ICMR to launch study on human-to-cattle TB infections

After cases of humans infecting cattle with tuberculosis emerged in southern India, a research institute under the Indian Council of Medical Research is planning to launch a study to check the prevalence of bovine TB in household cattle across the country since reverse zoonosis is an alarming development, which could adversely affect India’s goal of eliminating the infectious disease by 2025, as it poses a risk of cross-infection.

The Chennai-based National Institute for Research in Tuberculosis (NIRT) submitted a research proposal to Union health ministry in February to assess the magnitude of the spread. To screen a large number of animals for the study, the institute, which operates under the ICMR, has also sought funding from the National Institutes of Health in the US.

“This is the first time we have seen TB strains in cattle that are usually found in humans, such as mycobacterium tuberculosis. Their handlers were also tested and the same TB strain was found in them,” said Dr P Kannan, senior scientist at NIRT who studies zoonotic diseases. “On the face of it, there is strong evidence of humanto-animal transmission as these strains are not usually found in animals.”

“But we need to conduct a larger study to understand the magnitude of the problem, as these samples were lifted from four farms in Tamil Nadu,” Dr Kannan said. “Now we need to screen household animals to see if the disease is also found in that subset, and at different sites across the country, to get a pan-India picture.” The researchers screened 167 cattle with active symptoms during the study conducted from 2015-19, of which 21 came back positive for TB in intradermal tests.

Mycobacterium orygis, another TB strain, was found in two black bucks and one spotted deer in Guindy National Park in Tamil

Nadu. Four animals died during the course of the study and Kannan’s team conducted autopsies that found TB lesions in critical organs such as lungs, liver, udder and spleen. The findings of the study have been published in the medical journal Nature.

“Numerous lines of evidence confirmed that infection was due to M. tuberculosis rather than M. bovis (a common strain in animals), including whole genome sequence data that unequivocally placed cattle-associated isolates within a lineage, the most common cause of human tuberculosis in southern India,” the paper said.

The researchers are yet to determine how the transmission between humans and animals took place. “It will be interesting to know what genetic changes must have happened in the bacterium for it to jump to animals from humans. We also need to know the intensity of transmission and look for other critical answers,” said Dr Uma Devi, head of immunology department at NIRT. “The study that’s being planned will keep these questions in mind.”

The institute was first approached by Madras Veterinary College with suspected TB samples in 2015. After advanced testing, the samples returned positive.

Further study is required as there is no treatment available for such infected animals, Kannan said. “The research needs to be carried to open pathways for not only developing treatment of TB, but also bovine TB vaccine, as bovine TB will need to be controlled because it could be the next problem,” he said. “It at least has the potential.”

THE RESEARCHERS SCREENED 167 CATTLE DURING THE STUDY, OF WHICH 21 CAME BACK POSITIVE FOR TB IN TESTS