Newswise — As far as relationships go, parasitism may seem particularly selfish: one partner benefits at the expense of another. Many parasites even alter the behavior of their hosts to get what they need. Parasitoids are similar, but they usually spend a significant portion of their lives living inside or on their hosts’ bodies and controlling them from the inside-out, before ultimately killing and often consuming them.“My favorite example is a nematode parasite that infects ants” says Melissa Bernardo, a PhD student at Wesleyan University who studies how parasites and parasitoids manipulate their hosts. This nematode alters the ant’s color and behavior, basically turning the ant into a berry so that the nematode can pass into its next host—a hungry bird.Bernardo and her colleagues are interested in a more subtle, but equally complex type of host manipulation: how parasites and parasitoids influence feeding behavior and diet of their hosts. Diet manipulation could be reasonable because the parasite might need different types of nutrients than the host. But few researchers have studied host diet manipulation, and none have shown it conclusively. Bernardo says “There hasn’t been much evidence.”

Bernardo has been examining host diet manipulation to determine if any empirical evidence can back up what has largely been just a hypothesis. She’s found support that diet manipulation might be possible for one special parasitoid wasp, considered a “master manipulator”. About 14 days after laying eggs inside its host (the wooly bear caterpillar), the larvae of this wasp emerge, killing the caterpillar in the process. Bernardo has turned to this system because, unlike most caterpillars, the wooly bear caterpillar grazes on over 80 different plant species, meaning the parasitoid could have a veritable smorgasbord of diets to choose from. If diet manipulation exists, this would be a good place to look for it.

In a series of experiments, Bernardo found that when caterpillars were allowed to choose between a protein- or carbohydrate-rich diet, unparasitised caterpillars chose a protein diet, whereas parasitized caterpillars preferred a carbohydrate diet. In effect, Bernardo says, “The wasps are making their hosts carb-load.”

But why? It turns out that when caterpillars eat more carbs, the wasp larvae that chew their way out of the caterpillar’s carcass are bigger. Bernardo explains, “when these parasitoids are older larvae living in the host, they switch from feeding on host blood to feeding on specific host tissue.” This tissue is rich in lipids, which wasps can’t make, so they get the lipids from their hosts when they are larvae. By making the caterpillars eat more carbs, the wasps cause more lipids to accumulate in the caterpillars, leading to bigger wasps.

In case you’re thinking this sort of host manipulation is confined to distantly related insects, think again. Some evidence suggests that humans with Toxoplasma gondii infections show risky, self-destructive behaviors and may even be more prone to schizophrenia. In its natural system, rats infected with Toxoplasma are less afraid of cats, which facilitates the parasite passing into a cat host. Bernardo is more interested in how human parasites and gut microbes might influence diet. “There are hypotheses out there that predict that our feeding behavior would be altered depending not only on the parasites in our gut, but our gut microbiome in general,” she says.

Bernardo also thinks this research fits into a new field called ecoimmunology. Ecoimmunology uses concepts in ecology and evolution to investigate the causes behind diseases and specific problems in immune function. Bernardo’s research could add a new facet to this field—host manipulation. Obesity is a major health concern in the US, and specific kinds of gut microbes might not only influence how we digest food, but which foods we want to eat.

In other words, if you feel like skipping the gym and eating a donut instead, blame it on your parasites.

Bernardo presented her research at the 2016 annual conference of the Society for Integrative and Comparative Biology in Portland, Oregon.