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Epidemiology

The tones of the difference

Variations in the mating song show that there may be more than one transmitter of visceral leishmaniasis

Studies carried out by the Oswaldo Cruz Foundation (Fiocruz), of Rio de Janeiro, suggest that the phlebotomus mosquitoes nowadays called Lutzomyia longipalpis, the main transmitters of visceral leishmaniasis in humans in the Americas, may belong to several sister species of hematophagous insects – and not just to a single one, as used to be believed. An analysis of the genetic material of different populations, coming from different spots in the country, reinforces the thesis that there are at least three, perhaps four, distinct types of L. longipalpis.

The researchers have still not managed to pinpoint if each one of these varieties of L. longipalpis, which practically do not mate with each other, may come to be labeled as a new species. They are, however, convinced that not all the mosquitoes are exactly the same. “In Venezuela, consistent evidence has now been found that there are at least two sister species of L. longipalpis“, says Alexandre Peixoto, from Fiocruz, the coordinator of the studies, who worked with phlebotomus from the localities of Sobral (Ceará), Jacobina (Bahia), Lapinha (Minas Gerais) and Natal (Rio Grande do Norte.). “In Brazil, the same seems to be happening”. This kind of taxonomic information is important in the struggle against the disease, since the fight against visceral leishmaniasis requires better control – and knowledge – of its transmitting agents.

From the point of view of appearance (morphology), there are no significant differences between the specimens of the mosquitoes obtained in each one of these localities, but there are noticeable distinctions from the molecular and behavioral point of view. After sequencing and analyzing a region of a gene called period in populations of L. longipalpis from these four regions, Luiz Guilherme Bauzer, from Peixoto’s group, saw that this genetic material is very different in each one of the samples, which is evidence in favor of the idea that there are four distinct species.

With the collaboration of colleagues from two British universities (Keele and Leicester) and from Nataly de Souza (Fiocruz), Peixoto recorded and analyzed the content of the song intoned by L. longipalpis males from these different places. Once again, the result points to the existence of different songs, another indication that there may be a complex of species around the phebotomus mosquito that transmits visceral leishmaniasis. “We may be in the face of a situation like that of the Anopheles gambiae mosquito, which transmits malaria in Africa”, says Peixoto. By means of a series of studies, the scientists discovered that there is a set of seven species of mosquito around A. gambiae.

If it is proved that the mosquitoes capable of transmitting visceral leishmaniasis belong to a group of kindred species, the role of these unknown varieties of L. longipalpis in disseminating the disease will have to be discovered. Each one of these potential new species may have a different weight in the propagation of this illness, which is characterized by long-lasting fevers, loss of weight, and swelling of the spleen and liver. If untreated, the symptoms may lead to death in one or two years.

“Besides being fundamental for an evolutionary study of phlebotomus, the confirmation of the existence of a complex of sister species around L. longipalpis may have repercussions for the epidemiology of the disease”, Peixoto reckons. The most common way for a person to catch visceral leishmaniasis is to be bitten by an L. longipalpis infected by the Leishmania chagasi protozoon, the causal agent of the disease. In Brazil, there are about 2,000 new cases of visceral leishmaniasis a year, with a mortality rate close to 10%, according to the Ministry of Health.

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