Lutzomyia

Only the female Lutzomyia sand flies are blood-feeding, requiring the bloodmeal to provide the nutrients for the maturation of eggs. Therefore, it is only the females who have medical importance as vectors of disease. Both males and females require carbohydrate food as adults, the source of which remains unclear.[1]

The most medically important species for human disease transmission are those which predominantly favour humans as a source of bloodmeal. These species are known as anthropophillic and tend to feed at around dusk. However, there are examples of anthropophillic species that will attack in the daytime. The majority of anthropohillic Lutzomyia species in the Americas are exophillic, which means that they favour biting outside of homes.[1]

The resting behaviour of Lutzomyia sand flies, like many other aspects of their biology, is important to understand for targeted, vector-based control methods to reduce transmission of Leishmaniasis. For example, residual spraying of insecticides can be targeted at known resting sites to increase effectiveness. Lutzomyia sand flies generally rest outdoors, with the type of resting site varying between species and in response to the seasons and the availability of particular microhabitats. The largest resting microhabitat is the forest floor, but sand flies will also rest in a variety of other areas, such as in the nests and burrows of mammals, within the trunks of hollow trees and inside bat caves.[1] The resting position adopted by Lutzomyia sand flies is characteristic to the genus, with wings angling above the abdomen.[5]

The sand fly genus Lutzomyia includes all species responsible for transmission, in the New World, of the Leishmania parasite, the causative agent of leishmaniasis. Of the more than 350 Lutzomyia species identified, less than 10% are known or suspected to transmit leishmaniasis to humans.[6] The disease is in endemic in 22 countries of tropical and subtropical America, where it is generally considered a zoonosis.[7] That is, the parasite cycles between mammalian reservoir hosts and the sand flies, with humans serving as incidental, dead-end hosts. Common reservoirs in the Americas include a number of rodent species, as well as dogs, sloths and armadillos.[7] The Lutzomyia sand flies maintain transmission amongst reservoir species and allow the disease to 'jump' to humans, causing the visceral, cutaneous or mucocutaneous forms of the disease depending on the Leishmania species. Visceral Leishmaniasis is a serious form of the disease in particular that affects the internal organs. This specific disease is primarily transmitted by the species Lutzomyia longipalpis.[8] The human-sand fly-human cycle of transmission, known as anthroponotic, is limited to two Leishmania species endemic in the Old World and so does not involve Lutzomyia sand flies.[7]

Giemsa-stained amastigotes of Leishmania (Viannia) panamensis

The New World Leishmania species transmitted by Lutzomyia sand flies are often classified in the subgenus, Viannia.[9] These genetically distinct parasites show markedly different patterns of development within the New World sand flies when compared to those seen in the Old World Phlebotomus sand flies. The first stages of development are similar between all Leishmania species, with the sandfly taking up the amastigote form of the parasite following a bite of an infected host. However, unlike in the Old World, the replicated parasites then migrate to the hindgut of the Lutzomyia sand fly, a feature which is thought to be essential in allowing the parasite to become established.[10] The parasite then undergoes further development into the infective, promastigote stage as it migrates to anterior end of the insect, ready for inoculation into a new, susceptible host.

The high level of species diversity in the genus Lutzomyia, coupled with its high adaptability is leading to increasing risk of Leishmania transmission in the New World in response to global change. Lutzomyia sand flies transmit the disease in the tropics and subtropics, regions which are subject to high levels of deforestation associated with continual development. Deforestation, with the establishment of settlements at the periphery of primary or secondary forest, increases the risk of Leishmania transmission by creating a selection pressure for the adaptation of sand flies to these new peridomestic environments.[11] In southern Brazil, for example, levels of cutaneous leishmaniasis are increasing as a result of the adaptation of three, formerly forest-inhabiting species (Lutzomyia intermedia, Lutzomyia whitmani and Lutzomyi migonei) following deforestation.[12] Now, a disease which previously only affected forest workers is becoming more prevalent in urbanisations. Similar increases in risk factors are also driving higher rates of visceral leishmaniasis in north-eastern Brazil. Increased urbanisation, driven by climate change and socio-economic factors, is bringing the adapting sand flies into closer contact with both humans and the domestic canine reservoirs of the disease.[11]

Carrion's disease, also known as bartonellosis, is a disease caused by the blood-borne bacteria, Bartonella bacilliformis. The disease is transmitted by the sand fly species Lutzomyia verracarum, as well as lice and fleas, and is found in areas of Peru, Colombia and Ecuador.[13] The lifecycle of the bacteria within Lutzomyia sand flies remain largely unknown, with speculation that the bacteria are spread between sandflies sharing the same breeding grounds and water supplies. The existence of bartonellosis transmission in areas not inhabited by Lutzomyia verracarum suggests that secondary vectors, and potentially other Lutzomyia species, are important in the spread of the disease.[14]

A number of arthropod-borne viruses (arboviruses) are transmitted by Lutzomyia sand flies. One such virus of medical importance is the vesicular stomatitis virus (VSV) of the genus Vesiculovirus. Viruses of this genus are typically associated with flu-like symptoms, but have also been known to cause encephalitis.[1] While transmission of the Indiana serotype of VSV by Lutzomyia trapidoi has been demonstrated under laboratory conditions, little is known about the cycles of virus infection between vertebrates and Lutzomyia sand flies. However, they are thought to be supported by both horizontal and transovarial transmission in the sand fly.[15]