Saturday, April 26, 2014

The Microbiology of Zombies By Dr Yvette Merga

Zombies come in a variety of forms these days; fast ones, slow ones, flesh-eating ones, scary ones and funny ones but whatever the type of zombie, they remain one of the most scary and successful “monsters” in popular culture. The model zombie is thought to originate long ago in the voodoo cultures of Haiti and West Africa, where the dead were said to have been ‘raised’ by sorcerers or witches using voodoo magic. Prior to that, they get a mention in the Mesopotamian poem from about the 18th century BC, “Epic of Gilgamesh” in which a goddess proclaims she will “let the dead go up and eat the living, and the dead will outnumber the living”. Bearing in mind that the dead do actually outnumber the living by some 107 billion to 7 billion, this is some good science!

The original horror-movie zombies of George A. Romero’s “Night of the Living Dead” (in black and white!) and other classics such as Dawn and Day of the Dead are shambling, rotting beings that aspire only to eat the living (and provide cannon fodder for film-makers to try out new ways of splattering brains around!). Modern versions include zombies that run (e.g. Danny Boyle’s ‘28 days later’ and the recent World War Z film) and even telepathic zombies (Brian Keene’s “The Rising” novels), although most films and books tend to stick with the original slow-moving, groaning variety. Overall, zombies seem to be defined by the main “symptoms” of hunger for living human flesh, being dead-but-mobile and being generally quite scary. Transmission (‘zombification’) is usually via a zombie bite or contact with infected fluids (technically known as Goo).Now, as a horror fan and microbiologist I’ve often found myself wondering about the source of the various zombie plagues and thinking about how a zombie apocalypse might actually come about.

The vast majority of zombie stories are attributed to a mystery “infection”. Sometimes, thoughtful writers attribute an outbreak to a named infectious agent such as Resident Evil’s T-virus or Zombieland’s “Mad Zombie Disease”, while others are much more vague. However, are there any infectious agents out there that could actually create zombies? Could a zombie apocalypse really happen?

Possible candidates
One virus that quickly springs to mind is Rabies. Rabies is a type of infection known as a zoonosis, which means it can spread between animals and people. It’s a virus that spreads to humans when they are bitten or scratched by an infected animal, such as a dog (this sounds zombie-like!). It affects the central nervous system, causing anxiety, headaches, fever, muscle spasms, brain inflammation, fear of water and eventually death. In dogs, it is well known for causing them to aggressively bite humans (also like zombies!). There’s a very similar virus called Bat Lyssavirus that’s becoming more prevalent, with transmission occurring from bats to people via bites or scratches. So the idea of a zombie virus travelling from person to person via infective bites is not too unlikely. However, no virus on earth is capable of infecting and taking over a host as quickly as the ones seen in zombie films!
One type of organism rarely mentioned in zombie films/books is the parasite. As you may have read in previous SSA posts, parasites can range from teeny single-celled organisms to big tapeworms and flukes, and they have many interesting characteristics, one of which is mind control or behaviour modification. This is a relatively new area of research referred to by scientists as neuroparasitology.
The best-known example of a mind-altering parasite is probably the small, zoonotic parasite Toxoplasma gondii, which stops rats being scared of cats so that they are more likely to get eaten; the brain-dwelling parasites are then spread to the cat where they multiply and are released as oocysts in the cat’s faeces. These same parasites have also been associated with neurological changes in humans that can manifest as personality disorders and increased rates of suicide. Other examples include the trematode (flatworm) Euhaplorchis californiensis, which changes the behaviour of its fish host, increasing the likelihood of its consumption by birds and the tiny crustacean Gammarus, which is attracted to light only when infected with the spiny-headed worm Polymorphus paradoxus, making it more likely to be eaten by predators, spreading its parasitic guests to other hosts.
Scientists have also shown that Toxoplasma gondii changes its host’s behaviour by “hijacking” its sexual arousal pathways, making the odour of cats suddenly seem very attractive to the rat – maybe it could do the same in zombified humans, making chasing and eating the living even more appealing! It is also thought that the immune response of a person infected with T. gondii directly affects their behaviour, and that T. gondii also damages the areas of the brain responsible for processing fear.
So, while these examples of parasite-induced suicidal behaviour are not strictly relevant when considering zombies, they do show us that it is possible for an organism to change the way a person might behave, causing them to do things they otherwise wouldn’t (like eating braaaaaains).

Another feature of most zombie infections is the quick spread of the disease – someone may be bitten by a zombie and change over to join the zombie ranks in a matter of seconds, making fighting off a potential apocalypse rather challenging. Luckily, there are no viruses, bacteria or parasites that could spread or have such an overwhelming effect on your body that quickly. Viruses tend to be the fastest-acting types of bug, but even these need at least a few hours to get into your cells and replicate into large enough numbers to actually make you ill; a stage known as the incubation period.

Of course, there is no organism around (nor anything else for that matter) that could actually re-animate the dead, let alone make them walk around and eat people, but it’s fun to pretend, and anyone wanting to see this in action should check out the 1980’s classic Re-animator, complete with green-glowing serum and re-animated cat!

For more information on the current worldwide rabies status, click here:

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