Unlike people with drug-susceptible TB, those with multi drug-resistant TB (MDR-TB) are less likely to transmit disease to others living in the same household (also known as household contacts), a study published on June 23 in the journal PLOS Medicine found.
Although it may not be right to extrapolate the findings to the community level, within households, MDR-TB surely has “relatively low fitness [be less capable of spreading] compared with drug-susceptible TB.”
The study results agree with those of previous animal and laboratory studies, as well as molecular epidemiology studies that had estimated that the fitness level of drug-susceptible TB bacteria to spread within populations was higher than MDR-TB bacteria. But animal and lab studies do not take into account the clinical, environmental and socio-economic factors that influence infection. Hence, the latest study has great significance.
The study carried out in South Lima and Callao, Peru followed up people living in the same household as the index patient for three long years (2010-2013). The study tracked 213 MDR-TB index patients and 1,055 of their household contacts. In the case of drug-susceptible TB patients, the study followed 487 index patients and their 2,362 household contacts.
While only 35 of 213 of MDR-TB contacts developed MDR-TB disease, 114 of 2,362 drug-susceptible TB contacts developed disease. The hazard ratio for TB disease for household contacts of MDR-TB index cases was “half” that of the household contacts of those with drug-susceptible TB.
Since the study did not genetically compare the TB strains isolated from the index patient with strains isolated from household contacts, there is a possibility that the contacts could have got infected with TB outside the household.
As a rule, people who are in close contact for extended periods of time with a person who has TB disease — either drug-susceptible TB or MDR-TB — are at heightened risk of getting infected and suffering from TB disease. All the more when more number of people live in small households with poor ventilation.
This is reason why the WHO and the Indian TB control programme (RNTCP) insist that all household contacts of a TB index patient should be screened for TB disease when the index patient is diagnosed. Such active case finding greatly reduces the time lag between developing TB disease and testing. The time lag between developing disease and being correctly diagnosed is anywhere between three and four months in India.
The study found that male household contacts and those who slept in the same room as the index patient had higher incidence of active TB than those who did not share a sleeping room. Also, household contacts who had diabetes or HIV or who had previous history of active TB had higher incidence of active TB.
While contacts from the lower socio-economic strata were more likely to suffer from active TB, crowding (large number of people in a household) was “not significantly” associated with higher incidence of TB disease.
The results of the study indicate that there are greater chances of controlling the spread of MDR-TB due to its lower fitness. But there can be no room for complacency as the MDR-TB can become fitter with time and be equally transmissible as drug-susceptible TB.
“The low relative fitness of MDR-TB estimated by this study improves the chances of controlling drug-resistant tuberculosis. However, fitter multi drug-resistant strains that emerge over time may make this increasingly difficult,” writes Louis Grandjean, the first author from the Imperial College London. “The greater the relative fitness of drug-resistant TB, the greater the size of the drug-resistant epidemic.”