To Kill Parasites, Fruit Flies Self Medicate With Alcohol

Fruit flies seek out alcohol as a drug to kill parasites. Video by Emory University.

Infected fruit flies turn to alcohol to self medicate, a new study shows.

It’s no secret that fruit flies are fond of booze, and born with a naturally high tolerance to the stuff. In fact, the life of a fruit fly revolves around alcohol. It goes something like this: As fruit rots, yeast on the fruit breaks down sugars, creating alcohol.. The alcohol vapors signal to the flies that food is present. Adult flies are then drawn to the fermenting fruit, where they feed and lay their eggs.

“The flies in larval stage are swimming in alcohol,” said Todd Schlenke, assistant professor of biology at Emory University. “They’re really resistant to it.” Ideally, he said, they like their food with about 4% alcohol, or roughly the same alcoholic content as a bottle of beer.

But a new study published last week in the journal Current Biology takes the insect’s attraction to alcohol a step farther, showing that fruit flies infected by parasitic wasps are more likely to seek out higher concentrations of alcohol to kill off these parasites. This study adds to a growing body of literature showing that animals ranging from caterpillars to chimpanzees will seek out toxic plants and other materials in their environment to fight infections.

Many fruit flies are in a constant fight for their lives against endoparasitoid wasps. These wasps, not much bigger than edge of a dime, infect as many as half of the larvae Schlenke’s lab collects to study. They’re “pretty mean little aliens,” Schlenke said. They lay their eggs inside the body cavity of the baby fruit flies, and then they inject the infected insects with venom that suppresses their immune systems. The wasps feed on the flies, slowly eating them from the inside out, until the fly is gone and all that is left in the pupa is a wasp.

Schlenke along with graduate student Neil Milan and undergraduate Balint Kacsoh, both co-authors on the study, decided to find out if the fruit flies’ naturally toxic environment helped them to resist infection and fight off the predator once infected.

They bisected a Petri dish; half contained yeast and the other contained yeast with 6% alcohol. Within 24 hours, 80% of the infected flies had chosen the alcohol, compared to only 30% of the uninfected flies, indicating a preference among infected flies for the alcohol. The uninfected flies were also less likely to ultimately get infected, since the wasps couldn’t handle the alcoholic environment.

Plus, the alcohol appeared to effectively destroy the parasites. “If the flies had been eating alcohol, [the wasps’] guts would all kind of pop out of their anus,” said Schlenke. “That’s something we’ve never seen before.”

Robert Anholt, professor of zoology and genetics at North Carolina State University, who has studied the effects of alcohol on fruit flies, but was not involved in this study, calls this further evidence of Darwinian natural selection at work.

“The flies appear to have found a way to win the evolutionary arms race against the wasps,” he said. “The behavior and genetic architecture experience positive selection to survive in presence of a predator, in this case a pathogen.”

Humans and flies share many genes, especially when it comes to alcohol sensitivity and immune system responses, and Schlenke hopes this research could help inform studies on the prevention and treatment of parasites in humans. While the other medicinal effects of alcohol have long been studied, this study is the first to show that alcohol can be used to kill a blood-borne parasite, and protect against future infection, Schlenke said.

But some biologists are skeptical about the study’s application to other organisms, particularly mammals. While the study indicates flies have used a toxin to survive, it’s unclear whether that can be replicated among other animals, where fermented fruits or grains may not be as readily available, said Juan Villalba, associate professor of wildland resources at Utah State University.

“In nature, it’s difficult to replicate,” Villalba said. There’s evidence to support that humans have learned from animal behavior which plants to select for medicine, but it’s never been shown with alcohol, he adds.

Humans have used alcohol as a sanitation for thousands of years, Schlenke points out. It’s been used as a surface disinfectant, and history provides examples of humans preferring wine and beer when water alone could make them ill. His lab is now performing similar studies on other insects that feed on food containing alcohol to see if they will also seek out high doses to cure an infection.

But even among fruit flies, too much is too much. Even with a higher resistance and ability to process alcohol, prolonged high doses can cause fatty liver syndrome and other problems that we see in alcoholics, Anholt said.

“When you expose flies to saturated alcohol vapors, they act a lot like people do. They become animated and excited and then they fall over,” he says.