Aerosolized Transmission of Leprosy May Cause it to Persist

Leprosy, also known as Hansen’s Disease, is a slow-growing chronic condition that can result in painful skin lesions and sores, among other symptoms. Although uncommon in the United States, it remains a problematic disease worldwide, particularly in Brazil. Traditionally, untreated leprosy patients have been blamed for the spread of the disease. However, multidrug therapy treatment has been available around the globe for decades, and still, almost 212,000 new cases of the disease were reported last year. How is Mycobacterium leprae (M. leprae), the bacterium that causes leprosy, still managing to infiltrate new hosts? The answer, a new study suggests, can literally be found in the air.

Researchers at the Federal University of Uberlandia in Brazil and the University of California, Davis, examined samples gathered from 113 patients with untreated leprosy and 104 household contacts, defined as people who shared a home with a leprosy patient some time in the five years before the patient’s diagnosis. The samples, collected between the years 2003 and 2015, suggested that M. leprae sneaks into new hosts via the upper respiratory tract, specifically the nose. The researchers detected M. leprae in 66.4% of nasal swabs of leprosy patients and in 49% of nasal swabs of household contacts. When they looked at biopsies of subjects’ nasal turbinates, the researchers discovered that 71.7% of the leprosy patients had M. leprae DNA in their turbinates; 53.8% of the household contacts did, too. Blood samples revealed that 19.5% of patients had M. leprae DNA circulating in their blood, with 62.8% of patients exhibiting antibodies to the infection. Of the household contacts, 6.7% had M. leprae DNA in their blood and 18.3% had developed antibodies to the disease.

The aerosolized transmission of M. leprae appears to follow a certain path. Initially, the simple act of inhaling allows the disease particles to infiltrate the nasal passage, where they somehow evade airway epithelial clearance, a mechanism that normally protects the nasal passage against viruses and other invaders. The bacteria then colonize in the nasal passage and can be transmitted from host to host during inhalation and exhalation. The bacteria found in some subjects’ nasal turbinates suggest that the microbes often travel further into the nasal passage and establish a foothold in the body. They likely enter the lymph system via these turbinates and spread throughout the body, where they may be detected in blood samples.

For household contacts who don’t display any symptoms of leprosy, the threat of disease is still real. During the study’s multiyear follow-up period, 6.7% of the contacts developed leprosy themselves—most notably the contacts who displayed both M. leprae DNA and had developed antibodies to the infection in their blood. But even contacts who never become symptomatic are at risk to other contacts simply because they transmit airborne particles as they breathe in and out.

According to the US government’s Health Resources and Services Administration, there were 178 reported cases of leprosy in the country last year, appearing in 32 jurisdictions, including Puerto Rico. Most of the cases (72%) were located in just seven states—Arkansas, California, Florida, Hawaii, Louisiana, New York, and Texas. The highest number of cases reported in a single year was 456 in 1983, while in 2000, the number of reported cases dipped below 100.

With these findings, researchers have added to the arsenal of knowledge on leprosy. Knowing more about how the disease is transmitted, researchers may be able to develop more informed prevention strategies.

Laurie Saloman, MS, is a health writer with more than 20 years of experience working for both consumer and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.