Leishmania Parasite: Deadly For Humans, But Good For Flies?

Whatever you do, do NOT do an image search for "cutaneous Leishmania".

This is a sand fly, an insect which spreads leishmaniasis.Rod Dillon

No human would be inclined to think favorably of leishmaniasis, caused by a parasite spread by sand flies, which infects about 12 million people worldwide and kills 20,000 to 30,000 per year.

Leishmaniasis comes in two basic forms, cutaneous and visceral. The second is more serious, attacking the internal organs, and can lead to death if it's not treated. But cutaneous leishmaniasis is more visible, causing large (and egregious, unsightly) skin sores and lesion that can leave behind nasty scars. The cutaneous variety can also spread to the body's mucous membranes, creating sores in the sinuses and mouth--which can end up destroying them. Leishmaniasis is found in 90 countries, mostly in the tropics, from Latin America to Africa, the Middle East and South Asia. "Collectively the leishmaniases present a major global health problem, and are the second biggest parasitic killers worldwide after Malaria," Owens said.

But it turns out that this "parasite" may actually be beneficial for the flies that carry it, by helping them to fight off infection from a different type of pathogen, new research shows.

It was previously known that various species of the Leishmania protozoa can shorten the lifespan of sand flies, especially if they are stressed (hey, flies get stressed too)--but according to the new study, published in the journal Parasites and Vectors, nobody had looked to see if the microbe might have beneficial effects for the insect. But that's just what a team of Brazilian and British researchers has done. When they exposed sand flies to a form of Leishmania protozoa found throughout Latin America, then exposed the insects to pathogenic bacteria, many more of the protozoa-carrying flies survived. In fact, at least five times more of the Leishmania-carrying flies lived after exposure to the bacterium (known as Serratia marcescens), compared to flies free of the protozoa.

The Leishmania parasite "works as a kind of probiotic and reduces the mortality of the fly," said study co-author Rod Dillon, a researcher at Lancaster University in the United Kingdom.

"This is very interesting, as it is suggestive that similar mechanisms are operating here in the sandfly, as occurs in humans--i.e. that the ['good'] bacteria that inhabit your gut can protect you from pathogenic bacteria," said Ben Owens, an immunologist at the University of Oxford, who wasn't involved in the study. But in this case the Leishmania "is acting as a 'good' bug.'"

There are other instances of "parasites" having some beneficial effects for their hosts. For example, some helminths, or worms, can help regulate the immune system of animals that carry them, Owens told Popular Science. In fact, various helminths have potential to treat human autoimmune and gastrointestinal disorders like ulcerative colitis and Crohn's disease.

But not everybody is convinced. "I think it is really a stretch to say that the parasite has evolved to provide this protection," George Dimopoulos, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health in Baltimore told The Scientist. "It's more likely that Leishmania, as with all parasites that are transmitted by vectors, will turn on the sand fly's immune system, which in turn is going to provide some level of protection against any other type of microorganism." He added: "It's not something that is necessarily specific to [Leishmania]."

The team had originally been looking to see whether they might be able to halt the spread of leishmaniasis by exposing sand flies to bacteria (to kill the flies, but perhaps also make the flies less likely to carry the protozoa). But exposing the flies to this bacterium, could ironically do quite the opposite. "Sand flies not carrying Leishmania may succumb more rapidly to the biological control agent and this would lead to the development of a wild sand fly population containing an increased proportion of the surviving flies carrying the human disease", the authors wrote. A scary thought.

There is no vaccine for leishmaniasis, and it can be difficult to treat--the standard therapy to date usually involves injecting patients with an antimony-containing compound that can have bad side effects. But for sand flies, Leishmania is not the horror it is for humans.