Chennai: Researchers at Tamil Nadu’s National Institute for Research in Tuberculosis (NIRT) have sought funding from the US National Institutes of Health to conduct household screening of domestic animals in Chennai after they found instances of human-to-animal transmission of tuberculosis (TB).
Dr P. Kannan, Scientist D at NIRT, which comes under the Indian Council of Medical Research (ICMR), told ThePrint: “We screened about 162 cattle in four farms around Chennai and found tuberculosis in 20 of them. The handlers too were positive. This is the first instance where we have found evidence of reverse zoonosis of TB.”
Instances of zoonosis — transmission of a disease from animal to human — of the TB bacterium have been found in the United Kingdom but not in India. But now, the detection of TB in farm animals in India who contracted the disease from their handlers has raised apprehensions that the human-to-animal barrier in transmission has been breached.
“In a farm setup, the protocol is to isolate the animals because cow slaughter is not allowed. Now that we have evidence of reverse zoonosis, there are worries of animal-to-human transmission as well. That is why we are planning the household screening, to understand the prevalence,” Kannan added. “To control human tuberculosis, we need to control animal tuberculosis.”
During the study on farm animals, that lasted from 2015 to 2019, animals were screened for TB using the Mantoux test. In the course of the study, six animals died of the disease and autopsies found them to have TB lesions on the lungs, liver, spleen, udder, etc. The findings were published in the journal Nature.
Kannan’s team also detected Mycobacterium orygis, a member of the TB pantheon, in two black bucks and one spotted deer in Tamil Nadu’s Guindy National Park.
The NIRT researchers pointed to the need to take the environment into account in such cases.
“Nevertheless, it would be wise to take into account the environment where there exists a high chance of transmission due to the increased human-animal interaction. Since it is well known that the pathogen is capable of causing infection in both human and animal hosts, systematic surveillance and screening of spotted deer, black buck as well as humans in the vicinity is essential for successful implementation of the One Health approach,” they wrote in the journal Transboundary and Emerging Diseases.
Dr Uma Devi, head of the department of immunology at NIRT, said that evidence of TB zoonoses was found many years ago in the UK. “Mycobacterium bovis was found to have been transmitted from animals to humans, but no cases were found in India. But these fresh findings in farm animals are significant and need additional research,” she added.
Recommendation for BCG revaccination
With India having made BCG vaccination, which prevents TB, mandatory decades ago and yet struggling with the world’s highest burden of the disease, the NIRT has sent a recommendation to the government for BCG booster shots to household contacts of positive patients.
“We are studying the effect of BCG revaccination in children aged between 6 and 12 years. We will follow them for three years and see whether they catch the disease. Children tend to catch the disease in middle school,” NIRT Director Dr Padmapriyadarsini C. said.
“We have also had two meetings with the government on a proposal for revaccination of household contacts (of patients). For the trial, we plan to study 8,000 children.”
The trial has already been registered with the Clinical Trials Registry of India. The study’s summary reads: “Bacille Calmette Guerin (BCG) vaccine is one of the most used vaccines in the world to reduce the risks of natural tuberculous infection. The efficacy of BCG vaccination in newborns is well known and has a documented protective effect against meningitis and disseminated TB in children.
“However, there is considerable uncertainty on BCG revaccination. It is known that BCG revaccination enhances immune responses, but it is yet to be established if BCG revaccination can help prevent TB disease in household contacts.”
The primary aim of this study, the summary adds, “is to assess the efficacy of BCG revaccination compared to oral chemoprophylaxis in preventing TB disease in household contacts aged 6-18 years”.
(Edited by Nida Fatima Siddiqui)