Research Article: A Historical Overview of the Classification, Evolution, and Dispersion of Leishmania Parasites and Sandflies

Date Published: March 3, 2016

Publisher: Public Library of Science

Author(s): Mohammad Akhoundi, Katrin Kuhls, Arnaud Cannet, Jan Votýpka, Pierre Marty, Pascal Delaunay, Denis Sereno, Anne-Laure Bañuls.

Abstract: BackgroundThe aim of this study is to describe the major evolutionary historical events among Leishmania, sandflies, and the associated animal reservoirs in detail, in accordance with the geographical evolution of the Earth, which has not been previously discussed on a large scale.Methodology and Principal FindingsLeishmania and sandfly classification has always been a controversial matter, and the increasing number of species currently described further complicates this issue. Despite several hypotheses on the origin, evolution, and distribution of Leishmania and sandflies in the Old and New World, no consistent agreement exists regarding dissemination of the actors that play roles in leishmaniasis. For this purpose, we present here three centuries of research on sandflies and Leishmania descriptions, as well as a complete description of Leishmania and sandfly fossils and the emergence date of each Leishmania and sandfly group during different geographical periods, from 550 million years ago until now. We discuss critically the different approaches that were used for Leishmana and sandfly classification and their synonymies, proposing an updated classification for each species of Leishmania and sandfly. We update information on the current distribution and dispersion of different species of Leishmania (53), sandflies (more than 800 at genus or subgenus level), and animal reservoirs in each of the following geographical ecozones: Palearctic, Nearctic, Neotropic, Afrotropical, Oriental, Malagasy, and Australian. We propose an updated list of the potential and proven sandfly vectors for each Leishmania species in the Old and New World. Finally, we address a classical question about digenetic Leishmania evolution: which was the first host, a vertebrate or an invertebrate?Conclusions and SignificanceWe propose an updated view of events that have played important roles in the geographical dispersion of sandflies, in relation to both the Leishmania species they transmit and the animal reservoirs of the parasites.

Partial Text: Leishmaniases are vector-borne diseases caused by obligate protozoan parasites from the genus Leishmania (Trypanosomatida: Trypanosomatidae). Leishmaniases are endemic in large areas of the tropics, subtropics, and the Mediterranean basin, including more than 98 countries, where there are a total of 350 million people at risk and 12 million cases of infection. Canine leishmaniasis is a serious problem, and it is estimated that 2.5 million dogs are infected in the Mediterranean basin only [1]. Among the endemic regions on five continents, there is an estimated incidence of 0.7–1.2 million cases of cutaneous leishmaniasis (CL) and 0.2–0.4 million cases of visceral leishmaniasis (VL) in these countries [2]. The disease is absent in New Zealand and the southern Pacific. Leishmaniasis is transmitted by the bite of infected female sandflies, whose hosts are animals such as canids, rodents, marsupials, hyraxes, or human beings. Approximately 53 Leishmania species have been described (without considering the synonyms and including all five subgenera and complexes: Leishmania, Viannia, Sauroleishmania, L. enriettii complex, and Paraleishmania); of these, 31 species are known to be parasites of mammals and 20 species are pathogenic for human beings. Leishmania parasites cause four main clinical forms of the disease—according to the location of the parasite in mammalian tissues—referred to as visceral, cutaneous, diffuse cutaneous, and mucocutaneous leishmaniasis. The most common form is cutaneous disease, and the ten countries of Afghanistan, Algeria, Colombia, Brazil, Iran, Syria, Ethiopia, North Sudan, Costa Rica, and Peru together account for 70% to 75% of the global estimated CL incidence [2]. Regarding visceral leishmaniasis, more than 90% of all cases occur in just the six countries of India, Bangladesh, Sudan, South Sudan, Brazil, and Ethiopia [2]. Leishmaniasis currently constitutes a major global public health problem, showing an increasing burden over the last decade [2].

Among more than 800 recognized sandfly species, approximately 464 species are found in the New World and 375 in the Old World [3,4]. The classification of both Old and New World sandflies has historically been based mainly on a phenetic approach to identifying overall similarity relationships between genera and subgenera, rather than on ancestor–descendant relationships. This approach has led to a proliferation of taxa, particularly at the subgeneric level, and to the simplification and incorporation of higher taxonomic categories into species. Sandflies belong to the order Diptera, suborder Nematocera, family Psychodidae, and subfamily Phlebotominae. Initially, studies on phlebotomine sandfly taxonomy were exclusively based on morphological aspects of dead specimens. Because of the introduction of several new methods, such as chromosome analysis, multivariate morphometrics, laboratory rearing and colonization, isoenzyme, molecular and phylogenetic analysis and, more recently, mass spectrometry, our knowledge of phlebotomine sandfly systematics has increased. These advances have led to better identification and classification of sandfly specimens, which together with an appreciation of sandfly flight range (approximately 1.5 km per day), have helped to clarify the intraspecific and interspecific variations within sandfly subgenera and populations. A large portion of the literature regarding phlebotomine sandfly systematics addresses their general classification and relationships with other groups [3,5–8] as well as the phylogenetics of the Psychodidae, based on insect fossils [9], phlebotomine sandfly evolution [5], phenetic and phylogenetic analyses of phlebotomine sandflies [10], and the molecular systematics and phylogenetic relationships of phlebotomines using DNA analysis [11]. Many classification systems for phlebotomine sandflies have been proposed since that of Newstead 1911, including those of Abonnenc, Davidson, Fairchild, Leng, Lewis, Quate, and Theodor. However, despite this extensive literature, there is no universal agreement regarding the ranking of taxa above the species level.

The Old World sandflies include three genera: Phlebotomus, Sergentomyia, and Chinius, which are found in the Palaearctic, Afrotropical, Malagasy, Oriental, and Australian regions.

The New World sandflies include three genera: Lutzomyia, Warileya, and Brumptomyia, which are found in the Nearctic and Neotropical regions:

Fossils, including the remains of living organisms from the past, are one of the best forms of evolutionary evidence. They allow for comparisons with current organisms and are of particular importance in allowing knowledge of primitive character states (plesiomorphic) and derived specialized states (apomorphic) to be obtained. Fossils provide information about the origin of vector flies in relation to infectious agents, host coevolution, and geographic locations. Therefore, research on sandfly fossils is of great importance for highlighting the evolution and phylogeny of these insects. As mentioned above, phlebotomine sandflies are found in a wide range of ecozones, which could be due to their long evolutionary history with their origins in the Palaeozoic or Mesozoic eras [68].

The Trypanosomatidae family consists of three dixenous genera (life cycle in vertebrates or plants and invertebrates)—Trypanosoma, Phytomonas, and Leishmania—11 monoxenous genera (life cycle in invertebrates only)—Leptomonas, Crithidia (together with Leishmania form the subfamily Leishmaniinae), Blastocrithidia, Herpetomonas, Sergeia, Wallacemonas, Blechomonas, and Jaenimonas—and three genera that are characterized by the presence of endosymbiotic bacteria and form the subfamily Strigomonadinae: Angomonas, Strigomonas, and Kentomonas [84–88].

Leishmania belongs to the phylum Kinetoplastida, which is likely related to the phylum of Euglenids [132]. Both of these groups belong to the eukaryotic supergroup Excavata, for which fossil evidence suggests emergence during the Ordovician [133]. Leishmania might have originated during the Mesozoic, prior to the separation of Gondwana [106]. The first Leishmania fossil record was Paleoleishmania proterus, a digenetic Leishmania species associated with a blood-filled female of the sandfly P. burmitis in Burmese fossil amber (Cretaceous, 100 MYA) (Table 3) [134]. Within the alimentary canal of this sandfly, amastigotes (n = 20), promastigotes (n = 393), and paramastigotes (n = 64) of digenetic leishmanial trypanosomatids were observed. The observation of these different parasitic stages in the alimentary tract of the insect suggests that their presence was likely the result of a blood meal and that they were multiplying within the midgut. The blood cells were later identified as being of reptilian origin. They also described the development of putative amastigotes within whitish, spherical-to-oval vacuoles associated with some blood cells. The second fossil of Paleoleishmania species described was P. neotropicum, which was found in Dominican fossil amber (20 MYA). A large number of promastigotes (n = 20) and amastigotes (n = 20) were found in the gut of L. adiketis. Additionally, four promastigotes, two paramastigotes, and several amastigotes of P. neotropicum were found in the proboscis of L. adiketis. The presence of amastigotes demonstrated the digenetic life cycle of P. neotropicum, as this parasitic life stage is considered to be present only in the vertebrate host, and no monogenetic flagellates are known to colonize sandflies.

A Palaearctic origin of the genus Leishmania was proposed by Lysenko in 1971 [142]. Fossil evidence indicates that both phlebotomine sandflies and murid rodents originated in the Palaearctic [5,143], making it likely that Leishmania, along with its vectors and reservoirs, could have evolved in the Palaearctic during the Cenozoic period and dispersed to the Nearctic during the Oligocene (Eocene), when the Bering land bridge was intact. These species then dispersed into the Neotropics across the Panamanian land bridge during the Pliocene, when the climate was sufficiently warm to permit further dispersal of Leishmania (Fig 2) [82,142,144,145].

In 1998 [149], Noyes suggested a Neotropical origin of Leishmania during the Palaeocene or Eocene period (36–46 MYA). Subsequently, the parasites invaded the Nearctic ecozone via the Panamanian land bridge and the Palaearctic via the Bering land bridge during the Miocene. The greater diversity observed among New World Leishmania species compared with those from the Old World provides some circumstantial evidence arguing for a Neotropical origin of Leishmania [19,150]. Nevertheless, if this hypothesis is true, then Sauroleishmania might have evolved later during the Miocene, either in the Nearctic or the Palaearctic area, as a result of adaptation to reptiles [149]. Sloths (Xenarthra) might have served as the first vertebrate reservoirs of Leishmania in the Neotropics. Also, it has been suggested that a number of monogenetic and digenetic trypanosomatids can grow in the rectal glands of marsupials. After adaptation to rodents during the Eocene, infected porcupines would have carried the parasites across the Panamanian land bridge to the Nearctics and across the Bering land bridge to the Palaearctic during the Miocene in an unspecified mammalian reservoir (Fig 2) [83,149,150].

According to this theory, the genus Leishmania is divided into two sections: Euleishmania (Leishmania and Viannia subgenera and Sauroleishmania) and Paraleishmania (L. hertigi, L. deanei, L. colombiensis, L. equatorensis, and L. herreri) [114,139]. It is also speculated that the separation of Gondwana in the Mesozoic resulted in the evolution of the Leishmania genus into Leishmania and Sauroleishmania in Africa, and Viannia and Paraleishmania in South America [139]. The origin and the evolution of Leishmania would have been related to the origin of humans in eastern Africa, with Leishmania following the dynamics of the human population in the Palaearctic (Asia, Africa, and Europe) ecozone. An African origin of Leishmania was emphasized by Momen and Cupolillo [139], based on the importance of the origins of its vectors and reservoirs as evidence for this hypothesis and citing the restricted habitat of Arvicanthis rodents and Phlebotomus sandflies in Africa. According to this hypothesis, the Old World Leishmania species (e.g., L. donovani/L. infantum, L. tropica, L. major, and L. aethiopica) exhibit an African origin. L. aethiopica is present only in the Ethiopian and Kenyan highlands. Because of its restricted geographical distribution, it is reasonable to assume an African origin for this species as well as for the other L. (Leishmania)–hyrax systems that occur in Africa [128]. The origin of humans from eastern Africa suggests that Leishmania species with anthroponotic transmission, i.e., L. tropica and L. donovani, may also have originated in eastern Africa (Fig 2) [152].

The term “coevolution” was first used to demonstrate a particular type of relationship between Leishmania and sandfly species in the Old World [147]. Leishmania and sandflies have survived over many millions of years under selective pressure, depending on natural ecological changes. A close relationship has been demonstrated between some sandfly and Leishmania species, such as L. major and P. papatasi. This longstanding evolutionary history of Leishmania and sandflies has resulted in a similar distribution. However, there is not always a clear distinction between coevolution and certain other concepts, such as coassociation (meaning that the transmission cycle exhibits a distinctive landscape epidemiology), interaction (the molecular and immunological relationship between the sandfly midgut and the parasite’s external surface), or vector–parasite cospeciation or co-cladogenesis [37]. Most Leishmania parasites are more restricted regarding the range of sandfly vectors that can transmit them than in the range of mammalian hosts/reservoirs they are able to infect, suggesting a much closer coevolutionary relationship with sandflies than with their vertebrate hosts, although it is sometimes difficult to interpret this coevolutionary relationship [153]. For example, there is a specific relationship between P. papatasi and L. major because of the presence of specific midgut receptors [154], and these two species show strong distribution sympatry. Nevertheless, such high specificity of Leishmania for its sandfly vector appears to be restricted to P. papatasi or P. duboscqi and P. sergenti. However, the appearance of Leishmania interspecies hybrids might have consequences in terms of specificity and transmission efficiency [155,156].

Phlebotomine sandfly systematics, particularly at the supraspecific level, have always been controversial [34,53]. Originally, this family was composed of a single genus: Phlebotomus Rondani. In 1948, Theodor proposed subdivision of the sandfly family into four genera: Phlebotomus and Sergentomyia in the Old World and Lutzomyia and Brumptomyia in the New World. A “stable” classification of the phlebotomine sandflies was proposed in 1977 by Lewis and colleagues [14], who retained the well-known family, subfamily, and genus names. It was also proposed that the subgenera and species groups be used as a model to put forward a new proposal. A “flexible” classification was proposed by Ready and colleagues in 1980 [162]. These researchers challenged the “stable” classification through a comparative analysis of characters that were described as “exclusive” characters for their proposed genera, e.g., Phlebotomus, Sergentomyia, Brumptomyia, Warileya, and Psychodopygus, but no such characters were found for Lutzomyia. The absence of unique characters for the genus Lutzomyia is certainly the weakest point in their comparative character analysis. New discoveries in later years led to the erection of new subgenera or genera. One of the difficulties in sandfly classification concerns the position of sandfly species at the genus or subgenus level. There is no general agreement regarding the definition of some groups at the genus or subgenus level. Idiophlebotomus in Phlebotomus, as well as Parrotomyia, Rondanomyia, and Grassomyia in Sergentomyia were classified by Quate and Fairchild [163] at the subgenus level, whereas Abonnenc [164] considered Idiophlebotomus to be genus and Sergentomyia to be a subgenus. Abonnenc and Minter [165] did not include Parvidens as a subgenus of Sergentomyia, whereas Abonnenc [164] considered Parvidens to be a subgenus of the Phlebotomus genus. Lewis [5] declined to recognize generic status for Spelaeophlebotomus and Idiophlebotomus, whereas Artemiev and Neronov [166] considered them at the genus level. Similarly, for New World sandfly species, Young and Duncan [8] classified Bichromomyia, Dampfomyia, Deanemyia, Evandromyia, Expapillata, Martinsmyia, Micropigomyia, Migonemyia, Nyssomyia, Pintomyia, Psathyomyia, Psychodopigus, Trichophoroymyia, Trichopigomyia, and Viannamyia to be subgenera of the Lutzomyia genus, whereas Galati et al. [66] elevated these groups to the genus level. These conflicts in classification are mainly due to (i) differences or variations in the criteria and the methods used for classification, such as criteria that are now considered to be outdated or scarce, e.g., the presence of erected or recumbent abdominal setae; (ii) morphological similarities between species and some uncertainty in species identification, such as the existence of cryptic or sibling species and the similarity of morphological characters among females that makes species identification dependent on male characters (e.g., Adlerius); (iii) the inadequacy of the reported species descriptions; and (iv) the massive increase in the number of sandfly species described. The construction of a well-supported phylogeny of the generic and subgeneric groups in the Phlebotominae subfamily will likely require a supermatrix analysis. This matrix must include molecular information on several nuclear genes combined with mitochondrial genes—as well as other criteria related to biology—and ecology, which has been successfully applied for the classification of the Drosophilidae family [167]. This type of analysis would provide a firmer basis for the classification of Phlebotominae sandflies, in addition to resolving the problem of the proposal of classifications suggested for the Old World and New World sandflies. Therefore, a more extensive molecular phylogenetic analysis, e.g., focussing on gene flow and the phenotypes of specimens, awaits the development of an accurate and valid protocol for sandfly classification.

The evolutionary relationship between sandflies and Leishmania has implications for leishmaniasis interventions and control. It is therefore necessary to obtain information on the origin of Leishmania and the Phlebotominae sandflies and their chronological history of coevolution. Understanding these evolutionary relationships between different Leishmania and sandfly species is of epidemiological importance for the future prediction of Leishmania transmission patterns.