A Bad Case of the Zombies: Could a virus really cause World War Z?

A zombie playing the sousaphone. I wanted one playing the trombone because of the tenuous trombone/bone/zombie connection. Ho hum.

A zombie playing the sousaphone. I wanted one playing the trombone because of the tenuous trombone/bone/zombie connection. Ho hum.

The other day I went to see the film, World War Z. It was fine and thus ends my review of my enjoyment of it. Anyway, the real World War Z will of course be between those who pronounce it “zed” and those who pronounce it “zee”. World War Z is based on the 2006 novel by Max Brooks (a follow-up to his 2003 book, The Zombie Survival Guide). Both books are excellent and if you’re not too bored of zombie-based fiction then you should read them. I say this because there seems to have been a recent upsurge on things about zombies of some kind. The zombies are everywhere, which I suppose is ironic. The film stars Bradley Pitt as a retired United Nations employee who must travel the world to find a way to stop a zombie-like pandemic.

In the film being a zombie (Zombieism? Esprit de corpse? Zombosis?) appears to be caused by a viral infection, primarily caught by being bitten by a zombie. Those who are bitten appear to die within about 30 seconds and then reanimate with slightly cloudy eyes. They then become very aggressive and begin to chase down victims to bite them and spread the infection. They do not appear to eat their victims; rather keep on going just generally being runny and a bit bitey. I say runny as in they run a lot rather than hinting at any advanced state of decomposition. Although eventually the zombies do appear to go a bit rotten.

I accept that all this doesn’t have to be dead-on realistic (ahem) but there are a few problems with the concept. It’s assumed that the mass zombification is caused by a viral pandemic. Yet time from being bitten to turning into a zombie appears to be too rapid for this to be the case. It would take a bit longer for whatever virus it is to circulate, invade cells, hijack their genetic machinery and start producing copies of the virus and manifest symptoms. Especially given that the virus appears to completely take over the host’s central nervous system and musculature while leaving the rest of them deceased. Like a more infectious version of Britain’s Got Talent.

While viruses certainly can be deadly they generally need the thing they’re in to be alive to make more virus and spread them. This might be by sneezing in their co-worker’s face, not washing their hands, licking fruit bowls etc. The zombie virus doesn’t appear to need this. It kills the host and still somehow has them running around. Where is the host’s energy coming from? Could the humans all just hide and wait for the zombies to fall apart? Granted this would make the film quite dull. Nobody wants to watch a film where people eat sandwiches in a bunker waiting for their enemy to decompose. Although Panic Room is OK.

The idea however that an infection can control its host’s behaviour to help its spread is well established in nature. For example, malaria is an infectious disease spread by mosquitoes, caused by one of five species of the Apicomplexan parasite, Plasmodium. Most deaths from malaria are caused by Plasmodium falciparum. It really is an awful disease with the WHO estimating that in 2010 there were 219 million cases of malaria resulting in 660,000 deaths. As I’ve hinted, Plasmodium can change the behaviour of mosquitoes to spread itself faster and wider.

The common Plasmodium Puppet. Also known as the mosquito.

The common Plasmodium Puppet. Also known as the mosquito.

Once in a mosquito, Plasmodium needs time to move to the mosquito’s gut to mate and reproduce to form ookinetes. These are a sort of mobile egg. The story of Plasmodium really ruins Humpty Dumpty. Ookinetes develop into sporozoites (Literally: “animal seed”. Don’t go planting your hamsters though!) and travel to the mosquito’s salivary gland. Prior to this it doesn’t do the Plasmodium much good for the mosquito to bite someone with the risk the mosquito might get killed during the attempt. So Plasmodium tries to alter the mosquito’s behaviour to prevent this. For a mosquito to get your blood it has to drive its proboscis through your skin and find a blood vessel. The longer this takes the greater its chances of being noticed and squashed. Like if McDonalds killed you if you queued too long rather than years later of heart disease. If a mosquito finds it too difficult to draw blood they’ll quickly give up.  A mosquito with ookinetes in it will abandon biting quicker than an uninfected one.

However once the sporozoites reach the mosquito’s mouth, it benefits Plasmodium for the mosquito to bite as much as possible.  The Plasmodium at this stage appears to make the mosquito “hungrier”, causing it to drink more blood and visit more hosts to get it. In these ways and more Plasmodium is manipulating its hosts behaviour to reproduce itself and spread more easily.

Some species of tapeworm live in the three-spined stickleback but also spend part of their lifecycle in the birds that eat these fish. The tapeworms can alter the behaviour of the fish making it more likely they’re caught and eaten. As you’d expect, sticklebacks try to keep away from heron. They stay away from the surface and if a heron appears they dart away. Sticklebacks infected with tapeworm appear to become more fearless, staying near the surface to feed even if a heron is about. These are more likely to be eaten and the tapeworm gets where it wants to go; into the heron.

Similarly, Toxoplasma gondii, a protozoan of “don’t go near the litter tray if you’re pregnant” fame, needs to move between rats and cats and back again to complete its lifecycle.  A healthy, uninfected rat will normally become anxious when it smells cat urine staying away from where they smelled it.  They will literally piss off. Rats infected with Toxoplasma however do not become anxious when they catch the scent of a cat, do not avoid it and increase their chances of becoming dinner.

Toxoplasma also appears to alter the psychology of humans it infects. Men infected with Toxoplasma become less willing to follow rules and less worried about being punished for breaking these rules. Women infected with Toxoplasma become more outgoing. Toxoplasma: the party protozoa! I probably shouldn’t get into marketing. It is not fully known how this occurs although there is some evidence that Toxoplasma increases production of the neurotransmitter dopamine and in males, increases testosterone levels.  It should be noted that this evidence is largely from rats. A lot of evidence is.

Afraid? Are you a man or a mouse? Or are you infected with Toxoplasma?

Afraid? Are you a man or a mouse? Or are you infected with Toxoplasma?

All of our examples have been parasites, but the infection is World War Z is cited as a virus, which I guess technically can be seen as a parasite. Can a virus alter its host’s behaviour to aid its spread? You bet your hot butter on toast it can! The baculovirus, infects the caterpillars of the European gypsy moth and causes them to climb to the tree-tops. Once there they die and liquefy, releasing thousands of viral particles to rain down and infect more unfortunate caterpillars. In this way Lymantria dispar forces the caterpillar to turn itself into a piñata and explode itself, raining down sweets i.e. a nasty virus, on other unsuspecting future piñata-pillars.

Rabies is another viral disease that manipulates its hosts’ behaviour. Rabies causes acute encephalitis in warm-blooded animals, including humans. More than 55,000 people, mostly in Africa and Asia, die from rabies every year. There are three stages of rabies progression. The first is characterised by behavioural changes and is known as the prodromal stage. The second is the excitative stage. This stage is also known as “furious rabies” as the infected animal is exceptionally aggressive, hyper-reactive and will bite with little provocation. The virus is present in the nerves and saliva and as such the route of infection is usually, but not always, by a bite. With the encephalitis induced aggression and biting, the virus’ manipulation to aid its spread becomes clear. The third stage is the paralytic stage (due to motor neuron damage) which is followed by death.

The excitative stage of rabies is the example we’ve seen that is most similar to our zombie virus and in fact in the film the zombie pandemic (a good name for a band) is initially mistaken for an outbreak of rabies. So could a virus cause the changes seen in World War Z and cause a zombie pandemic with Brad Pitt staring concerned across various international scenes? Probably not, but parasites and viruses can certainly manipulate their hosts behaviour in a variety of subtle and not-so-subtle ways. Although ultimately it might be preferable to have your emotions and behaviour manipulated by watching a film. Panic Room is OK.

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