Leprosy spreads to muscles and other tissues through a stealth operation in which it reprogrammes key cells in the nervous system, a new study stays.

Tests on mice show that the parasitic germ which causes the disfiguring disease is extraordinarily skilful in hijacking so-called Schwann cells, according to researchers from the University of Edinburgh in Scotland.

These are fatty cells which wrap around nerves, acting as insulation to protect the electric signals that pass through the nerve strands.

The Mycobacterium leprosae bug then reprogrammes the cells to an immature state before converting them to muscle-like cells, according to the research published in the journal Cell on Thursday.

It does this by turning off genes that cause the cell to operate in its adult state and by switching on genes that are expressed when the cell is in a premature, embryonic state.

The probe may explain a long-running mystery as to how the bacteria is able to spread to other tissues after starting in the peripheral nervous system, say the authors.

The discovery could open the way to drugs that hit the bacteria before the reprogramming stage, researcher Anura Rambukkana of the University of Edinburgh said in a press release.

Leprosy is an ancient disease that carries an additional curse of stigma. In 2012, 182,000 people had the condition, mainly in Asia and Africa, according to the World Health Organisation.

The disease takes about five years to incubate and symptoms can take as long as 20 years to appear.

It is curable with antibiotics. But without treatment, it causes crippling damage to the hands, skin and eyes.