Visceral leishmaniasis and arsenic: an ancient poison contributing to antimonial treatment failure in the Indian subcontinent?

Antimony and arsenic are elements that have a long history of use as poisons, therapeutic agents, or cosmetics. For over a century, compounds containing pentavalent antimony (antimonials) have formed the basis of treatment of the leishmaniases worldwide. Antimonial preparations remain first-line drugs for visceral leishmaniasis in Sub-Saharan Africa and Brazil [1], but in the hyperendemic state of Bihar, India, the cure rate of antimonial compounds has declined over the past 30 years from over 85% to less than 50% (Figure 1) [2] and resistance in parasites has been demonstrated [3]. This marked decrease in efficacy has been attributed to long-term, widespread misuse of antimonials, with patients undergoing inappropriate treatment courses often administered by the largely unregulated Indian private health care system [4], [5]. Here we propose an additional hypothesis to explain the substantially lower efficacy of antimony in Bihar compared to other regions in the world. Figure 1 Results of consecutive clinical studies of antimonials at 20 mg/kg dosing in Bihar, India. Since the 1970s, millions of inhabitants of Asia have been chronically exposed to naturally occurring arsenic on a daily basis through the large-scale insertion of multiple shallow tubewells which were originally installed for provision of clean and safe drinking water [6]. Antimony and arsenic are metalloids belonging to Group 15 of the periodic table that share many structural and chemical properties [7]. Antimony resistance in Leishmania parasites can be induced experimentally by exposure to stepwise increasing concentrations of sublethal concentrations of trivalent arsenite in culture [8]. If an individual who is chronically exposed to arsenic is infected with Leishmania, the parasites would be exposed to arsenic due to its presence within organs of the lymphoreticular system [9]. This could lead to the development of an arsenic-resistant Leishmania strain that would be cross-resistant to antimonial therapy. This viewpoint article will expand on, and discuss the evidence for, the possible contribution of arsenic to decreased antimonial efficacy in Bihar.