symptoms and cannot spread the disease. In October last year, WHO released a report indicating that death from tuberculosis infection is beginning to rival the death toll of HIV/AIDS as the leading cause of death from infectious disease: clearly necessitating increased funding and research efforts to reduce the global burden of TB.

Most recently, a little-studied pathogen called Mycobacterium mungi was identified in a population of wild banded mongooses in Northern Botswana. M. mungi, which has caused significant mortality in this mongoose population, was confirmed via DNA sequencing to be part of the Mycobacterium tuberculosis complex (MTC) – the group of species known to cause tuberculosis.

It was accepted knowledge that TB is spread through oral exposure. But scientists have discovered that this newly identified M. mungi species spreads through urine and anal gland secretions (nice!) as part of olfactory communication among mongooses. This means that – at least in the case of these mongooses – transmission can occur through injuries in the skin and nose, when the animals stumble upon this pathogen in the environment. This new finding is challenging our understanding of potential TB transmission mechanisms.

Broadening our understanding of TB from all angles is crucial, as multidrug-resistant TB continues to spread and TB-HIV co-infection continues to rise. This new finding might shed some light on the risk of TB outbreak potential at the wildlife-livestock interface, and might even shed insight into some mysterious reported outbreaks of TB in cattle.  Most importantly, though, this newly reported route of transmission has expanded our knowledge concerning how this pathogen can spread.

This new insight will challenge scientists to compare the genetic differences between M. mungi and MTC species that infect humans, and examine what exactly is different about M. mungi that allows it to infect different tissues. To put it another way, mongoose butts may be a unique tool for our arsenal (ahem) in the fight to understand TB infection.

– Kellie Vinal. 

Kellie is based in Atlanta, Georgia, and is finishing up her PhD at Emory University. Connect with her on Twitter at @kellievinal!

Kellie2* This can be found in in mBio, the American Society for Microbiology’s open access online journal –  Alexander, K.A., Sanderson, C.E., Larsen, M.H., Robbe-Austerman, S., Williams, M.C., and Palmer, M.V. 2016. Emerging tuberculosis pathogen hijacks social communication behavior in the group-living banded mongoose (Mungos mungo). mBio 7(3):e00281-16. doi:10.1128/mBio.00281-16.

Additional TB Resources:

Global Tuberculosis report 2015: http://www.who.int/tb/publications/global_report/gtbr15_main_text.pdf

WHO TB Factsheet: http://www.who.int/mediacentre/factsheets/fs104/en/

CDC TB Factsheet: http://www.cdc.gov/tb/topic/basics/default.htm

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