Distribution of the 20 Leishmania species pathogenic for humans. The species name has been shown as abbreviation. A: L. aethiopica; Am: L. amazonensis; B: L. braziliensis; C: L. colombiensis; D: L. donovani; G: L. guyanensis; Gh: 'Ghana strain'; I: L. infantum; La: L. lainsoni; L: L. lindenbergi; M: L. major; Ma: L. martiniquensis; Mx: L. mexicana; N: L. naiffi; Pa: L. panamensis; P: L. peruviana; S: L. 'siamensis'; Sh: L. shawi; T: L. tropica; V: L. venezuelensis and W: L. waltoni. The species with question marks need to be confirmed by further genotyping studies.
Leishmaniases are vector-borne diseases caused by obligate protistan parasites from the genus Leishmania (Trypanosomatida: Trypanosomatidae) which are endemic in large areas of the tropics, subtropics and Mediterranean basin, globally spanning more than 98 countries. There are ̴ 350 million people at risk and ̴ 12 million cases, with an estimated worldwide annual incidence of 0.7-1.2 million cases of cutaneous leishmaniasis (CL) and 0.2-0.4 million cases of visceral leishmaniasis (VL) (Alvar et al. 2012). Hence Leishmaniases constitute a major public health problem with an increasing burden over the last decade, and among tropical infections ranks as the 2nd and 4th most common cause of death and disease, respectively (Bern et al. 2008).
Leishmania infections have six clinical forms, defined by the location of the parasite in the infected tissues: visceral (VL), post-kala-azar dermal leishmaniasis (PKDL), cutaneous (CL), diffuse cutaneous (DCL), mucocutaneous (MCL) and mucosal (ML) leishmaniasis. The most common form is CL: over 90% of cases are distributed across three main regions: (i) Afghanistan, Iran, Saudi Arabia and Syria; (ii) Algeria and Tunisia; and (iii) Brazil and Peru. Over 90% of cases of the less common but more pathogenic VL occur in another three geographic foci: (i) Bangladesh, India and Nepal; (ii) Ethiopia, Kenya and Sudan; and (iii) northeast Brazil (Alvar et al. 2012).
Currently, 54 Leishmania species [excluding synonymous species (the synonymy based on molecular typing results e.g., MLEE (Multilocus Enzyme Electrophoresis), MLMT (Multi-locus microsatellite typing) and sequencing) and including Sauroleishmania] are known and at least 20 of them are pathogenic to humans in the Old World and in the New World. Leishmania flagellates are transmitted to vertebrates by the bite of infected female phlebotomine sandflies and are frequently hosted by canids, rodents, marsupials, mongooses, bats and hyraxes; therefore the disease may be zoonoses, anthropozoonoses or anthroponoseses, although few species are strictly anthroponotic.
Due to the wide spectrum of non-human reservoir hosts and the large number of vectors involved in the transmission, improved diagnosis of Leishmania parasites is an important feature of control programs (Stauch et al. 2014). Only a minority of infected humans develops the disease: most are infected at a sub-clinical level (Singh et al. 2002). These asymptomatic hosts help sustain VL transmission in endemic areas and represent major challenge for infection control. Given that virulence, pathogenicity and clinical manifestation varies among Leishmania species, and that the clinical outcome of the disease also depends on the sandfly vector type and the host’s genetic background, highly accurate diagnosis is essential for successful elimination of these versatile parasites because it is the mainstay for bridging molecular epidemiology and therapy choices (local/systemic and drug).
Updated classification of Leishmania species
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