Biology concepts – neural parasitology, domoic acid
toxicity
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Zombies
don’t have a choice in how they behave. Free will
is
a thing of the past; they don’t even have the ability to resist
a
dance routine with Michael Jackson. Michael seems awfully
at
ease in the midst of the undead. Creepy….yes. Scary…. yes.
A thriller?…..maybe no.
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Don’t scoff at this; nature is full of examples where one
organism can cause another organism to change its behavior – just think of all
the silly things boys do trying to impress girls. But first a couple of stories
where a change in behavior has less to do with parasitism.
In August 2013, residents of Moscow began reporting that the
pigeons were acting odd. They would walk around in a funk, not get out of the
way of traffic, and not fly away from danger. One family reported that their
dinner one evening was disrupted by a pigeon on their window ledge that lost
its balance and fell into their kitchen.
These zombie pigeons (the pecking dead, as one website
called them) were freaking out the population, so the scientists went to work.
It seems that many of the dead and affected pigeons were carrying salmonella bacteria
and/or had Newcastle disease. The virus that causes this disease,
unimaginatively called the Newcastle disease virus (NDV), can be transmitted to
humans, so it's a good thing the population got freaked out.
The virus causes the birds to stagger about, stumble around
in circles, and turn their heads upside down – much like vodka does in humans.
However, when humans get NDV, they most likely will just have a flu-like
episode.
Zombie birds also led to a famous movie. Alfred Hitchcock’s
classic film, The Birds, is the story of a terrifying attack on a small fishing
village by many flocks of different birds. They attacked people, flew into car
windows and houses, and caused deaths and damage. Seems silly doesn’t it, being
killed by a shore bird? I think I would have them give some other reason in my
obituary.
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Tippi
Hedrin was the female lead in Hitchcock’s The Birds. Hitch
had
seen her in some commercials and chose her over Grace
Kelly…..
GRACE KELLY! Later on , he developed an unhealthy
obsession
with Tippi, and who wouldn’t, with all that running and
screaming and bird doo?
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In 1987, it was recognized that a toxin produced by certain
species of marine algae was responsible for the zombie like behaviors. Called domoic acid, the toxin is produced by
the algae and accumulates in marine organisms that feed on phytoplankton or
algae that are contaminated. Normally, levels of domoic acid are too low to
cause problems, but in years where the algae overgrows, called a bloom, the
levels will rise dramatically.
Although the acid seems to have no affect on lower life like
shellfish, bigger animals are strongly affected, including humans. When the sea
lions or birds feed on contaminated food, they begin to display the bizarre
behaviors. In the case of the 1961 birds, there happened to be a collection of
samples from the bay that had been kept all these years. Tests on the shellfish
and algae samples from 1961 showed high levels of domoic acid.
In a strange coincidence, a new paper has been published
about how infections can move through a flock of birds. It uses a mathematical
model based on many predictors and factors. The model is called the Zombie-City model, based on how a
zombie population might grow in a population of unsuspecting humans. But we
want to focus on the loss of free will in nature’s creatures.
Free will in lower animals? It does exist. Most people believe that the behaviors of
insects and such are merely responses to environmental and situational cues,
and any variation in behavior is due to misreading of cues or random errors. But studies in
fruitflies show that they can pick out their own patterns of behavior when a
blank canvas is given them.
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The
Emerald Cockroach Wasp (A. compressa)
is solitary insect,
it
doesn’t live communally as many bees and wasps do. Only
the
females have stingers, so making zombies is definitely a
reproductive
strategy. In 1941, they were introduced to
Hawaii
to try and control the cockroach population, but it
didn’t work. They just don’t lay enough eggs.
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What the wasp steals is the roach’s ability to decide if it wants to
walk or run. Most wasps sting to kill, but the Jewel Wasp stings the cockroach
in the brain, altering its behavior with its venom. A 2010 study showed that
the wasp stings the roach continuously for up to three minutes, trying to
locate a particular part of the cockroach’s brain.
What it is searching for is called the subesophageal ganglion, the part
of the brain that allows the roach to initiate walking and running movements.
When that part of the brain is flooded with venom, the cockroach stands still,
with no will to begin leg movements. It isn’t paralyzed – it’s just a zombie.
Another study has started to investigate just how the wasp venom robs
the cockroach of its will to walk. There is an insect neurotransmitter called
octopamine that is released by some of P.
Americana’s neurons. It is this transmitter that allows the cockroach to initiate
walking.
The study hasn’t pinpointed just how the venom interrupts the octopamine
signaling, but they know if they deplete the amines in the brain, they see the
same affect. If they add back octopamine, they can rescue the cockroach’s
natural behavior. However, the study also showed that the venom doesn’t reduce
octopamine levels and it doesn’t prevent is release, so there's still more
work to be done.
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The
wasp first stings the cockroach in the abdomen, just a
quick
sting to temporarily weaken the front legs. Then it
stings
the brain and when the cockroach stops
moving,
it cuts off part of one antenna. I don’t know why. It
grasps
the antenna in its jaws and herds to the roach to its nest.
Instead
of pulling, they should evolve saddles.
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So why not just kill the cockroach with the sting and lay the egg? The
larva need fresh meat, and a dead cockroach rots in one day. To make the meal
satisfactory for the eight days needed, the cockroach must remain alive, but in
a state where it can’t attack the wasp or the larva; hence the zombification.
It gets even creepier. The wasps have gotten so good at this strategy
that they now go to the trouble of cleaning their meal. A 2013 study shows that
the wasp larvae produces several antimicrobial chemicals that rid the cockroach
of any contaminating bacteria or parasites as the larvae munch on it. I know I’d
clean a zombie before I ate it.
There are several other examples of theft of free will, including a couple
of fungi that make ants stop their normal work and climb high in trees to allow
for the best spread of the fungal spores as they mature. There’s also a
hairworm that forces grasshoppers to commit suicide by jumping into water, just
so the worm can complete its life cycle. But I don’t want to leave this subject
without hitting the king of neural
parasitology – Toxoplasma gondii.
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The
spiny ant is the zombie victim of the O.
unilateralis fungus.
When
infected, the ant stops doing its job for the colony and
falls out of the tree canopy. After wandering the forest floor, it
will
bite the underside of a leaf and never let go. It just stays
there
waiting to die. Then the fungus sprouts a fruiting body
with
spores out the top of its head, and the spores shoot of
into
the air. The low altitude (less humid) and under leaf
position
give the fungus the greatest chance to survive. Fossil
evidence
shows that this has been occurring for at least 48
million years.
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T. gondii is a
single-celled eukaryotic parasite that has a complex life cycle. It can reproduce asexual
in any of the hosts it infects, but can only reproduce sexually in cats, of all
things. This is important because sexual reproduction is an obligate life cycle
stage for the parasite and contributes to its evolutionary health.
The parasite has taken steps to insure that it finds its way into cats
by changing the behaviors of the mice and rats it finds itself inside. It messes with rodent brain chemistry (since it tends to form cyst organisms in
the brain) that makes rodents unafraid of cats. In fact, a recent study found
that the organism confuses the rodents into believing that cat urine smells
like a potential mate!
T. gondii
activates a certain neuronal transcription factor, which leads to increased
production of different proteins in the brain. In rodents and humans, this
leads to an increase in dopamine (similar to octopamine in the cockroach) production and a decrease in tryptophan usage.
Because the cysts target areas of the rodent brain that
control fear, the change in behaviors are involve, but are not limited to fear.
There isn’t any evidence that the cysts have a selective range in the human
brain, but considering the changes that occur in men as described below, it is a possibility.
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On
the left are four T. gondii parasites
and on the right are the cysts
that
they can form in the brain. Recent ecvidences are showing
that
cysts of T. gondii can be linked to
increased chance of suicide
attempt
and more violent attempts, more depression and
neurotic
behavior, and an increased chance of having children
that
will develop schizophrenia. In humans, this may have
something
to do directly with the parasite, or it may be that high
levels
of immune modulators lead to the changes in thoughts and
behaviors.
Scary thought, our immune system could drive us
to kill ourselves.
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In general, men with a long-term T. gondii infection show lower IQs,
are taller (about 3 cm on average), and are more likely to break rules, take
risks, be jealous, and exhibit anti-social behaviors. Right now – I’m not so
proud to be a guy.
On the other hand, women with long-term toxoplasmosis infections tend to
be more outgoing, friendlier, more promiscuous, and more attractive to men.
Wow, Mars and Venus to the nth degree! The question still remains – how do the
forced behavior changes in humans benefit the organism?
Next week, we return to our discussion of nucleic acid exceptions
by discussing instances where organisms can rewrite their genetic code.
For
more information or classroom activities, see:
Moscow
zombie pigeons-
Domoic
acid toxicity –
Jewel
wasp and cockroach –
Toxoplasma gondii –
Fungal
parasites and ants –
Hairworm
and grasshopper -
Zombie
apocalypse case study for class -
