The burden of visceral leishmaniasis in South Asia

Abstract

This supplement presents a collection of studies that were undertaken in India and Nepal on the burden of visceral leishmaniasis (VL), investigating issues related to the epidemiology and economic impact of the disease. Since 2005, the governments of India, Nepal and Bangladesh are engaged in a collaborative effort to control and eliminate visceral leishmaniasis from the Indian subcontinent. With 60% of the global VL burden in South Asia, this disease remains an important public health problem in the region. VL not only leads to tremendous human suffering since untreated cases result in death, but also has a profound impact on the livelihood of affected households (Boelaert et al. 2009). Renewed interest given to VL over the past decade resulted in important innovations in diagnosis and treatment. However, many aspects of the VL burden of disease are still not clear. One of the major challenges for the elimination initiative is the absence of reliable epidemiological surveillance data on VL morbidity and mortality in the region. Without these data it is difficult to assess whether the initiative is achieving its stated goal – the reduction of the annual incidence of VL to <1/10 000 population at the district level in Nepal, and the sub-district and upazila level in India and Bangladesh respectively (World Health Organization 2005) – or whether the various control interventions have their intended effect. The current officially reported figures are obtained through passive case detection in government health services and usually do not include cases detected by the private for-profit sector, which constitutes a majority of the health providers in the Indian subcontinent. These figures therefore largely underestimate the actual number of cases. For example Singh et al. (2006) documented underreporting by a factor of eight in a community development block in Muzaffarpur district in Bihar, India in 2001–2003 and more recently (Singh et al. 2010c) estimated underreporting by a factor of four in Vaishali district, also in Bihar. This ‘decrease’ in underreporting is attributed by some to the effect of the VL elimination initiative and the recent contribution of several not-for-profit NGOs operating in the area with specific VL care programs. Population-based surveys such as those described above provide us with valuable information on local incidence rates, and repeating these surveys over time might allow us to not only monitor progress of the VL elimination initiative, but also to assess the effectiveness of VL control strategies. This supplement includes several populationbased surveys that can shed further light on the true frequency of infection and disease in the region. VL incidence rates (and underreporting) vary greatly between countries (Mondal et al. 2009) as well as between districts and within districts as shown by Das et al. (2010) in this supplement. Moreover, VL cases tend to cluster within certain sections of the village at hamlet level and further at household level (Ahluwalia et al. 2003). Incidence rates can be ten times greater at hamlet level (in the range of 1– 2% per year) than the aggregate figures recorded at district level (in the range of 1–2 per 1000 per year). Some of the work presented in this supplement by Singh et al. (2010b) and Rijal et al. (2010) has documented precisely what the burden is in such so-called ‘hot spots’ at hamlet level in endemic districts. The impact of VL in these affected communities tends therefore to be much stronger than would seem from aggregate figures, because those communities tend to be the most deprived ones, of lowest cast, and lowest economic status (Boelaert et al. 2009). Singh et al. also point to some gender-specific differences in burden of infection, a theme which is under-researched so far, apart from work in Bangladesh (Ahluwalia et al. 2003). Our understanding of the risk factors for VL in SouthAsia has greatly improved in recent years. Identified risk factors have been fairly similar across studies, with the presence of a previous case of kala-azar in the household as the strongest predictor of risk and living conditions as a recurrent theme. Killick-Kenrick coined the term ‘paradise for sandflies’ to describe the housing conditions in endemic villages. Damp earthen floors, mud-plastered walls with cracks, proximity to water bodies, soil littered with organic matter, sleeping on the ground etc. are all linked to poverty as well as to increased exposure to peri-domestic sandflies. Most of the published studies assessed the risk for VL Tropical Medicine and International Health doi:10.1111/j.1365-3156.2010.02564.x