SIMLEP: a simulation model for leprosy transmission and control

SIMLEP is a computer program for modeling the transmission and control of leprosy which can be used to project epidemiologic trends over time, producing output on indicators such as prevalence, incidence and case-detection rates of leprosy. In SIMLEP, health states have been defined that represent immunologic conditions and stages of leprosy infection and disease. Three types of interventions are incorporated: vaccination, case detection and chemotherapy treatment. Uncertainties about leprosy have led to a flexible design in which the user chooses which of many aspects should be included in the model. These aspects include natural immunity, asymptomatic infection, type distribution of new cases, delay between onset of disease and start of chemotherapy, and mechanisms for leprosy transmission. An example run illustrates input and output of the program. The output produced by SIMLEP can be readily compared with observed data, which allows for validation studies. The support that SIMLEP can give to health policy research and actual decision making will depend upon the extent of validation that has been achieved. SIMLEP can be used to improve the understanding of observed leprosy trends, for example, in relation to early detection campaigns and the use of multidrug therapy, by exploring which combinations of assumptions can explain these trends. In addition, SIMLEP allows for scenario analysis in which the effects of control strategies combining different interventions can be simulated and evaluated.

Lot quality assurance sampling (LQAS) for monitoring a leprosy elimination Program

In a statistical sense, prevalences of leprosy in different geographical areas can be called very low or rare. Conventional survey methods to monitor leprosy control programs, therefore, need large sample sizes, are expensive, and are time-consuming. Further, with the lowering of prevalence to the near-desired target level, 1 case per 10,000 population at national or subnational levels, the program administrator's concern will be shifted to smaller areas, e.g., districts, for assessment and, if needed, for necessary interventions. In this paper, Lot Quality Assurance Sampling (LQAS), a quality control tool in industry, is proposed to identify districts/regions having a prevalence of leprosy at or above a certain target level, e.g., 1 in 10,000. This technique can also be considered for identifying districts/regions at or below the target level of 1 per 10,000, i.e., areas where the elimination level is attained. For simulating various situations and strategies, a hypothetical computerized population of 10 million persons was created. This population mimics the actual population in terms of the empirical information on rural/urban distributions and the distribution of households by size for the state of Tamil Nadu, India. Various levels with respect to leprosy prevalence are created using this population. The distribution of the number of cases in the population was expected to follow the Poisson process, and this was also confirmed by examination. Sample sizes and corresponding critical values were computed using Poisson approximation. Initially, villages/towns are selected from the population and from each selected village/town households are selected using systematic sampling. Households instead of individuals are used as sampling units. This sampling procedure was simulated 1000 times in the computer from the base population. The results in four different prevalence situations meet the required limits of Type I error of 5% and 90% Power. It is concluded that after validation under field conditions, this method can be considered for a rapid assessment of the leprosy situation.

Trends in leprosy case detection rates

BACKGROUND: A systematic review of the trends in leprosy incidence is lacking. The question of whether leprosy transmission has declined remains, therefore, unanswered. This study investigates trends in new case detection rates (NCDRs) in selected leprosy-endemic areas from different continents. METHODS: A literature search using specific inclusion criteria was performed. Average annual rates of change in NCDRs were obtained by exponential curve fitting. The variation in trends within individual areas was investigated using direct and indirect information on leprosy control activities. RESULTS: This review covers 16 areas in the Pacific, Asia, Africa and Latin America. For 10 out of the 16 areas, the trend was seen to be declining consistently over the last 10 years or longer. Near stabilization or stabilization after decline was observed for two areas. For three areas, interpretation of recent NCDRs was difficult due to changes in control, but two of them showed a decline over the study period. A consistently increasing trend was observed over the last 20 years in the one remaining area. The observed downward trends could not be attributed to reduced control activities or changed diagnostic criteria. A general acceleration of downward trends in the NCDR after the introduction of multidrug therapy (MDT) has not so far occurred. CONCLUSION: Our main conclusion is that despite many differences between the studies and study areas, the review demonstrates a considerable tendency of downward NCDR trends. Lack of information and changing control conditions necessitate caution in interpreting NCDR trends in individual areas. A general impact of MDT on NCDR trends is so far not visible. The coming years will be crucial for MDT-based control to prove its ability to reduce leprosy incidence.

Regional lymphadenitis following antileprosy vaccine BCG with killed mycobacterium leprae

Phase-II and extended Phase-II studies were conducted in three different sets of the population in Thiruthani Taluk, Chengalpattu District, South India, involving BCG and killed Mycobacterium leprae (KML) combination vaccines to ascertain the acceptability of the vaccines. In the Phase-II study, 997 healthy volunteers were vaccinated on individual randomization with one of the vaccines arms: BCG 0.1 mg + 6 x 10(8) KML, BCG 0.1 mg + 5 x 10(7) KML, BCG 0.1 mg + 5 x 10(6) KML, BCG, 0.1 mg or normal saline. Blood samples were taken and the serum was tested for antibody levels against phenolic glycolipid-I (PGL-I) and the 35-kDa protein of M. leprae. In this study, we observed regional suppurative adenitis in 6% (6 out of 100), 3% (3 out of 100), and 3% (3 out of 100) of the vaccinees in the BCG 0.1 mg + 6 x 10(8) KML, BCG 0.1 mg + 5 x 10(7) KML, and BCG 0.1 mg + 5 x 10(6) KML vaccine arms, respectively, in the 13-70 year age group. Earlier BCG scar status, skin-test reactions to lepromin-A, Rees' MLSA, and serum antibody levels against PGL-I and the 35-kDa protein did not help to identify the group at risk of developing suppurative adenitis. Suppurative adenitis appears to have a different relationship between the age of the subject and the dose of the vaccine. In order to overcome the problem of regional suppurative adenitis and to know the mechanism involved, an extended Phase-II study was conducted in similar groups of the population by reducing the BCG and KML doses, i.e., with BCG 0.05 mg + 6 x 10(8) KML, BCG 0.05 mg + 5 x 10(7) KML, and BCG 0.01 mg + 5 x 10(7) KML. Biopsy specimens were collected from lymph nodes of the suppurative adenitis cases and were subjected for culture and histopathological examination. The observations showed that regional suppurative adenitis could be reduced to 1% in the BCG 0.05 + 6 x 10(8) KML group, 0.5% in the BCG 0.05 + 5 x 10(7) KML group, and 0.5% in the BCG 0.01 + 5 x 10(7) KML group. This phenomenon of suppurative adenitis appears to be related to the total dose of mycobacterial antigens. Suppurative adenitis was seen by weeks 18 and 20 post-vaccination in the latter two lower doses; whereas it was seen by week 8 in the higher dose of the combination vaccines. No case of suppurative adenitis was observed in the BCG 0.1 mg group. Culture and histopathology ruled out the possibilities of progressive BCG infection and superadded infection. Considering the above results, BCG 0.05 mg + 6 x 10(8) KML was acceptable for a large-scale vaccine trial in South India.