MATHEMATICAL MODELS :
The dynamics of transmission and Control of Lymphatic filariasis is influenced by at least 21 parameters. Direct estimates are available only for 8 parameters and the rest are to be estimated indirectly by using mathematical models.
LYMFASIM: a stochastic micro-simulation model, has been developed in collaboration with the Department of Public Health, Erasmus University Rotterdam, The Netherlands. The model describes the individual life histories of human and the parasite and simulates the course of prevalence and intensity of infection in a dynamically changing population.
Different control strategies can be simulated and optimized for coverage, duration and frequency of treatment, pattern of compliance under chemotherapy, and impact of vector control on the transmission and prevalence of infection. The model parameters have been quantified by simulating the impact of 5-year Integrated Vector Management programme in Pondicherry. The predicted trends showed that at least 13 years of vector control is required to reach zero prevalence of infection. This model has been used for determining the risk of acquiring new infection with different levels of coverage and duration of mass treatment with DEC
Costing model: has been developed using unit cost menu to cost interventions against filariasis. The model is useful to assess the impact of control programmes by comparing costs and effectiveness based on LYMFASIM predictions. This can be used as a decision support tool to optimize the programme by rationalizing the input and improving the effectiveness.
EPIFIL: is a deterministic model developed jointly with the Parasite Epidemiology group, Oxford University, U.K. This model assumes the acquisition of immunity is a function of infective larvae and worms cause damage to lymphatic system leading to disease progression. This model has been used to quantify the relationship between parasite population dynamics and morbidity of lymphatic filariasis.
Probability matrix : was constructed using the relationship between community mf-load (cmfl), vector density and risk of infection index (RII). This matrix can serve as a ready reckoner for deciding intervention strategy depending on vector density and cmfl in an endemic situation. In an endemic area, parasite control should be initiated to bring down the cmfl < 2 and maintain resting density < 30 per man hour by introducing vector control so as to interruption of transmission.
Delimitation and epidemiological mapping of filariasis: Development of rapid assessment procedures (RAP) for delimitation of filariasis endemic areas is important since the traditional night blood examination is cumbersome, costly and unpopular. Two rapid assessment procedures, viz., questionnaire method (QM) using key informants and physical examination by the health workers (PEHW) were developed and compared with conventional methods of case detection. QM is most reliable in endemic areas with mf prevalence over 5%. In view of low cost and more reliability, PEHW is suitable for rapid delimitation of areas for bancroftian lymphatic filariasis.
A grid sampling technique:
A grid sampling technique (with grid size of 25X25 Km) using RAP including ICT day blood antigeneamia, covering an area of 200 X 200 Km2 spread over 13 districts in four states was developed for Rapid Epidemiological Mapping of Filariasis (REMFIL). A sequential application of the RAP (QM followed by PEHW followed by ICT) is recommended for identification of endemic areas. No significant auto-correlation of filariasis prevalence was observed in the study area. This suggests that either absence of spatial pattern or if present, that may capture in grid sizes smaller than 25 Km2. QM is cheaper (US $ 4) followed by PEHW (US $ 6) and ICT (US $ 79)