From haunted houses to video games, movies and trick-or-treaters, zombies are everywhere this time of year. But zombies aren’t real — or are they?

While the flesh-eating undead portrayed on television are just fiction, there are clear examples of parasites that have evolved to manipulate their hosts, often in ways that affect host behavior to favor parasite survival and spread, said Theresa Gildner, an assistant professor of biological anthropology in Arts & Sciences at Washington University in St. Louis. Some even live inside the brains of their host — eek!

“One way parasites manipulate their host is through influencing the release of neurotransmitters — the chemical messengers of the nervous system — like dopamine, epinephrine, serotonin and other chemicals that directly impact mood and behavior,” Gildner said.

“Given how common parasites are in the world and how well they have evolved to manipulate us — usually without the host knowing they are being influenced — there’s a chance many of us are already ‘zombies,’” Gildner said.

Below, Gildner answers questions about parasitic infections — including those that try to hijack the brains of their host — and explains why it’s unlikely you will need that zombie apocalypse survival plan. Read on — if you dare.

What are parasitic infections?

A parasite is typically defined as any organism that relies on a host for its essential nutrients, without any benefit to the host, and is therefore considered harmful. There are several types of parasites, including single-celled protozoas that cause Taxoplasmosis and malaria; helminths, parasitic worms like hookworm or tapeworms; and ectoparasites, parasitic species that live outside the human body, like ticks, fleas and lice.

Parasitic infections are extremely common globally, both for humans and animals. If you have a pet, you’ve probably given them flea or deworming medications to prevent parasite infections. But human parasitic disease is also still widespread, especially in low-resource communities that lack the resources and infrastructure to effectively prevent and treat infection.

With the exception of select parasitic diseases, like malaria, most are not lethal. However, these conditions often impair physical and cognitive function and result in suboptimal growth and development patterns. Overall, parasitic disease contributes to the perpetuation of structural health inequities, especially among marginalized communities where lack of access to key health determinants — medical care, functional sanitation systems and adequate nutrition — both increase parasite exposure and compound the negative impacts of infection.

Effective preventative measures — usually linked with effective sanitation infrastructure — as well as medical tests and treatments do exist for many parasite species, but not everyone has access to these resources, including in the United States. So, even though we have the tools to control parasitic disease in many cases, we are still nowhere near close to eradicating most.  

Could parasitic infections cause a zombie apocalypse?

Host-manipulating parasites are real, but it seems unlikely that parasites will cause a zombie apocalypse on the scale of those on “The Last of Us” or “The Walking Dead” for a few reasons. First, in order to spread easily from person to person, the parasite would need to be specific and well adapted to human hosts. In reality, many parasites have relatively long, complex life cycles that involve spending part of their development in the environment and/or in another species besides humans to successfully mature and reproduce.

Second, for the scenarios depicted in these shows, the parasite would have to incapacitate or kill its host quickly following infection, but this is an ineffective strategy for most parasites since they need time to mature and reproduce in the human host. If the parasite infection was this fast and severe, the infected human might die before coming into contact with other potential hosts, preventing future transmission to new human hosts.

Third, along these same lines, an infection this serious would almost certainly be readily apparent to uninfected people, leading to disease prevention measures — such as quarantine — that decrease the risk of future spread. Many human parasites therefore seem to impact the human host in more subtle ways, so we’re typically able to go about our daily lives and unknowingly spread the parasite to other people over a longer period of time.

But examples of zombie-like behavior in animals do occur, right?

There are many examples of parasites that hijack the minds of infected animals and insects to achieve a very specific aim. One well-known mind-altering pathogen is rabies, which impacts mammals like dogs, raccoons and sometimes humans, making them more aggressive. Rabid animals are more likely to bite other animals and people, which allows transmission through the saliva. The virus has also been shown to make the host afraid of water, leading them to avoid consuming water, which might dilute viral load in saliva and reduce disease spread.

Another example is the trematode Dicrocoelium dendriticum, a type of parasitic worm. This parasite ultimately wants to get to a grazing animal so it can complete its life cycle and reproduce, but it must go through an intermediate host — the ant — to do this. The parasite affects the brains of ants, causing them to climb to the top of a blade of grass at night instead of returning to the ant colony. This behavior increases the likelihood that a grazing animal accidentally consumes the infected ant during the night while it is grazing.

There’s also the Hymenoepimecis argyraphaga, a parasitic wasp that infects spiders. Female wasps will first paralyze the spider and then lay an egg in its abdomen. The egg hatches and larva feeds on the spider’s blood while the spider is still alive. The spider will behave normally for several days, but then the wasp larva injects a chemical into the spider that causes it to build a unique type of web and sit motionless in the middle. Then, the larva kills the spider host with poison, eats the spider and builds a cocoon in the middle of the web for protection until the adult wasp emerges and the cycle continues.

The Toxoplasma gondii parasite typically spreads between rodents and cats, although the CDC estimates that more than 40 million people are infected with Toxoplasmosis in the U.S. Generally, rodents become infected after consuming contaminated food or water. Once in the rodent intermediate host, the parasite continues to mature and forms cysts in the rodent’s tissue. Cats become infected after consuming infected rodent tissue. Mature parasites live inside cats. Infected cats also shed the parasite in their feces, where it continues to mature and become ineffective.

The interesting part of the Toxoplasma life cycle is that some of the parasite cysts form in the rodent’s brain, potentially concentrated in the part of the brain that regulates fear, some researchers believe. This directly affects rodent behavior, making them less fearful of cats. Some evidence suggests they might even be attracted to the smell of cat urine, rather than fearful, increasing their risk of encountering a cat and being consumed. This is ultimately the best outcome for the parasite: for the intermediate rodent host to be consumed so the parasite can get into a cat and complete its life cycle.  

Well, this is terrifying!

The good news is that we have evolved with these parasitic species for a very long time, and our immune systems are generally effective at keeping infections in check. We also have many effective medical treatments available to help treat infection and many people in the world today have access to important resources — such as clean water and food, sanitation systems and well-constructed houses — that help shield them from serious infections.

However, parasites still infect millions of people around the world, so we still have a lot of work to do to make sure all people have access to the resources and infrastructure needed to prevent continual infection and related poor long-term health outcomes.  

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