Leishmaniasis is a sometimes fatal disease marked by skin ulcers, fever, and spleen and liver problems. It currently affects about 12 million people, mostly in the developing world, with about 2 million new infections annually. It’s caused by a parasite, which is spread by sand flies.

There is not yet a good vaccine against leishmaniasis. But researchers have shown that it may be worthwhile to target not the parasite—but its fly host. The report good results in non-human primates in the journal Science Translational Medicine. [Fabiano Oliveira et al, A sand fly salivary protein vaccine shows efficacy against vector-transmitted cutaneous leishmaniasis in nonhuman primates]

Vaccines usually trigger our immune systems to rally against a pathogenic agent. If that agent should actually show up, the immune system can disarm it.

But in this case, scientists aimed at the carrier—the sand fly. When an infected fly bites a victim, it injects parasites and its own saliva. So the researchers subjected monkeys to repeated bites from uninfected sand flies, then to bites from infected sandflies. And the monkeys bitten first by uninfected flies were partially protected when confronted with the leishmaniasis parasite. Seems that exposure to fly saliva alone could elicit an immune response.

The researchers then determined that one particular protein in the sand fly’s saliva kicked the immune system into action. A vaccine based on that protein offered partial protection, with 7 of 10 monkeys that got it developing smaller ulcers and a stronger immune response than their unvaccinated peers when infected with the parasite.

This protein can be generated and purified easily from E. coli, which means that it’s possible to get large quantities of it to produce a potential future human vaccine against leishmaniasis. Which would make this saliva-based protection nothing to spit at.

—Cynthia Graber

[The above text is a transcript of this podcast.]