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How do sand flies spread leishmaniasis and sandfly fever?

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Housefly — Diptera An adult housefly (*Musca domestica*) on a doughnut

Sand flies, which belong to the order Diptera, transmit both leishmaniasis and sandfly fever through the bites of bloodsucking females.12

Leishmaniasis

Leishmaniasis is caused by Leishmania protozoan parasites. When a sand fly feeds on an infected vertebrate, it ingests cells containing the parasite's nonmotile amastigote stage. Inside the fly's digestive tract, these develop into motile, flagellated promastigotes (or leptomonads), which multiply and migrate to the fly's mouthparts. During a subsequent blood meal, the fly injects these parasites into a new host, initiating an infection.34

Sandfly Fever

Sandfly fever (or pappataci fever) is caused by a phlebovirus. A sand fly becomes infected by biting an individual who is already carrying the virus. After an incubation period of 7 to 10 days, the virus resides in the fly, which remains infectious for the rest of its life. The virus is then transmitted to humans through the bite of the infected fly.2