Opinion New Strategies and Biomarkers for the Control of Visceral Leishmaniasis Ana V. Ibarra-Meneses, 1 Javier Moreno, 1, * and Eugenia Carrillo 1 Effective diagnosis and treatment of visceral leishmaniasis, together with the study of vectors and reservoirs, can lead to a better understanding of the parasite transmission dynamics and the development of more efficient control measures. Recent studies have applied new method- ologies and biomarkers, and these have contributed to the early and rapid diagnosis of the dis- ease; assessment of success of pharmacological treatments; efficient monitoring of immunosup- pressed individuals; and to population screening for field trials of vaccine efficacy. This opinion article proposes an update to the diagnostic tools for visceral leishmaniasis and their rational and combined use to establish the real prevalence of infection or of exposure to Leishmania in endemic areas. Unveiling the Complexity of Visceral Leishmaniasis Leishmaniasis is a vector-borne infectious disease caused by parasites of the genus Leishmania. Globally distributed, it is poverty-related and is among the deadliest of the neglected tropical dis- eases (NTDs). Visceral leishmaniasis (VL), caused by Leishmania. donovani and Leishmania infantum, is the most severe clinical form. It affects internal organs and is fatal in 95% of cases if not successfully treated. On some occasions, after an episode of VL caused by L. donovani, the patient may develop a post-kala-azar dermal leishmaniasis (PKDL). So far, little is known about the mechanism by which a patient with VL develops PKDL [1]. The overall incidence of VL has declined in recent years, mainly because of the elimination efforts carried out in South Asia [2]. However, the incidence of VL has increased alarmingly in the Americas, where the recent report by the Pan American Health Organiza- tion (PAHO) and the World Health Organization (WHO) indicates that VL is expanding geographi- cally: the number of cases has increased by 26.4%, while the fatality rate and number of deaths have grown progressively since 2014 [3]. In addition, epidemic outbreaks have appeared in Europe, the Indian subcontinent, and Eastern Africa [4–7]. The transmission dynamics (see Glossary) of Leishmania are complex and variable, and are depen- dent on environmental conditions, the distribution and biology of the vector, the reservoirs involved, and on the health, social, and economic aspects that affect the human host [2]. In the absence of an effective vaccine, the control of VL has been based on the prevention of sand fly bites, the elimination of animal reservoirs (if the VL is zoonotic), and the early detection and effective treatment of human cases [8]. Nevertheless, in regions endemic for VL, most infected individuals remain asymptomatic. Their possible role as ’parasite carriers’ with capacity to infect sand flies has been suggested and is under active consideration [9]. Individuals who suffered previous VL infections for which treatment was not fully effective can also remain asymptomatic and subsequently relapse later. Additionally, im- munosuppressed patients can remain asymptomatic after VL therapy, because they receive second- ary prophylaxis; however, they can still act as reservoirs, as confirmed by xenodiagnoses [10]. This complex scenario for the transmission dynamics of VL makes it even more difficult to establish effective control measures, and highlights the clear need for improved tests that are able to distin- guish between all the different conditions (Box 1).  Diagnostic tests have to provide an immediate, reliable, confirmatory diagnosis of active VL cases independently of a central laboratory. Improved tests are also necessary to assess treat- ment success, a fundamental measure for predicting and avoiding relapses. This requires a specific test that goes beyond the clinical recovery of the patient, and the nondetection of the parasite, and confirms cure [11,12]. 1 WHO Collaborating Centre for Leishmaniasis, National Centre for Microbiology, Instituto de Salud Carlos III, Majadahonda (Madrid), Spain *Correspondence: javier.moreno@isciii.es Highlights Leishmaniasis is a complex of dis- eases, transmitted by sand flies, that mainly affects poor and ne- glected populations. Leishmania infection presents different situations requiring different diagnostic approaches. Faster, simpler, more sensitive, and more robust field tools are needed for early diagnosis of visceral leishmaniasis and monitoring of patients during and after drug treatment. Very little is known about asymp- tomatic Leishmania infections, although they represent 25–80% of the infections. It is important to identify these in order to determine the real prevalence of leishmaniasis. Trends in Parasitology, xx, Vol. xx, No. xx https://doi.org/10.1016/j.pt.2019.10.005 ª 2019 Elsevier Ltd. All rights reserved. 1 Please cite this article in press as: Ibarra-Meneses et al., New Strategies and Biomarkers for the Control of Visceral Leishmaniasis, Trends in Para- sitology (2019), https://doi.org/10.1016/j.pt.2019.10.005 Trends in Parasitology