Transmission of leprosy within households.
As part of the leprosy control program, population follow-up studies were carried out during 1962 to 1970 in Gudiyatham Taluk, an administrative unit in South India (population: 400,000). More than 97% of the 23,285 contacts from 5,088 families having a leprosy patient were clinically examined using accepted methodology and confirmed as having or not having leprosy. New cases developing among household contacts of leprosy (secondary attack rates) were determined and studied in relation to characteristics of the index case and that of contacts. The secondary attack rate is defined as the number of new cases discovered in the contacts of leprosy patients per 1,000 person-years, which is equivalent to observing 1,000 persons per year. Nearly two-thirds of all new cases were of the tuberculoid type of leprosy and another one-sixth of indeterminate type. Lepromatous and borderline cases each constituted about ten percent of the total new cases. The total secondary attack rate was 6.8 per 1,000 person-years. Compared to an annual incidence rate of 0.8 per 1,000 in the total population, this incidence rate among the contacts is nearly ten times higher. Such enhanced risks are observed clearly and consistently when studied by the number of patients within a family for both sexes and in various age-groups. The rate for females (6.3 per 1,000 person-year) though less, was not significantly different from that of males (7.1 per 1,000 person-year). The differences observed between males and females for each type of leprosy were also not statistically significant. The risks for children (less than 15 years) are significantly higher than those for adults. Among boys, the maximum risk was observed in the age-group 5-14 years, whereas for females the risk is highest in the age-group 5-9 years, dropping down significantly after that. Furthermore, it was observed that significant differences existed between children and adults only in males but not in females. The secondary attack rates almost doubled when there were multiple index cases in the family. Regardless of the number of index cases, the male-female differences were not statistically significant. Attack rates were significantly enhanced when there was a bacilliferous type of leprosy (lepromatous or borderline) in the family. This was true for the specific attack rates of each type of leprosy too. However, a significantly higher proportion of lepromatous and borderline types is also seen when there is a bacilliferous type of leprosy present. The study reiterates the differences in susceptibility to leprosy among males and females, especially during younger ages. Further immunological studies are necessary to determine the differences in host responses in males and females that produce such a characteristic sex-ratio in prevalence of leprosy. There is still a great need to obtain more data on incidence rates both in general population and among contacts on the basis of prospective observation using acceptable statistical technics in design and analysis.