Research finds a reason leprosy has persisted

The bacteria that cause leprosy can survive for months inside amoebae that are common in water and soil, and even in human eyes and noses, scientists at Colorado State University have found.

The discovery may help answer a question that has puzzled tropical disease experts for years: Why does the number of new leprosy cases around the world not decrease even though thousands of victims are now on drugs that make them less infectious and eventually will cure them? There are about 2,00,000 new infections each year in Brazil, India, Angola, Madagascar, Myanmar, Indonesia, the Philippines and a few other countries.

The study was published in PLOS Neglected Tropical Diseases. Leprosy is caused by Mycobacterium leprae, slow-growing bacteria related to tuberculosis that target nerve cells beneath the skin.

They cannot be cultured in the laboratory, and exactly how they infect is unclear. Because leprosy spreads in families and among people in prolonged contact, researchers have long assumed that it always moves between human hosts. “But we do get novel cases that don’t seem to be related to others,” said William H Wheat, a microbiologist at Colorado State University and one of the study’s authors. M. leprae are engulfed by five kinds of common amoebae, including some that can live in mucus and eye fluids and can resist being digested. When the amoebae form cysts to avoid drying out, the study found, the bacteria can survive inside them for months and then still infect laboratory mice.

That, William said, may explain how the bacteria persist and turn up even where no infected humans are found.