Venom: Why the alien symbiote isn’t as scary as a real parasite from Earth

The 2018 independent arthouse film Venom follows the profound and hard-hitting story of Eddie Brock, a mild-mannered journalist who is infected by an extraterrestrial parasite. It begins to eat his organs, it tries to take over his mind, it gives him superhuman abilities, at one point Tom Hardy jumps in a lobster tank. It’s the greatest film ever made.

But with the release of its sequel, Venom: Let There Be Carnage, come questions on how realistic its portrayal of a parasite infection is.

“As far as I am aware, [parasite] infections don’t lead to the growth of large teeth or the desire to eat one’s friends,” says Michael White, a professor in the College of Public Health at the University of South Florida. But, he adds, that doesn’t mean that they don’t have disturbing effects.

White studies Toxoplasma gondii, a single-cell parasite which typically passes through felines and rodents but infects humans too. For people with competent immune systems, it’s barely noticeable; it could just sit dormant in the brain. But the chilling aspect of T. gondii is that not only do most people not know that they are infected (which could, according to White, account for up to 30 per cent of the world population), but once they are infected they’re infected for life.

“That’s going to be a problem for anybody who has any kind of immunocompromised state,” says White. “You might be healthy today, but what happens tomorrow? You may be undergoing cancer chemotherapy in 20 years or you might have an organ transplant. During the AIDS epidemic it was a huge problem. It destroyed patients’ brains. They couldn't recognise their families. It was horrifying.”

There have been various studies and suggestions that T. gondii can alter the behaviour of its human host, causing personality changes or mental illness. One 2012 study, for example, links the parasite to a higher likelihood of schizophrenia. But White is reluctant to be drawn on whether such claims are true or not.

Read more about parasites:

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“I don't discount those studies,” he says, “but it’s a very hard to find a definitive answer. With a mouse, you can do it. You can take an inbred mouse and you can infect it and you can put some drugs in it and test it. So you can go through the Koch postulates” –a criterion for testing whether a microbe causes a disease – “with mice, but you can't do this in humans.

“So when someone does a correlation study and finds that men who are toxo-infected have higher traffic accidents, which is a paper I have read, no one can say that for sure. You have to take it with a grain of salt.”

What is fairly certain, however, is that parasites like T. gondii do alter the behaviour of animals.

“An interesting example is people who had been working with the hyena population in Africa,” says White. “They were noticing that some young hyenas were more susceptible to lions because they didn't seem to be afraid of them. And when they went in and looked at their blood samples, the ones that appeared to be less afraid of predation were positive for toxo.”

It’s a change that also been observed in infected mice, who seem to be far easier for their intended hosts, cats, to catch. “It is pretty much known if you put a powerful odour like cat urine in a corner an infected mouse would still go over to the odour, when they normally wouldn’t. So there's no doubt that it's shutting down the anxiety system.”

Venom: Let There Be Carnage picks up with Eddie Brock and his parasite having come to something of an understanding, transforming their dynamic from a parasitic relationship to a symbiotic one. There is some precedent for this. A 2014 paper from the Canadian Institute for Advanced Research, for instance, argued that Diphyllobothrium latum, a species of tapeworm, can boost your immune system in exchange for crashing on the couch of your intestines. The paper’s co-author even ingested three of them to prove it (he said he felt fine).

As far as T. gondii are concerned, however, there is no bargaining with it, no reasoning. It is simply terrifying. “They are hugely resistant,” says White, who explains that the parasite is impervious to being stored in substances such as 2 per cent sulphuric acid and dichromate, “which is the stuff chemists use to strip everything off a glass. These things stay in the environment, including in uncooked food, which is our biggest risk of catching it. If you like steak tartare, that's probably not a good thing.”

Hmm, maybe making a deal with an extraterrestrial parasite with a taste for flesh wouldn’t be so bad after all.