The medical and scientific community has historically argued that tuberculosis can only be spread once the disease is activated in the patient, but a new study shows that it can even be transmitted when it is dormant, before developing the first symptoms.

This is the main conclusion of a research led by Spanish scientists, published in PLOS Medicine. The study included 785 patients from the Valencian Community that were followed during the years 2014-2016.

The research leaders are scientists from the Fisabio Foundation and the Biomedicine Institute of Valencia (of the CSIC), in collaboration with 18 hospitals in the Valencian Community, as well as researchers from Imperial College London and Simon Fraser University of Canada.

Tuberculosis is a disease that usually affects the lungs and is caused by a group of bacteria that make up the so-called Mycobacterium tuberculosis complex. It is transmitted from one person to another through the air by coughing or sneezing and it have risk factors, such as suffering from HIV or diabetes. According to data from the World Health Organization, it is one of the ten leading causes of death worldwide.

It is estimated that last year 10 million people became ill with tuberculosis, of which 1.2 million died –most of them because they did not have access to the standard treatment of four antibiotics or because they were interrupted. In Spain, 4,400 cases were registered in 2018.

Historically, it has always been said that if you are infected by the dormant bacteria –time spam between the moment when the disease is acquired and the first symptoms appear–  you cannot transmit it, and only if it is active is it when you can spread it,” explains Iñaki Comas, one of the authors of this work.

However, in this study, the researchers observed that a percentage of latent cases are infectious and they wonder why this happens.  According to Comas, understanding this would change how the disease is perceived and controlled.

For the study, the scientists developed a method that combines epidemiological data of the patient with the sequencing of the complete genome of the bacteria causing tuberculosis. In total, 785 patients were analyzed, of which 35% had been recently infected. The method was applied in 117 individuals involved in 21 transmission outbreaks.

To accurately determine the date on which an infected case transmitted the disease, the complete genome of the bacteria was sequenced and compared in the people involved in these outbreaks. Mutations in the tuberculosis bacteria accumulate over a period of time and researchers know the speed with which this occurs, so comparing one genome and another and knowing the date of diagnosis are able to establish whether that patient was the ‘index case’ (the first documented patient in the onset of an epidemiological investigation) and the date of the infection.

Thus, when analyzing the samples, it was discovered that the origin of the transmission was not always in the patients previously identified as ‘index cases’. In addition, about 30% of patients seem to have transmitted the disease before developing symptoms, which has traditionally been known as latent tuberculosis. Further analysis will be necessary to corroborate this data and how it is extrapolated to other cohorts.

This work corroborates that latent infection is more complex than we knew and is in line with other recently published works,” adds Comas.

The team also revealed a possible relationship between the ability to transmit it and suffer from type 2 diabetes mellitus, a finding in which, according to the authors, we must continue to deepen.

For Irving Cancino-Muñoz, another of the authors, this research supports the need to change active tuberculosis control policies and expand preventive controls in at-risk populations in order to identify possible sources of transmission, beyond the patient’s environment. These results, he adds, are of special relevance in high-incidence countries where disease transmission is still one of the major sources of new cases of tuberculosis.


Source: Agencia ID