Identification of asymptomatic Leishmania infections: a scoping review

Abstract

Background: Asymptomatic Leishmania infection may play an important role in the transmission of the parasite in endemic areas. At present there is no consensus on the definition of asymptomatic Leishmania infection, nor is there a safe and accessible gold standard test for its identification.

Methods: This paper presents a scoping review to summarize definitions of asymptomatic Leishmania infection found in the literature, as well as to detail the approach (molecular, serological, cellular, and/or parasitological tests) used by researchers to identify this asymptomatic population. A scoping review of published and gray literature related to asymptomatic Leishmania infection was conducted; retrieved citations were screened based on predefined eligibility criteria, and relevant data items were extracted from eligible articles. The analysis is descriptive and is presented using tables, figures, and thematic narrative synthesis.

Results: We conducted a screening of 3008 articles, of which 175 were selected for the full review. Of these articles, we selected 106 that met the inclusion criteria. These articles were published between 1991 and 2021, and in the last 5 years, up to 38 articles were reported. Most of the studies were conducted in Brazil (26%), Spain (14%), India (12%), Bangladesh (10%), and Ethiopia (7%). Of the studies, 84.9% were conducted in the immunocompetent population, while 15.1% were conducted in the immunosuppressed population (HIV, immunosuppressive drugs, and organ transplantation population). We report 14 different techniques and 10 strategies employed by researchers to define asymptomatic Leishmania infection in an endemic area.

Conclusions: The definition of asymptomatic Leishmania infection is not unified across the literature, but often includes the following criteria: residence (or extended stay) in a Leishmania-endemic area, no reported signs/symptoms compatible with leishmaniasis, and positive on a combination of serological, molecular, cellular, and/or parasitological tests. Caution is recommended when comparing results of different studies on the subject of asymptomatic infections, as the reported prevalence cannot be confidently compared between areas due to the wide variety of tests employed by research groups. More research on the importance of asymptomatic immunosuppressed and immunocompetent Leishmania-positive populations in leishmaniasis epidemiology is required.

Keywords: Asymptomatic; Blood donor; Cellular test; Leishmania; Leishmaniasis; Molecular test; Serological test.

Fig. 1

Flow chart of literature search and screening. *Unclear study characteristics: indicates works where study population inclusion/exclusion criteria, diagnostic methods, and/or overall conceptualization was ambiguous

Fig. 2

Yearly frequency of studies published on asymptomatic Leishmania infection

Fig. 3

Distribution by WHO geographic region and countries of studies included in the scoping review. #A study co-conducted in Spain and Bangladesh; *a study co-conducted in India and Nepal

Fig. 4

Summary of diagnostic tests employed for the detection of asymptomatic Leishmania infection among the included studies

Fig. 5

Studies included in the scoping review using a rapid diagnostic tests (RDT), b ELISA, and c molecular tests for detection of asymptomatic Leishmania infection. Pie slice size represents the percentage of tests of each type with the indicated target/brand, while numerals denote the number of papers represented by each slice

Fig. 6

Network demonstrating comparisons of various tests employed for the detection of asymptomatic Leishmania infection within articles included in the scoping review. In the network diagram, the nodes represent the test type and the edges (lines) indicate that two test types have been compared in at least one study