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23
May

The Urbi et Orbi of Leishmania parasites

Trypanosomatids are recognised as one of the most ubiquitous groups of protozoa on Earth. Most, if not all, vertebrate and invertebrate organisms could potentially be infected by a member of the Trypanosomatidae family. Three articles published in Memórias do Instituto Oswaldo Cruz  (Cutolo et al. 2014, Samy et al. 2014, Soares et al. 2014) contributed novel information on the extensive thread of the connections established by the trypanosomatids that are the most important in public health, the members of the genus, Leishmania. Some species of Leishmania are responsible for visceral leishmaniasis (e. g., Leishmania donovani, Leishmania infantum) or cutaneous leishmaniasis (Leishmania braziliensis, Leishmania panamensis etc). The vectors implicated in the transmission of parasites to vertebrate hosts are female sandflies (Diptera: Psychodidae) belonging to two genera: Phlebotomus, in the Old World, and Lutzomyia, in the New World. This host parasite relationship could be summarised as the net result of a favourable environmental relationship, by which infective Leishmania cells (promastigotes) are transferred by an invertebrate feeding on a vertebrate (an one-million-year old vertebrate host or a newly introduced vertebrate host, referred to as an intermediate host, e. g., humans). The infection cycle of Leishmania and the similar infectious cycle of Trypanosoma species (e. g., Trypanosoma cruzi, Trypanosoma brucei) developed in a human-free environment and the relationship between these parasites and humans has been evolving for more than 100 million years (Hamilton et al. 2012). Regarding humans and their economic activities, social interactions, cities, roads and machines, there has been an inevitable transformation of the environment by the action and needs of billions of individuals occupying a finite space. Infection by Leishmania species is considered to be a huge threat to a large population living in rural and urban areas in economically and socially constrained regions. Altogether these facts represent an amazing adaptation of a parasitic cycle that has been fine tuned through an evolutionary race to a reduced set of vectors, among the hundreds of species of sandflies, less than 70 are able to transmit infectious Leishmania forms (Ready 2013). Some questions accompany traditional entomological research regarding the feeding habits of these flies. The most important question concerns the methodology for the unequivocal detection of the blood source in cases in which a fly is captured and analysed in its natural environment. This question could be partially answered by Soares et al. (2014), researchers at the federal universities of Piauí, Alfenas and Pará and the São Paulo State University at Botucatu, Brazil. These authors describe the identification of the cytochrome B gene by a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay to determine the source of the blood found in the intestine of a sandfly. Currently, most methods for identifying a eukaryotic species based on DNA amplification require a DNA sequencing step, which limits their use in a large-scale survey of field samples. The use of non-sequencing methods such as PCR-RFLP remains a good choice for the identification of unknown samples if the species to be identified exhibits genetic distance, which is the case with Lutzomyia sp. blood sources; sylvan mammals, cattle, horses, pets and humans have sufficient mutations in their mitochondrial DNA to provide a specific band pattern after enzymatic digestion of an amplified DNA fragment. This approach might be quite affordable for the mass screening of sandflies from field samples, particularly in those regions in which expensive equipment is not readily available. If food searching is one of the driving forces impelling organisms such as the sandfly and associated parasites to spread their genes, the visible result of this action might be adaptation to a new environment, i.e., urban and periurban areas. The results reported by Cutolo et al. (2014) reports on this question and the public health concern represented by an increase in urban leishmaniasis. As a new element in this long-standing evolutionary race between Leishmania parasites and their vertebrate hosts, humans brought genetic and environmental novelty, which works as a mechanism for selecting parasite populations. The meeting of human and Leishmania genomes generates a type of evolutionary novelty (yet to be revealed) whose outcome we are only observing from the medical side in leishmaniasis. A starting point for forecasting the evolutionary consequences of this host parasite relationship is the cycle described by Cutolo et al. (2014), as follows: the opossum, a marsupial, is one of the oldest mammals on earth and might harbour molecular events that could explain the success of trypanosomatids infection in vertebrates. Major events with evolutionary implications do not happen at the individual level. Thus, leveraging knowledge regarding the population structure of Leishmania and its vectors is essential for a correct understanding of this evolutionary relationship. Parallel to this goal is the identification and characterisation of Leishmania populations in environments such as deserts. Samy et al. (2014) present a new perspective on the resilience and adaptability of Leishmania parasites to hosts and vectors under extreme conditions. We might speculate that the plasticity of the trypanosomatid genome encodes much more than nucleotide substitutions, deletions and insertions. Positioned in the middle of the prokaryote-eukaryote transition, trypanosomatids might represent a milestone, eventually leading to major evolutionary transitions (Szathmáry & Smith 1995).

 

Adeilton Alves Brandão | Publisher Editor

 

REFERENCES

Cutolo AA, Teodoro AKM, Ovallos FG, Allegretti SM, Galati EAB 2014. Sandflies (Diptera: Psychodidae) associated with opossum nests at urban sites in southeastern Brazil: a risk factor for urban and periurban zoonotic Leishmania transmission? Mem Inst Oswaldo Cruz 109: 391-393.

Hamilton PB, Teixeira MMG, Stevens JR 2012. The evolution of Trypanosoma cruzi: the ‘bat seeding’ hypothesis. Trends Parasitol 28: 136-141.

Ready PD 2013. Biology of phlebotomine sand flies as vectors of disease agents. Annu Rev Entomol 58: 227-250.

Samy AM, Doha SA, Kenawy MA 2014. Ecology of cutaneous leishmaniasis in Sinai: linking parasites, vectors and hosts. Mem Inst Oswaldo Cruz 109: 299-306.

Soares VYR, da Silva JC, da Silva KR, Cruz MSP, Santos MPD, Ribolla PEM, Alonso DP, Coelho LFL, Costa DL, Costa CHN 2014. Identification of blood meal sources of Lutzomyia longipalpis using polymerase chain reaction-restriction fragment length polymorphism analysis of the cytochrome B gene. Mem Inst Oswaldo Cruz 109: 379-383.

Szathmáry E, Smith JM 1995. The major evolutionary transitions. Nature 374: 227-232.

 

 

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