The vector that spreads visceral leishmaniasis 

Dr Matthew Rogers: In this section, we will begin by introducing you to important features of sandflies and their lifecycle that will assist you in identifying them and appreciating their role in leishmaniasis. Sandflies– Diptera, Phlebotominae– are the only proven factor of disease-causing Leishmania parasites. Globally, there are over 700 species of phlebotomine sandflies, although only 50 so far have been proven to transmit disease to humans.

Sandflies not only transmit leishmaniasis. They can transmit sandfly fever caused by phelboviruses and bartonellosis caused by the bacterium Bartonella bacilliformis. Sandflies transmit Leishmania by bite when the female sandfly takes blood to mature their eggs. Sandflies deposit the infectious form of Leishmania, the metacyclic promastigote from within the gut of the sandfly by regurgitating parasites into the skin as they blood feed. The infection begins when deposited parasites are phagocytosed by cells of the monocyte macrophage lineage. Inside these cells, Leishmania transform to the aflagellate amastigote stage and begins replicating. Natural transmission by infected sandflies is very efficient, as it is assisted by sandfly-derived factors, such as the sandfly’s saliva and a parasite-secreted gel that blocks the sandfly gut.

Phlebotomine sandfly vectors are tropical and subtropical insects. In the Old World, leishmaniasis is transmitted by sandflies of the genus Phlebotoumus, and in the New World by the genus Lutzomyia. Sandflies are small– up to 4 millimetres long, which make them difficult to identify. Phlebotomine sandflies are characteristically hairy, even on their wings. This enables them to survive in semi-arid environments. Their legs are long, spindly, and their wings rest in an upright position in a V-shape. Sandflies fly short distances, and have a recognisable hopping flight. Their antennae are long, and have 16 segments. Leishmaniasis is not transmitted by other biting insects.

The term sandfly has been loosely ascribed to biting midges– figure 1– and blackflies– figure 2– where they are biting nuisances on sandy beaches. Unlike in most biting Diptera, sandflies live and breed on land rather than the water. They require humid and organic-rich environments to lay their eggs. In the wild, immature stages of sandflies have rarely been found, making it difficult to identify breeding sites. However, animal sheds and rodent burrows are thought to be favoured sandfly habitats. Larvae hatch and pass through four instars before pupating. Although the duration of the life cycle in nature has not been determined, at ambient temperature, colonised sandflies develop slowly, and take between four to six weeks from egg laying to adult emergence.

In contrast, mosquitoes take two weeks to complete their life cycle. Some Palaearctic species undergo diapause as larvae to survive the cooler months. Sandfly eggs are small, ovoid, and dark. When freshly laid, they are pale, but quickly darken with exposure to air as the eggshell or chorion melanises. Closer inspection reveals that they have a sculptured egg shell. Eggs are laid in moist, organic material-rich environments to support the developing larvae. A sandfly can lay up to 40 eggs per blood feed. Sandfly larvae develop as four instars and have an off-white colour with a dark head capsule. Those of the first instar can be distinguished by the presence of two dark caudal bristles protruding from their last segment. All subsequent instars bear four.

Though the larvae have no legs, they have the characteristic bristles on each segment within large tips known as matchstick hairs. Their function are unknown, but they are thought to act as sensory organs and to aid larval movement. The pupae are golden to reddish brown in colour, and are fixed to the surface of the substrate in which they developed by the final larval exuvium. Shortly before emergence, the wings and eyes turn black.