Towards a Sustainable Vector-Control Strategy in the Post Kala-Azar Elimination Era

Abstract

Visceral leishmaniasis (VL) is a potentially deadly parasitic disease. In the Indian sub-continent, VL is caused by Leishmania donovani and transmitted via the bite of an infected Phlebotomus argentipes female sand fly, the only competent vector species in the region. The highest disease burden is in the northern part of the Indian sub-continent, especially in the state of Bihar. India, Bangladesh, and Nepal embarked on an initiative, coordinated by World Health Organization, to eliminate VL as a public health problem by the year 2020. The main goal is to reduce VL incidence below one case per 10,000 people through early case-detection, prompt diagnosis and treatment, and reduction of transmission using vector control measures. Indoor residual spraying, a major pillar of the elimination program, is the only vector control strategy used by the government of India. Though India is close to its VL elimination target, important aspects of vector bionomics and sand fly transmission dynamics are yet to be determined. To achieve sustained elimination and to prevent a resurgence of VL, knowledge gaps in vector biology and behavior, and the constraints they may pose to current vector control methods, need to be addressed. Herein, we discuss the successes and failures of previous and current vector-control strategies implemented to combat kala-azar in Bihar, India, and identify gaps in our understanding of vector transmission towards development of innovative tools to ensure sustained vector control in the post-elimination period.

Keywords: Bihar; India; Kala-azar; Phlebotomus argentipes; elimination; vector control; visceral leishmaniasis.

Figure 1

Association of indoor residual spray (IRS) coverage with visceral leishmaniasis (VL) case incidence in Bihar, 2007–2018. The incidence curve shows no sign of ‘bending’ either with increased IRS coverage or with the introduction of synthetic pyrethroids (SP) and liposomal amphotericin B (LAmB). Miltefosine, first introduced in 2008, replaced Sodium Stibogluconate as the first line of treatment by 2012. The transition from Miltefosine to LAmB began at the end of 2014 and was completed by the end of 2015. Assessment of IRS coverage, expressed as a percentage (%) of targeted households (HH) sprayed during one round of insecticide application, was conducted from 2014 onwards, through large-scale household surveys conducted by specially constituted measurement teams of CARE India, independently of program implementers, after each round of IRS. IRS coverage before 2014 (lighter shaded columns) is extrapolated from available data at 30–40%, and based on observations of all components of IRS operations during that period. R1 and R2 refer to the first and second rounds of spraying during the year. Data sources: CARE India IRS coverage surveys; Kala-azar Management Information System (KAMIS), National Vector Borne Disease Control Program (NVBDCP).

Figure 2

Potential role of indoor residual spraying (IRS) in controlling transmission of visceral leishmaniasis by Phlebotomus argentipes sand flies. Arrows indicate all possible scenarios of visceral leishmaniasis transmission taking into consideration the change in Ph. argentipes behavior from endophilic and endophagic to exophilic and exophagic. The accepted peridomestic transmission cycle assumes that Phlebotomus argentipes has an endophagic-endophilic behavior (red-solid lines); in this scenario indoor residual spraying is effective (green tick). Sand flies with an exophagic–endophilic behavior (orange-dotted lines) may rest inside the walls of IRS-sprayed houses and cattle sheds after taking a blood meal outdoors; in this scenario IRS is also effective. Other suspected scenarios of sylvatic transmission include a change in Ph. argentipes behavior from endophilic to exophilic, where a sand fly would rest outdoors after taking a blood meal indoors (endophagic–exophilic:blue-dotted lines), or outdoors (exophagic–exophilic:green-solid lines); in these two scenarios IRS is not effective (red cross).