In the August 2023 issue of the Emerging Infectious Diseases journal, the United States Centers for Disease Control and Prevention (CDC) published a case report on leprosy in central Florida in 2022.

The report from Drs Aashni Bhukhan, Charles Dunn and Rajiv Nathoo of Kansas City University addresses a case of leprosy in a 54-year-old male and the context of rising cases of domestic leprosy in the US since 2000. The authors note that leprosy cases in Florida have been increasingly frequent among residents of the state compared to cases in migrants: the diagnosed incidence in Florida residents has outstripped that of foreign-born individuals since 1998 (~4% and ~2.5% respectively) and diagnosed incidence in Florida has been increasing while the diagnosed incidence in foreign-born persons has been decreasing since 2002. An increasing proportion of leprosy cases in Florida have also been without clear transmission since 2015. The authors assert that central Florida, which contains ~81% of all cases in Florida, can therefore be classified as an endemic location of leprosy.

No clear mechanisms for disease transmission defines endemic as “native to”, “confined within”, or “widespread within”, and the word refers to a location or population. When a disease is endemic to an area, that means there is a relatively constant level of infection occurring for a long period of time. This can make the disease difficult to eradicate due to the presence of multiple consistent chains of transmission.  Drs Bhukan, Dunn and Nathoo argue for endemic status in central Florida due to its high proportion of leprosy cases as compared to the rest of the US, and increased cases in Florida residents with no clear mechanism of transmission.

Leprosy, also known as Hansen’s disease, is caused by mycobacterium leprae. M leprae is a non-tuberculous mycobacterium that infects the skin, eyes, mucous membranes and peripheral nerves. It is carried by nine-banded armadillos, which are common in Florida. Those handling armadillos are at increased risk of contracting leprosy, but this can be mitigated with hygiene practices such as the wearing of gloves and frequent hand washing.

Leprosy can also be spread person-to-person, which accounts for the transmission of cases globally. Historically most cases of leprosy in Florida were found in persons migrating from endemic areas, such as parts of Africa, Southeast Asia and Brazil. The proportion of cases found in migrants has decreased significantly since 2002 while cases in US residents have risen.

Treatment halts leprosy but cannot reverse effects

Leprosy is a disease of the skin and nerves, and early symptoms of infection include discolouration of the skin and growths and ulcers which are numb or painless. Further neural issues include muscle weakness and paralysis. Because M leprae reduces or eliminates sensations of pain, there is an increased risk of injuries going unnoticed and causing long-term damage. Eventually the disease can cause blindness, nose disfigurement and the re-absorption of fingers and toes. Treatment can halt the effects of leprosy but cannot reverse them, so early diagnosis is essential.

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The exact mechanism of leprosy transmission is not fully characterised, but the CDC notes that prolonged close contact – over months – with infected individuals is required to contract the disease. One theory of transmission is via fomites in coughs and sneezes, but casual contact and short-term shared space are not considered risk factors for infection.

60,000 cases in US by end of 2023

95% of the population has an innate immunity to M leprae and is therefore not at risk of infection. Leprosy can be cured with a course of multiple antibiotics over a period of six months to two years, and those undergoing treatment are not infectious. The CDC has not issued any travel advisories regarding Florida or surrounding areas due to the low transmission risk to travellers.

Leprosy can be difficult to diagnose. The bacterium is slow-growing, prefers cooler temperatures and cannot be grown in a lab under normal conditions. The disease itself can take three to five years to present symptoms and can often be confused with other illnesses. Diagnosis usually occurs through a process of elimination, due to resistance to treatment. Skin discolouration and the presence of acid-fast bacilli from skin biopsy are used to confirm the diagnosis. Dr. Richard Truman of the US Health Resources and Services Administration’s National Hansen’s Disease Program urges those suspecting leprosy to see their dermatologist.

GlobalData epidemiologists predict that there will be 60,000 diagnosed prevalent cases of non-tuberculosis mycobacterium infections in the US by the end of 2023. M leprae is responsible for only a small proportion of these cases – the National Hansen’s Disease Program reports a maximum of 216 cases annually from 2011 to 2020. Considering the difficulty of transmission and inherent population immunity, GlobalData epidemiologists do not anticipate a significant increase in diagnosed prevalent cases of leprosy. However, there may be delayed diagnoses of current infections of M leprae over the next few years.