SUMMARYMathematical models of transmission dynamics of infectious diseases provide a useful tool for investigating the impact of community based control measures. Previously, we used a dynamic (constant force-of-infection) model for lymphatic filariasis to describe observed patterns of infection and disease in endemic communities. In this paper, we expand the model to examine the effects of control options against filariasis by incorporating the impact of age structure of the human community and by addressing explicitly the dynamics of parasite transmission from and to the vector population. This model is tested using data for Wuchereria bancrofti transmitted by Culex quinquefasciatus in Pondicherry, South India. The results show that chemotherapy has a larger short-term impact than vector control but that the effects of vector control can last beyond the treatment period. In addition we compare rates of recrudescence for drugs with different macrofilaricidal effects.

In order to explore the relationship between acute and chronic disease, age-specific data on the frequency and duration of episodic adenolymphangitis (ADL) in patients with 3 defined grades of lymphoedema in bancroftian filariasis were examined. The age distribution of grades I and II exhibited a convex age profile, but that of grade III showed a monotonic increase. The mean duration of oedema increased with its grade (grade I, 0.3 years; grade III, 9.9 years). The mean number of ADL episodes in the previous year for all cases was 4.2 and it increased with grade (grade I, 2.4 and grade III, 6.2). The mean duration of each ADL episode for all cases was 4.1 d and it was independent of grade and age. The mean period lost to ADL episodes in the previous year was 17.5 d; it increased from 9.4 d with grade I to 28.5 d with grade III. The results imply that there is a dynamic progression through the grades of lymphoedema and that the frequency of ADL episodes is positively associated with this progression. However, the study design could not separate cause from effect.

Abstract. The lack of a quantitative framework that describes the dynamic relationships between infection and morbidity has constrained efforts aimed at the community-level control of lymphatic filariasis. In this paper, we describe the development and validation of EPIFIL, a dynamic model of filariasis infection intensity and chronic disease. Infection dynamics are modeled using the well established immigration-death formulation, incorporating the acquisition of immunity to infective larvae over time. The dynamics of disease (lymphodema and hydrocele) are modeled as a catalytic function of a variety of factors, including worm load and the impact of immunopathological responses. The model was parameterized using age-stratified data collected from a Bancroftian filariasis endemic area in Pondicherry in southern India. The fitted parameters suggest that a relatively simple model including only acquired immunity to infection and irreversible progression to disease can satisfactorily explain the observed infection and disease patterns. Disease progression is assumed to be a consequence of worm induced damage and to occur at a high rate for hydrocele and a low rate for lymphodema. This suggests that immunopathology involvement may not be a necessary component of observed age-disease profiles. These findings support a central role for worm burden in the initiation and progression of chronic filarial disease.Lymphatic filariasis continues to be a significant source of chronic morbidity in the developing world, with more than 120 million people currently thought to be infected with either Wuchereria bancrofti or Brugia malayi, the major lymph-dwelling filariae of humans. 1,2 Despite the considerable advances in intervention options, 3 attempts to control the infection have met with variable success, partially because of inadequate understanding of the epidemiology of transmission and disease. 4,5 This gap in epidemiologic understanding stems from the intricate relationships between infection, immunity and the development of filarial disease. [3][4][5][6][7] This situation may also reflect the lack of a quantitative framework to assist understanding of the long-term effects and costs of intervention. 8 Such mathematical frameworks have played an important role in improving understanding of the epidemiology and control of other helminthiases, including intestinal nematodiases, 9-11 onchocerciasis, 12 and schistosomiasis. 13 Simple epidemiologic models, based on the catalytic models devised by Muench, 14 have been in existence for filariasis since the 1960s.15-17 These models have provided useful insights into the dynamics of filariasis in human populations, including improving the understanding of the roles of host immunity and parasite biology in the epidemiology of infection. Less work exists on the modelling of filarial disease, 5,6,18 despite the importance of morbidity models in improving understanding of the health impacts of parasite control and thus the ability to rationally evaluate different control options in s...

SummaryThis year-round case-control study investigated treatment costs and work time loss to people affected by chronic lymphatic filariasis in two rural communities in south India. About three-quarters of the patients sought treatment for filariasis at least once and 52% of them paid for treatment, incurring a mean annual expenditure of Rs. 72 (US $ 2.1; range Rs. 0-1360 (US $ 39.0)). Doctor's fees and medicines constituted 57% and 23% of treatment costs. The proportion of people seeking treatment was smaller and treatment costs constituted a higher proportion of household income in lower income groups. Most patients did not leave work, but spent only 4.36 Ϯ 3.41 h per day on economic activity compared to 5.25 Ϯ 3.52 h worked by controls; the mean difference of 0.89 Ϯ 4.20 h per day was highly significant (P Ͻ 0.01). This loss of work time is perpetual, as chronic disease manifestations are mostly irreversible. An estimated 8% of potential male labour input is lost due to the disease. Regression analyses revealed that lymphatic filariasis has a significant effect on work time allotted to economic activity (P Ͻ 0.05) but not on absenteeism from work (P Ͼ 0.05). Female patients spent 0.31 Ϯ 1.42 h less on domestic activity compared to their matched controls (P Ͻ 0.05). The results clearly show that the chronic form of lymphatic filariasis inflicts a considerable economic burden on affected individuals. keywords lymphatic filariasis, economic, social, disability, rural areas correspondence K.D. Ramaiah, Vector