New route of TB transmission identified

"This is a game changer. We have known about this human and animal pathogen, TB, since ancient times, and it has always been considered something that is transmitted either through oral or aerosol exposure," said lead study author Kathleen Alexander, DVM, PhD, professor, Department of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, Virginia. "We have found that this species of TB is transmitted environmentally through urine and anal gland secretions used in olfactory communication, infecting mongoose through injuries in the skin and nose. This completely changes our understanding of the potential mechanisms for TB transmission."

Several different Mycobacterium species can cause tuberculosis, the most common being Mycobacterium tuberculosis. In 2000, Dr. Alexander was working as a government veterinarian in Botswana, when she encountered a sickly banded mongoose. When the mammal died, she determined the animal was infected with a novel tuberculosis pathogen, Mycobacterium mungi, closely related to the TB pathogen infecting humans in West Africa. Just how the mongoose became infected was unclear, as it didn't appear to be contracted by any of the usual routes.

Over the next 15 years, Dr. Alexander conducted research on similar banded mongoose deaths on the only known population of TB-infected mongoose, which spanned Northern Botswana and Northwestern Zimbabwe. The researchers conducted exhaustive research to determine how the mongoose were becoming infected with the bacteria. "It was clear they weren't eating or breathing it into their lungs. We looked places where you would expect exposure, but could not find any evidence of the pathogen," said Dr. Alexander. "We discovered the mongoose were getting lesions in their nose, so we thought it must be environmental, and we started looking through all of the different possibilities. Is the bacteria being transmitted through human sewage? Is the bacteria in the soil or feces of another animal?"

For years, researchers had no luck and could only cross possible routes of transmission off the list. "We looked everywhere. Finally, I checked the anal glands and that is where it was hiding," said Dr. Alexander. Banded mongoose engage in scent marking, somewhat similar to dogs, and the TB was being transferred through these markings used for social communication.

The secretions of the anal glands, an oily substance, present an ideal hideout for the bacteria, which doesn't thrive in a water-based environment (hydrophobic). This clever evolutionary mechanism allows the pathogen to hijack the social communication networks and connect to animals across social groups, where due to territoriality, contact necessary for transmission would not normally be identified.

The finding has critical implications for TB outbreak potential among wildlife and livestock. "Tuberculosis is a huge burden for the agriculture sector and environmental transmission between wildlife and livestock is an increasing concern," said Dr. Alexander. She pointed out that this route of transmission might help explain some outbreaks of tuberculosis in cattle that have mystified researchers.

"There is a lot more that we need to understand about this," said Dr. Alexander. "What is different about this TB organism that allows it to infect an animal through the skin or nose? How does it compare to the TB that infects humans? What is missing and what has changed in the genome?"

Pursuing these research avenues will provide further insight into the disease.

mBio is the open-access journal of the American Society for Microbiology.