Parasite Pursuit: Sand fly coughs up leishmania protozoan’s secrets of proliferation

Leishmania, a tropical parasite carried by sand flies, spreads prolifically to mammals by forcing the flies to regurgitate as they bite their prey, a new study finds.

While still in the sand fly, the parasite secretes—and then multiplies in—a gel that obstructs the fly’s throat. When the fly bites a person or another mammal to feed on blood, the insect is forced to expel the parasite-laden blob of gel into the victim’s bloodstream, the researchers report in the July 22 Nature. Inside the new host, the gel appears to promote the parasite’s survival.

A fly remains infected for life. So, by obstructing the sand fly’s digestive tract, the parasite could also be forcing the insect to take more blood meals than normal, prompting more bites and hence greater spread, hypothesizes Matthew E. Rogers, a parasitologist at the University of Liverpool in England.

He and his colleagues studied Leishmania mexicana, a New World species that causes skin lesions and disfigurement if untreated. Other more dangerous species of the leishmania protozoan parasite can cause internal-organ damage and death.

To ascertain the importance of the gel itself, the scientists exposed some mice to leishmania via either the bites of infected sand flies or laboratory injection of a solution containing the parasites but not the gel. Leishmania delivered by a single sand fly bite caused skin lesions much sooner than the injected parasites did. Fly saliva and the gel accompanying the parasite boost the virulence of the ensuing infection, Rogers says.

In a subsequent test, the scientists compared leishmania injected along with the gel against the parasite injected with saliva. They determined that the gel results in a more virulent infection.

Rogers and his coworkers also found that the active ingredient in the gel is a compound called filamentous proteophosphoglycan (fPPG), a protein coated with sugar molecules. Without fPPG, the gel doesn’t boost the virulence of an infection. However, the mechanism by which fPPG helps the parasite thrive in its new host remains under investigation, Rogers says.

The compound might tamper with the immune system of the mammalian host by activating certain chemical signals or shutting down others, says Jesus G. Valenzuela, a biochemist at the National Institute of Allergy and Infectious Diseases (NIAID) in Rockville, Md.

Shaden Kamhawi, a medical entomologist also at NIAID, says the new research “is great, because we really don’t know the intricacies of what’s going on [in leishmania] transmission.”

Such details might provide leads for researchers working to develop a vaccine containing components of saliva and the gel, Rogers says. No anti-leishmania vaccine is currently beyond the experimental stage (SN: 8/11/01, p. 85: Available to subscribers at Insect-saliva vaccine thwarts parasite).

Roughly 12 million people worldwide are infected with leishmania parasites. The lethal species in the Leishmania genus—which can cause fevers, anemia, and organ damage—are most prevalent in India, Sudan, and Brazil.

Drugs for leishmaniasis can be expensive, and some must be regularly injected for weeks or months.