The age group above 46 years old prevalent in the group of patients and with 8,7 times greater chances of developing the disease (OR = 8,792) corroborates with previous studies, which show that it is common in Brazil that leprosy has its highest concentration rates among the older age groups (7). The long bacillus incubation period, the lack of diagnosis and / or late diagnosis can be influencing factors on the high number of older people with the disease (8).
Likewise, the multibacillary manifestation in this study stood out in individuals over 46 years old, in line with the research by Nobre et al. (2017), who sought to identify groups at higher risk for the multibacillary form of leprosy in Brazil, in which they describe that more than half of the Brazilian states have the highest numbers of multibacillaries as age increases. The authors associated this relationship of multibacillary activity with older age groups with the hypothesis that leprosy transmission is decreasing and that these cases reflect a transmission that occurred a while ago and not recently, even because M. leprae has a long incubation period (9).
Silva et al. (2018) analyzed the new cases reported from a city in the state of Maranhão (Northeast of Brazil ) in the period between 2003 and 2015 and observed that most individuals aged between 50 and 59 years old (59.2%) and, mostly, over 60 years (72.3%) were within the MB classification. The authors proposed as a possible explanation for the high number of sick people at older ages, in addition to the incubation period, the delay in diagnosing these patients, a factor that even keeps them as active sources of infection transmission for long periods of time. This common delay in the diagnosis of leprosy in developing countries was observed in an epidemiological and clinical study with elderly people over 60 years of age, in southern Brazil, in which almost 40% of the patients were in grade II disability, which is a classic late diagnosis feature (10, 11).
In the population of the present study, the contact group was predominantly female, while in the patient group and in the multibacillary group there was a male predominance (p < 0,05). Likewise, the chances of developing the disease were 2 times greater for males (OR = 2,065), as well as the chances of developing the multibacillary form were 2 times greater for this gender (OR = 2,197). The relationship found by this research between the male sex and the occurrence of leprosy and, specifically, the development of the MB form of the disease, is also observed by research conducted in northeastern Brazil (7). This relationship raises the social issues surrounding men's health, such as the image of virility and incompatibility of the working hours of health units with the free time of work for these individuals, but it may also be influenced by physiological differences between men and women, because testosterone has immunosuppressive activity in both humoral and cellular responses, while estrogen stimulates the production of TNFα, an important cytokine of the Th1 cellular response. Thus, men and women can be equally exposed to the bacillus, with a higher proportion of men among the cases, due to the men having some susceptibility to develop the disease after being infected (12–15).
Another explanation may be due to a greater exposure of men to the environment compared to women, since men are more inserted in professions that come into contact with soil, water and animals. A study carried out in Ceará demonstrated that the fact that the individual is male increased the chances of having M. leprae DNA in the nose by 6,2 times, which may reflect a greater probability of sex in acquiring the disease (16). In a cohort study by Bakker et al. (2006), it was shown that males are twice as likely to develop leprosy as compared to women (17).
Despite the non-specificity of the bacillus Calmette-Guérin (BCG) vaccine for M. leprae, it is approximately 50% effective in protecting against leprosy (3). Brazil, together with Colombia, Peru and Australia, makes up the list of a few countries that use the application of a second dose of BCG vaccine in contacts of new cases of leprosy as a national prophylactic measure. Although not recommended by the World Health Organization (WHO), the application of this vaccination specifically for leprosy prophylaxis is recognized by the WHO for its significant contribution to the decline in the incidence of the disease (5, 18). The BCG vaccine induces the activation of T-cell clones that recognize specific M. leprae epitopes, providing a protective effect against disease progression, including leading to negative PGL-I serological tests that were previously positive (4, 19).
In the present study, 83,33% of healthy household contacts of patients were vaccinated at least once. As for the patients, almost half did not receive any dose of the vaccine (Table 1). The group of individuals who did not take any dose of the vaccine was 8.4 times more likely to develop leprosy (OR = 8,431). Studies point out that the second BCG vaccination increases protection against the disease, reaching a 95% decrease in the relative risk with the application of the second dose (18, 20). Among the patients participating in the present study, only 7,1% had received two doses of the vaccine.
In addition, observing the general vaccination situation of the individuals in the study, there is a divergence by gender. The greater number of female individuals vaccinated in relation to the male sex corroborates the idea that, in general, women are more willing to take care of their health than men (13). A similar result was observed in a survey conducted in the Southeast Brazil, in which the percentage of men who did not receive doses of the vaccine was higher than the percentage of women (21).
Thus, BCG vaccination seems to be an indispensable component of any program that aims to control or eradicate leprosy, because, despite important advances in the study of the molecular biology of M. leprae, specific vaccines against leprosy are still at an early stage initial development and evaluation. Although both BCG vaccination and treatment with the index case reduce the risk of contacts contracting leprosy, the changes in the immune response induced by these two measures, which could explain the resulting protective effect, still need to be investigated in detail. Thus, in view of the complex current difficulties encountered in eradicating this disease, we continue to need urgent measures to reveal the hidden side of the leprosy “epidemiological iceberg” in order to reduce its morbidity and the physical disabilities resulting from this disease.