Because the leishmaniasis is not usually found in temperate areas, we really have no data to go on as to whether the a ‘regular’ flea, a mosquito, or one of our tick species could pass it from one mammal to the next. Normally in the sand flea, the protozoans divide and increase in numbers, to be transmitted when the sand flea later bites the next mammal. It is possible that another insect may pass along infected blood through a bite, even if they did not serve as incubators for the organism.
The clues as to whether the leish would be able to become endemic in the US are hidden in its biology. Unfortunately, I know little about it, about how the organism is transmitted or how it reproduces in the sand fly. I cannot, therefore, make predictions as to whether this disease could become endemic. I do not expect that any insect other than sand flies would be able to transmit it.
When biting insects feed, everything goes to the stomach. In order to transmit a disease, the organisms have to be present in the saliva. Take malaria as an example. The parasite, plasmodium, has a very complex life cycle in which it enters the mosquito’s gut and grows until it reaches a certain life stage at which point it travels to the salivary glands and awaits being injected into the next person the mosquito bites (mosquitoes inject saliva when they bite; it contains an anesthetic so you don’t feel the bite). The life cycle of plasmodia are very specific to a few species of anopheles mosquitoes; no other insect is known to transmit malaria. Even though we eradicated malaria in the US, the mosquitoes are still here, and we therefore always have the risk of malaria becoming endemic again.