Leishmania parasites are transmitted through the bites of infected female phlebotomine sand flies, which feed on human blood to produce eggs. An estimated 31 species of Phlebotomus and 47 species of Lutzomyia sand fly species are considered the proven vectors of human leishmaniases (visceral and cutaneous).
In its global efforts to control the impact of leishmaniases, the World Health Organization (WHO) has published for the first time two key guidance documents for programmes and researchers: a report of a multi-centre laboratory study and WHO expert consultations on determining discriminating concentrations of insecticides for monitoring resistance in sand flies,1 and an operational manual on leishmaniasis vector control, surveillance, monitoring and evaluation.2
Vector control in the fight against leishmaniasis prevention, control and elimination is perceived to be the weakest component in terms of inadequate resources, scientific evidence of the usefulness of interventions and low capacity for quality-assured implementation. The operational manual provides practical tools, techniques and procedures to strengthen sand fly control and surveillance in order to improve the implementation of leishmaniases control programmes. The manual provides a rationale for programme managers in different geographical regions on the types of vector control interventions to be used in different epidemiological and environmental settings and how to measure their impact. Its scope also includes concepts of vector surveillance, methods and procedures, and policy considerations for insecticide resistance monitoring and management.
“We have endeavoured to prepare a global manual to address key aspects of sand fly vector surveillance, control, monitoring and evaluation. This is a much-awaited manual on this subject,” said Dr Saurabh Jain, Scientist and Focal Point for Visceral Leishmaniasis, WHO Department of Control of Neglected Tropical Diseases (WHO/NTD). “This work is the result of extensive global expert consultations and contributions, and a peer review conducted in all six WHO regions.”
“Concepts and standard operating procedures explained in this manual will assist national leishmaniasis control programmes in setting up key reporting variables, especially in the prevention and control of cutaneous leishmaniasis,” said Dr José Antonio Ruiz Postigo, Medical Officer, Skin NTDs, disability management and inclusion, WHO/NTD. “We look forward to receiving inputs from national programmes, vector control implementers, researchers and academia on the usefulness of this manual.”
New study results on determining discriminating concentrations of insecticides for monitoring resistance in sand flies
Insecticide resistance is monitored in field populations of mosquitoes by conducting bioassays with filter papers impregnated with a standard concentration of an insecticide (known as the diagnostic or discriminating concentration, DC).3
Although there is a global database on the resistance of malaria vectors to insecticides, none is available for sand flies. A major limitation to monitoring the frequency of phenotypic resistance of wild populations of sand flies is the lack of WHO standard test procedures and of availability of filter papers impregnated with insecticide-discriminating concentrations for major sand fly vector species. Furthermore, no WHO-validated bottle bioassay was available to test the susceptibility of sand flies to insecticides that cannot be impregnated onto filter papers, and a standard method had to be developed.
WHO conducted a multi-centre laboratory study in 2020–2022 to determine the DCs of selected insecticides in major class groups that are used for leishmaniasis vector control. Laboratories participating in this study belonged to the African Region, the Region of the Americas, the South-East Asia Region, the European Region and the Eastern Mediterranean Region.
“These two documents fill a major historical gap in WHO’s guidance for vector surveillance and control, and monitoring insecticide resistance in field populations of sand fly vectors in line with the Global Vector Control Response 2017–2030,” said Dr Rajpal Singh Yadav, Scientist, Veterinary Public Health, Environment and Vector Control, WHO/NTD, who coordinated the multi-centre study.
“Entomologists, researchers and the public health community can benefit by procuring insecticide test papers and carrying out resistance studies before implementing insecticide-based control programmes,” added Dr Raman Velayudhan, Unit Head, Veterinary Public Health, Environment and Vector Control, WHO/NTD. “We hope that Member States will generate and share data to create a global database on sand fly resistance and develop threats maps for resistance management.”
Leishmaniasis – the disease
Leishmaniasis is caused by a protozoan parasite from over 20 Leishmania species. Over 90 sand fly species are known to transmit Leishmania parasites. There are three main forms of the disease:
Visceral leishmaniasis:4 Fatal if left untreated, it is characterized by irregular bouts of fever, weight loss, enlargement of the spleen and liver and anaemia.
Cutaneous leishmaniasis:5 The most common form, causes skin lesions, mainly ulcers, on exposed parts of the body, leaving life-long scars on the skin and resulting in serious disability or stigma.
Mucocutaneous leishmaniasis:6 Leads to the partial or total destruction of the mucous membranes of the nose, mouth and throat.
———————————————
1
2
3 WHO has defined an insecticide DC as twice the lowest concentration that results systematically in 100% mortality after a 60-min exposure and a holding period of 24 h of a susceptible mosquito strain. The DC can also be defined as twice the 99% lethal concentration (LC99) as determined in a relevant statistical model against a susceptible strain of an insect.
4 Most cases of visceral leishmaniasis occur in Brazil, East Africa and in India, with an estimated 50 000 –90 000 new cases worldwide annually; only 25–45% of cases are reported to WHO. Visceral leishmaniasis remains one of the leading parasitic diseases with outbreak and mortality potential. In 2020, more than 90% of new cases were reported to WHO from 10 countries: Brazil, China, Ethiopia, Eritrea, India, Kenya, Somalia, South Sudan, Sudan and Yemen.
5 About 95% of cases of cutaneous leishmaniasis occur in the Americas, the Mediterranean basin, the Middle East and central Asia. It is estimated that 600 000 to 1 million new cases occur worldwide annually, but only around 200 000 are reported to WHO.
6 Over 90% of mucocutaneous leishmaniasis cases occur in Bolivia (Plurinational State of), Brazil, Ethiopia and Peru.
