Dead Likely? The Science Behind The Zombie Apocalypse

Maddy and Gavin team up to discuss the possible agents behind a zombie uprising. A common feature in many sci-fi movies, games and books, is the zombie a scientific possibility?

The zombie apocalypse will happen. At least that is what we are led to believe. It may be bioterrorism or an evil pharmaceutical company’s twisted experiment gone wrong. Either way, the media has led us to believe that this deadly scenario is not just a possibility but a probability. From the many computer games to the masses of films, this topic has been covered from every angle, but what is the science behind the stories?

The likelihood of reanimated corpses rising to fulfil their bloodlust is minimal but there may be some feasibility in stories involving infections. This has been explored in many movies and videogames including the 28 Days Later franchise, Left 4 Dead, Dead Island and Resident Evil. These plots are still far-fetched but at least have their roots grounded in science. So, how likely is a zombie apocalypse and how may it come about? It’s definitely worth exploring — after all, it might just happen.

Zombie ants

Though it may scare some to know, zombies already exist. Carpenter ants in Thailand have recently been discovered to be plagued by zombification caused by a parasitic fungus which infects the ants and manipulates their behaviour in order to increase its own transmission. The fungus, a species of Ophiocordyceps, is absorbed in its mycelium form (the vegetative phase) and thrives on the organs of the ant, releasing unidentified chemical signals which penetrate the central nervous system and allow the fungus full control over behaviour.

The infected ant displays unusual activity, often found astray from the group, wandering on nearby vegetation. When the fungus has found the perfect spot where conditions are ideal for optimal transmission, it induces a ‘death grip’ response in the ant, which bites down on the stem of a plant, locking its body in place. As the fungus finally starts to feed on its brain, the ant dies. A fungal growth called an ascocarp erupts from its head and releases spores, potentially infecting new victims.

So far the fungus has not made the leap to human transmission and no other known fungus exists that could. However there is always the possibility that on some undiscovered island, a Cordyceps fungus exists with the ability to jump the species barrier. Let’s hope not.

Toxoplasma gondii

Parasites, like the cordyceps above, always have a way of manipulating their host in order to ensure their own continuation. Toxoplasma gondii, a small protozoan parasite, is no different and is frighteningly common, affecting one third of the world’s population. Although primarily a cat parasite, T. gondii has intermediate hosts such as humans, livestock and rodents. However to reproduce, the parasite must be in the definitive host, so it manipulates the behaviour of its intermediate hosts to make this happen.

Rodents are timid creatures, normally averse to cats, their natural predators. Laboratory tests have shown that mice infected at early post-natal timepoints display increased activity and become ‘bolder’ – more readily exploring new territory and preferring to stay in more open, exposed areas of the testing chamber. Furthermore, mice have an amazing sense of smell and normally avoid areas marked by cat urine. Infection with T. gondii can not only cause mice to lose this life-saving instinct but also to become attracted to the smell of cat urine. The mechanism behind the alteration of behaviour is unknown but it makes these mice far more vulnerable to attack from cats, a method the parasite employs in order to breed. The parasite has even been shown to alter the personality of humans but unfortunately not in such a way as to create the flesh eating zombies that we know and love ¹.

Prion diseases

As infectious agents go, prions are indeed a novelty. Not a parasite, virus or bacterium, prions are misfolded proteins. These proteins can be transmitted from one person to another through ingestion of, wait for it … BRAAAIIINS. This major prion protein, or Prp, exists in two isoforms: the normal physiological isoform, PrPc and the misfolded form PrPsc, named after the prion disease scrapie found in sheep. Once the misfolded PrP protein has been ingested, it corrupts all PrPc production, leading to a build up of the scrapie form which is protease-resistant and cannot be removed by the body. Plaques begin to form in the brain, much like Alzheimer’s disease and a similar irreversible neurodegeneration occurs. Clinically, these patients undergo behavioural changes, with symptoms including depression, hallucinations and increased aggression. Muscular abnormalities are also common. Some form of prion outbreak is plausible as history has already shown us, with a UK epidemic of bovine spongiform encephalopathy (BSE, human form termed new variant Creutzfeldt-Jakob disease) beginning in 1987 and killing nearly 200 people over the next 2 decades.

Rabies and rabies related viruses

By far the most used virus in any storyline is rabies. This can be attributed to the virus’s ability to transmit between multiple species, how it changes the behaviour of those infected and the fact that there is no definitive cure. The combination of these qualities provides the perfect zombie story, although minor tweaking may be required to produce one of apocalyptic proportion. Both rabies and rabies related viruses fall under the genus Lyssavirus and are very closely related, although the latter affects insectivores more than other mammals. The fact that rabies has remained prominent for thousands of years is partially due to its ability to cross between species. Humans often contract the disease via bites from infected animals as the virus is found in high titres in saliva. Once infected the virus may replicate in muscle cells before invading the peripheral nervous system. It does so by binding to acetylcholine receptors, which is similar to the binding mechanism of many snake toxins. Once in the peripheral nervous system the virus moves towards the central nervous system where it disseminates into other cells. Victims initially present with flu like symptoms followed by huge behavioural changes, in particular disorientation. This is followed by extreme aggression and intense hydrophobia with an inability to swallow.

Rabies is almost always fatal and treatment is normally preventative. Following a bite by a possibly infected animal, the wound can be cleaned and treated with several doses of rabies vaccine. This must be carried out within 24 hours however or all is lost … almost. There have been a few cases where people have successfully survived rabies through the implementation of a treatment dubbed the Milwaukee Protocol. A drug-induced coma is used to protect the brain as the immune system mounts its response to destroy the virus. Several people have survived rabies through this treatment but its requirements wouldn’t be easy to implement if bitten by a zombie.

In terms of a zombie apocalypse rabies is an excellent candidate for an infectious agent. The localisation of the virus in saliva coupled with the intense aggression experienced by the infected gives rise to the idea of humans running rampant in the streets biting one another. It should be noted that there have been no recorded cases of humans transmitting rabies to one another via bites, however the possibility still remains. The incubation of the virus also differs widely between individuals with some people not experiencing symptoms until at least 2 years after initial infection. However with a few minor modifications, let us say through genetic modification by an evil corporation, this virus would be excellent for bringing down civilisation.

So, what are the chances of all this happening? Well, as expected, they are very slim but the science does exist to make it possible. Who knows, whatever the mechanism, one day this may all occur and the science fiction may become a lot less fictional. Until then, you can prepare yourself using this knowledge and hopefully the scientists of the world can prevent the unthinkable happening — but if you’re anything like these two authors you should make your zombie plan now before the zombies start chasing you through George Square and World War Z really does come to Glasgow.

1. Flegr J, 2007. Effects of Toxoplasma on Human Behaviour. Schizophrenia Bulletin 33 (3) 757–760