Persistence of Mycobacterium leprae in the oral mucosa and its importance in transmission to healthcare workers

Hansen's disease (HD) is a chronic, infectious, curable, compulsorily notifiable disease caused mainly by Mycobacterium leprae which affects especially the skin, peripheral nerves and mucosa.⁽¹⁾

Although there are some controversies, studies indicate that M. leprae infects the body through the nasal cavity, this being the primary site of infection.⁽²⁾ The oral mucosa is another route of entry and exit of the bacillus in humans which has a potentially important role in the transmission of the microorganism.^(3,4,5)

Colonisation of the oral mucosa occurs through the presence of resistant alcohol-acid bacilli in secretions from the nasopharynx and oropharynx which reach the oral mucosa. M. leprae may be present in the oral mucosa without any morphological alteration or clinical manifestations and, because of the resistance of this tissue to the development of lesions, only more sensitive laboratory tests can detect the presence of the bacillus.⁽³⁾

Some authors do not consider the oral cavity to be a relevant site for M. leprae transmission,⁽⁶⁾ even though a person affected by the untreated multibacillary form of HD can eliminate a large quantity of bacilli into the environment by speaking, coughing, sneezing or spitting.⁽²⁾ These microorganisms can survive for several days in the environment under certain conditions and consequently infect people or animals.^{(7, 8)}

Some infected people who have not developed HD represent a subclinical stage. These individuals would be sources of bacilli dissemination because they present a transitory period of bacillary excretion.⁽²⁾ However, they present a lower risk when compared to multibacillary forms of the disease. An important factor for the manifestation of the disease is the immunological susceptibility of the individual - M. leprae has high infectivity and low pathogenicity, meaning that it has the capacity to infect a great number of people, but few develop the disease.⁽⁹⁾

Lack of knowledge about the definitive aspects of M. leprae transmission promotes apprehension among some healthcare professionals around the risk of contamination through direct contact with lesions.⁽¹⁰⁾ This misinformation leads many healthcare professionals to prioritize physical distance from the patient rather than the use of individual protection,⁽¹¹⁾ an approach that is contrary to the fight for the end of the stigma about the disease. World Health Organization and Brazilian biosecurity regulations stipulate the use of individual protection equipment.^{(12, 13)} This minimizes the risk of cross-infection, especially when performing invasive and aerosol generating procedures on untreated patients affected by the multibacillary form of HD.⁽⁹⁾ It is important to remember that dentists, doctors and nurses, especially those working in HD-endemic regions often come into contact with people diagnosed with HD who are not receiving multidrug therapy.⁽¹⁴⁾

The presence of non-specific oral and dental conditions such as periodontal disease and tooth loss, and specific signs such as papules, plaques and ulcerations in the oral cavity, are common clinical events in the course of HD, mean that dental surgeons have an important role in HD control programmes. The use of ultrasonic rotating instruments and working in the oral cavity means that dentists are particularly exposed to M. leprae in oral mucosa. Here the adoption of biosafety methods, such as the use of gloves, masks, aprons, and goggles during procedures that produce aerosols minimizes the risk of infection. The identification of individuals suspected of HD during dental care is paramount, and patients should be referred to a doctor who can confirm a diagnosis and start treatment or exclude the possibility of the disease.⁽⁹⁾