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Backgrounds: ultibacillary patients are the major source of infection in leprosy. Nevertheless, the risk is higher in household contacts between multibacillary patients than paucibacillary patients and in the general population. Household contacts are in close genetic relationship with the index case-patient., Objective: To evaluate epidemiological data of the following variables: age, gender, education level, genetic proximity, and type of contact with the index case-patient (household or not) in 107 families with leprosy., Methods: Home visits were conducted to clinically examine family members. The medical charts of index case-patients and co-prevalent cases were reviewed., Results: The controlled analysis of variables such as type of contact and genetic proximity revealed that household contacts and first-degree kinship are independently associated with a higher chance of contracting the disease., Conclusion: Household contacts are often genetically closer to the index case-patient. To investigate the independent relevance of these risks in leprosy surveillance contact studies has been a challenge. Our results confirm literature data that show the influence of genetics in the susceptibility to leprosy per se.

The lysogenic capacity of human macrophages facing M. leprae in vitro may be dependent on an important genetic component. Although the family aggregation of the trait is demonstrated, this is a necessary but not sufficient condition to prove genetic influence. The data do not fit some simple genetic models (autosomal dominant or incompletely dominant gene; dominant or recessive sex-linked gene). The results obtained are consistent with the hypothesis that the macrophages' lysogenic capacity is mainly due to a major gene with variable expressivity. This hypothesis may be too simple to account for the whole variability detected and therefore must be considered just as a working hypothesis.

The in vitro phagocytic capacity of human macrophages facing M. leprae was analysed in a selected sample of healthy and hanseniasis patients (Virchowian and tuberculoid) individuals. Families where at least one of the spouses presented Hansen's disease were selected and 176 white people were studied. The analysis of the macrophages' behavior was performed by reading the slides in the 5th, 10th, 15th, 20th and 30th days after inoculation of M. leprae. The classification is discussed. The results showed a high variability. The main conclusions reached are: (a) people differ in their capacity to lyse M. leprae, as measured by their macrophages' behavior in vitro; (b) the results obtained give support to Beiguelman's theory of the lysis threshold.